Trailer cruiser revisited... as a trimaran

[marshmat]

I now have free time again, for the first time in eight months. Thus, it is project time once again.

Some of you may recall my last project ( http://www.boatdesign.net/forums/projects-proposals/playing-around-10-m-trailer-cruiser-17597.html ) and I very much appreciate the excellent discussion and heaps of advice that came from that one.

This is the next stage of that project. The planing monohull hasn't been rejected- it's still in the running. But between the computer simulations and the model tests, I'm not convinced it would run well in the "hump speed" range with the kind of load (1 to 1.5 t of payload) I'm talking about. And the need for a 150 hp engine is a bit off-putting.

Recently, I've been working on some Godzilla optimizations for a trimaran in this size. It'd be a folding tri, less than 1.5 tonnes on the trailer and capable of carrying 1 to 1.5 t of people and gear. LOA is 8.5 m, beam would be about 4 to 4.6 m with the amas out and less than 2.5 m with them retracted. I know that's a fair bit wider than most power tris, but for a reason- we want a lot of space out on the wings for carrying bulky stuff (kayaks, lumber, trusses, etc.) or playing (fishing, swimming).

I want this to be a relatively straightforward, quick build, so it's all developable shapes for taped-seam plywood/glass. The shapes are based on Godzilla optimizations, adjusted for developable construction while keeping the sectional area curve, key dimensions and coefficients the same.

As shown, at 2.5 t total displacement, the drag at 10 m/s (20 kt) is about 3.1 kN. Even with the flattened stern I don't think speeds much higher than this are feasible, but 16 to 20 knots on the top end should be plenty. More importantly, 16 knots loaded down / 20 knots empty would be achievable with a 50-horse outboard, and a very economical cruise at lower speeds should be possible.

Comments are welcome on any of the following points:
- Feasibility
- Folding mechanisms
- Styling (the blockiness shown here will not survive)
- Practicality
- Possible performance issues
- Anything else you feel is worthy of comment

Please keep in mind that this is only the 9th iteration of the design, and the first iteration in which the complete tri form is assembled. The intended use is mainly on canals, inland lakes, and the coastal areas of the Great Lakes. Crew would be 2 for a week, or 4 for a weekend.

[Fanie]

Hi Mars,

My 2 bits -

I like the size, 8m500 is a very nice size.

The length of the ama's also looks good, I don't like these short thingy's they put on some tri's.

The ama's is too low. I like the extra bouyancy and they can be used to store lots of stuff in which will free hull space up. It will also keep the trampoline up a bit to make for a dryer ride. There seems to be the idea the leeward ama must bury... I say no, keep it up and dry.

Personally I would look at a 6m BOA on an 8m500 tri. More stability, and more room to move around and space to do stuff on. Never heard anyone say they have too much space, usually not enough.

I've been playing with the idea of having the fuel, gass bottles in the ama's as well as the batteries. Piping and wiring goes through the beams to the hull, could also save some space in the hull, although I haven't thought this through yet.

One thing a tri does have a lack of is solid areas one can walk on. Maybe the ama's 'decks' could be widened towards the hulls. I remember seeing a tri that has solids between the hull and ama's, I liked it more than the tramps you cannot fish from properly ;) Even a part solid would be a bonus.

At 16 knots there won't be much drag from the stern, the hull would just plane over.

The 2 for a week sounds good, who is she :D

[Willallison]

I'll be intereseted to see how you manage to squeeze those 2-for-a-week accomodations in. This has always been the problem with small multi's - there simply isn't enough room in the hull volume to move around. And moving all the accomodation onto the bridgedeck makes the boat very high. Having said that, if you can get around that little snag, the gains are definitely there in terms of efficiency and stability - though multi's aren't exact;y renowned for their ability to cope with big variations in payload....

[Fanie]

Eh Will, it is quite possible imo. Why do you think I asked who is she, you gotta think one on top... oh never mind :D

The place for the pipe berths is where the hull flares wide, and if you make them telescopic or foldable they are away when you are awake, and you bring them out when you want to sleep. I like the foldables since you don't have to waste fishing time putting sleeping bags and blankets away, you just squish them to the side of the hull and you are done.

The hull is long enough for 4 berths and some storage in that area. If the hull bottom which is flat which I like is around 600 / 700mm wide you can walk there, maybe passing is less comfortable but that depends how big one (or two) is. A standard house door is 800mm wide.

There is place for a head ahead and a shower unless you plan to keep Wet-Ones for all accasions :D

What have you in mind for a sail Matt ? There's got to be one.

[Willallison]

What have you in mind for a sail Matt ? There's got to be one.

I fear you may be disappointed Fanie.... Matt lurks on the dark side with me;)

No arguing about whether it's possible to fit the accomodations in.... Just a question of whether it results in so much compromise that the gain in efficiency over a similar sized mono makes it worthwhile....

[marshmat]

Could probably slap a mast on her, yes.... unfortunately that would make many of the canals and lock systems around here virtually inaccessible. I might include fittings for a kite, though.

I'm thinking of a convertible cockpit seating area / dinette / double berth for the flared part, with a windscreen and small hardtop immediately forward of this at the helm.... the amas would fold in and under for trailering (I'm working on the 4-bar linkage to do that, they definitely fit quite well but the linkage is a bit tricky).

I've done 5 people / 5 weeks in a tent trailer, and several camping trips in my 5-metre Bolger Diablo (using a shore-based tent for sleeping). So we're quite fine with tight quarters.

An earlier series of attempts with a catamaran led to disappointment accommodation-wise; the hulls just couldn't be made wide enough to fit bunks without compromising the efficiency that made the cat attractive. I think with the folding tri, I can pull it off.

Fanie, I like the idea of getting a bit of solid decking instead of just trampoline. The trick would be to get it to fold with the amas, but it would definitely be a huge advantage when it comes time to haul stuff (or partiers).

[Fanie]

On the cat, Matt :D I don't see how you couldn't fit the accomodations. You should look at making the cat foldable as well, something like I'm doing and cat2fold already did. Means your hulls can be around 1.2m wide each and still trailer legally. The folding beams on an 8m cat is going to be considerably less than an a larger cat 10m or longer.

Want to reconsider ? The only drawback the folding cat have is you cannot add a single outboard, you have to go double, but the advantages are numerous over a tri.

On the tri you could look at widening the aft beam to make a solid area. I have no idea what your folding system is going to look like though. If you are going to fold a narrow beam you can probably just as well fold a wide one.

As for adding or not adding a sail, imo it's not even a question. It's half the fun on either... just motoring is fun for just a short while but it gets boring quickly, sailing is quiet, you keep buzy, it's fun, less expensive... and you can go non stop and not worry about fuel any other than your own.

Have a look at the sail setup I used for the little tri, easier than that you can't get, any one even a novice can sail it.

[marshmat]

Fanie:

Like so, only folding?
I rather liked the performance simulations of this design (2.5 tonnes at 20 knots with only 2.5 kN drag, so twin 20-25hp outboards). Never thought about making it fold, though. The hulls are quite narrow and, as drawn, the overall beam was trailerable. Flaring out the hulls top provide enough space in there for a few bunks resulted in the hull centrelines being too close for the stability I'm after, and any increase in the waterplane beam- only 67 cm- increases the drag noticeably.
I will take another run at that one with folding beams. The bridgedeck cabin would have to be scrapped but since the hulls could then be widened more above the waterline, it might work....

As much as I like sailing, a mast would be rather difficult here unless it can be folded and re-stepped by one person in a minute or two. There are just too many low bridges in my preferred cruising grounds.

[Fanie]

I disagree with your weight on this size boat, unless that's everything... the bare boat should weigh less.

That's the idea. The beams fold into cavities in the hull sides, so the two hulls sit side by side. To launch they are floated apart and the deck is folded over the area from beam to beam. This keeps the hulls from overtaking one another and is the tennis court I mean deck area.

A hull that is 8m500 long and 1m200 wide and say 1m500 high is quite a big hull. They seem narrow but they will float with a few people on them. You should have at least 8 tonnes or more of floatation per hull, draft around 300mm. A normal double bed is only a few mm off on 1m200, you can check...

You're not really limited by the BOA, the decks can also be 1,2m each if there's 2 per hull you'll end up with a 8m500 LOA cat that has a BOA of around 7m200 :D

The very first cat I drawed was this size, and that was before I knew about cat2fold and that it was illegal to make a cat that wide :rolleyes: I still like the idea though... Tri's can be square, why not cats :D

With two 40's it will fly, two 28's or 30's is plenty and even 2 x 15's is good enough.
You can add two shorter stand alone masts ie 10m each and still have a large sail area, again more manageable than one huge sail.

The initial 'cabin' I designed was a canvas tent, but tents are cold, noisey in wind and hot in the sun. I later tried to design a foldable cabin, but actually making it would be something else. The current design for my cat is a cabin trailered on a second trailer, we are always going with more than one vehicle anyway so.

The sail setup I used works quite well imo. I'm not the sailing expert around here, but I think I got some fair results.

[Fanie]

Pic 1 there is Dugout, the name I gave the roughly built experimantal little tri. I built it to test the 'aft mast' (stolen from Brian Eiland) and single sail with. The sail furls up in a second and is out of the way. Deploys as easy.

Pic 2 is where the mast fits in. You clip the sail to the mast head, just push the foot into the slot there, all done at ground level, and push the mast up. This is easy in this case because the mast is only 6m in total which is actually 2 to 3m short of what a single mast should be for this size a tri. On a larger mast I would do this with a gin pole and a winch, just as easy. The slot prevents the mast from swinging in any direction, it can go only up or down. Once up you clip the furler in place and this holds the mast upright.

Pic 3 is on the Vaal dam. The wife announced 'that boat is following us' and by golly that mono tried to catch us. I was actually just cruising with a slack sail then, soaking it in, he he... some minor adjustments later we were leaving it behind. My sailing area is 12m^2

LOA 5m500, BOA 4m :D Told you I like square things ;)

Manie also sailed with me once, you could ask his opinion on it too. He has some dinghy experience so I guess anything ese should feel good :D

http://www.youtube.com/watch?v=buLlFWcjRUo

[marshmat]

Thanks for posting the details on "Dugout", Fanie. Your folding system is roughly what I had in mind for my trimaran, although I'd be thinking of a slightly more bulky vaka hull to give some interior room.

2.5 tonnes is at design displacement for both the cat and the tri versions of what I'm working on now. No more than 1.5 t trailering weight.

The aft-mast idea looks rather appealing. I saw some of Brian's articles on that a while back, but they were all about very large, sophisticated boats and I didn't really think to extend the concept to a little folder. But an 8 m tall aft-stepped mast should be able to fold forward within the trailering length of an 8 m tri or cat. I don't like the idea of trying to engineer a folding hinge for the cat that would also support a mast step... but one in each hull, now there's an idea... aft-mast bi-plane....

(Am I crazy yet?)

More on the cat coming this afternoon....

[Fanie]

I saw some of Brian's articles on that a while back
A pity he didn't persue it some more. Personally I think there is some real merit in them. A few ovelooked simple trick of which I 'discovered' a few of, adds quite a bit of functionality.

I also didn't like the idea of the mast on the beams, and you will also have to stay it. The hull provides a very nice guide and support to hold the masts.

If you consider that I have a small 12m^2 sail on only a 6m mast of which 700mm is in the hull, then if it was a cat there would be 24m^2 on a double setup.

A single 8m mast is going to give you 18m^2 sailing area. To reach the same sailing area as the 2 x 6m masts you will need a 10m mast...

A more proprtional mast would be 8m in length, then 2 masts would give a 36m^2 sailing area. I can live with that :D and probably fly a bit with it too :D

Sorry, I know the stuff on my tri must be boring, it is only an experiment, remember. It's going to be better on the cat tho.

If there's crazy here then I'm in too...

[marshmat]

36 square metres of sail Fanie.... on a tonne or two of cat, that would certainly tend to move you along. I would think the forestay angle on the aft-mast rig would also tend to encourage the sail to pull upward as well as forward, much like some of the more elaborate spinnaker setups you see on offshore racers.

See more cat below.... 8 m on the waterline, draught 30 cm at a hair over 2 tonnes, hulls at 4 m centre-centre, and folds down to something that might fit on the trailer. Those hinges will be beasts, though....

[Fanie]

The forestay angle does make a lot of lift. On the Vaal dam the first day the wind was really up, at one stage I thought it was going to push us over. At no stage did the ama's even looked like it's going to go under.

That said, one must be carefull that you don't just make lift and no foreward power, it's a cat remember, not a hovercradft :D The longer mast improves this relationship some, modest lift with good foreward force. Sounds like a cam eh ;) It is kinda.

Brian's drawings all had their masts slanted foreward. It didn't work too well for me. The mast vertical worked best. You can change the cam some more if you keep the foot of the sail stiff so the wind gets dumped aft, makes a heck of a difference in speed as to allow dumping air off the foot. You can also sail higher to wind... and tack very fast. This was when I got the idea that you can actually sail directly to wind without tacking... you would need a wide cat though...

Do you plan to build this cat or is it just a design thing you're working on ?

[Fanie]

I'm sure you must have seen Orange... that big racing cat. Ever wondered how it can beat those big tri's :D

Be a sad day when the cats beat the tri's and nobody wants them any more.

[Rick Willoughby]

Mat
Can you post the light load linesplan for comparison.

Rick W

[marshmat]

Rick,

For the purposes of direct comparison, here's the tri and the cat both at 2.5 tonnes. The topsides have changed on both since I switched the models to Rhino, but from the waterline down there have been no major changes since these drawings. The undersides are pretty much straight out of Godzilla- faired smooth for the cat and made developable for the tri. (A little bit of strip-planking on the bottom of the cat is fine by me, but I want to be able to use sheet ply whenever possible.)

I don't have a light-ship condition weight for either yet, but the aim is for no more than 1.5 t on the trailer.

At 2.5 t, the cat's resistance at 10 m/s is 2.58 kN and the tri's is 3.07 kN, at least to the extent I trust Michlet calculations at such high a speed without accounting for sinkage and trim. 26 vs 31 kW indicated power to reach 20 knots, the fastest we would ever want to go in these.... I know that's a big gap by your uber-efficient standards, Rick (I've seen you fret on here about tens of watts), but for my purposes the values for the two are close enough to not be a factor in deciding which one gets built.

And yes, Fanie, my fiancee and I are planning to build one of these, in some form, hopefully soon. Sunset Chaser is starting to feel a little small, especially when we have to camp at lockstations in the rain.

[Fanie]

Question please... I work in Delfship. How accurate are the resistance compared to ie that of Rhino or Michlet ?

My cat shows a peak of 1400N at 8kn at 1990kg per hull. How bad or good is this ?

especially when we have to camp at lockstations in the rain
Sounds like perfect outdoor weather :D Back in the office the weather is usually perfect...

[marshmat]

Fanie,

At my maximum load of 1750 kg per hull, the "Godzilla's best" of the cat I've been mentioning here would have 1.16 kN resistance at 8 knots. So 1.4 kN at 8 kt / 2 tonnes would be very close to my calculations, and about the best one could probably hope for in a boat like this.

I am trying to figure out a way to get Michlet offsets out of Rhino, that would make life much easier.... I know it should be possible....

[Fanie]

Rick, what is the light load linesplan ?

[Rick Willoughby]

Mat
I am interested to see the light load linesplan for the tri to understand why you have them submerged so deep at full load. They seem much deeper than necessay. At light load I expect that it would be designed essentially as a monhull with the outriggers as surface skimmers.

Rick W

[Rick Willoughby]

Question please... I work in Delfship. How accurate are the resistance compared to ie that of Rhino or Michlet ?

My cat shows a peak of 1400N at 8kn at 1990kg per hull. How bad or good is this ?


...

Michlet will be within 2 to 3% of actual drag. If the others agree with Michlet then they are also accurate. If they do not agree with Michlet then they will be inaccurate.

Generally I have found using Delftship drag data for slender hulls is next to useless for design purposes.

Rick W

[marshmat]

Mat
I am interested to see the light load linesplan for the tri to understand why you have them submerged so deep at full load. They seem much deeper than necessay. At light load I expect that it would be designed essentially as a monhull with the outriggers as surface skimmers.

Rick,

The attached is at 1.25 tonnes total, which is in the ballpark of what I'd be aiming for in trailering condition (so including engine, fuel, etc. but not crew or cargo). I have not yet done a detailed weight estimate.

The relatively large amas are the result of a bunch of experiments with Godzilla. In my earlier drawings, they were fairly small and just skimmed the surface, each carrying less than 1/6 of the total displacement. Optimizing at various speeds in the 5 to 8 m/s range (10-16 knots) with various combinations of my own hulls and spacings versus Godzilla's unconstrained series-8 result, Godzilla would almost invariably make the amas wider and deeper than I had originally planned; at 2.5 tonnes displacement, each ama is carrying about 1/5 of the total weight.

I am not entirely finished with the Godzilla tweaking, and still need to do some runs on the tradeoffs between optimizing for different speeds and different goals. A 32-parameter unconstrained problem is a bit much for it to solve in a single shot. I may yet modify a few things, including the vaka/ama ratio.

The resulting wetted surface area is about 19 m^2 at 2.5 tonnes- quite high. Surprising to me, since viscous drag is usually so dominant in a fast multi. But constraining the amas to be small and the vaka to be large, given my length limit, always seemed to result in something with sufficiently higher wave drag to offset the reduced wetted area.

As drawn, the tri's wake with 2.5 t displacement is pretty tiny- see below. Despite being almost 10x heavier than my current boat for only a 60% increase in length, it can't make as big a wake as I can now...

[marshmat]

Ok, so a lot of tweaking and a heap of Michlet/Godzilla output files later....

See attached. This is at 2.5 tonnes displacement (1.9 t for the vaka and 0.28 t in each ama) with the ama centrelines offset 2.0 m to each side, vaka is up to 81 cm beam thus L/B = 10. These proportions are a bit closer to what I was originally thinking of.... Godzilla needed a bit of coaxing with tweaks to various constraints to get where I wanted to go. But all is well, and about 700 N of drag have been shaved since I first posted the design on here. We're down to a hair under 3 kN at 10 m/s. I've been smoothing and fairing things a bit in Rhino to make things close to developable, and can't yet get shapes from Rhino to Michlet, so there's probably about a 6 to 10 percent uncertainty on the resistance values mentioned above.

[Rick Willoughby]

Mat
The light load makes sense. Not quite what I expected but Godzilla has the habit of making sense once you reason it out.

My faux-tri is really a monohull with trainers. I have not looked carefully at a range of loading states.

Rick

[Fanie]

Hi Matt,

You may gain some space if you fit the shape of the ama's and the centre hull into one another, so folded there's no waisted spaces.

[marshmat]

Thanks for the advice, everyone.

Here she is at present (13th iteration): folded and open as viewed from the bow.

Displacement figures, to give an idea of how big she is and how that volume is distributed (corresponding to the two waterlines in the images):
35 cm draught, ama 44 kg, vaka 1180 kg, total 1268 kg.
50 cm draught, ama 272 kg, vaka 1939 kg, total 2483 kg.

Next up is to confirm loading and stresses on those crossbeams, and get their linkage figured out.

[Fanie]

Like your terminator look alike on the boat :D

Have you got a wire frame drawing you can post of the rh drawing ?

[Willallison]

Matt, have you considered using what is often referred to as a displaning hullform for the vaka? By this I refer to the type of shape originally conceived (I believe) by the late Malcom Tennant and widely adopted throughout the powercat world these days.... Basically a canoe stern below a wide flat transom / chine, like below...
There are a number of advantages, not least being their ability to resist squatting as speed increases. Also, I think there may be some benefit in terms of load carrying capability.

[marshmat]

Fanie,
More drawings will be coming, soon. Hold yer horses :)

Will,
I'm somewhat familiar with Tennant's 'displaning' stern, but had not considered it for this case. I'm under the impression that it's more of a pure-powerboat shape: tracking like it's on rails and performing at its best when pushed to relatively high speeds. I'm not sure how such a hull would perform under sail (I would like to be able to sail this boat, although it is more on the motor side of motorsailer)- but any insight would be appreciated :)

[Fanie]

Matt,
Make your ama's decks higher so they come out level with the beams. This will increase booyancy, add storage space and make the solids easier to join to the hull. Will also make for a dryer ride.

I don't like the 'displaning' stern, you lose booyancy and the nose tends to lift up. The bass guys like it though. Some of them cannot see over their bows when they're seated. I guess it makes them feel much faster and must launch up at about 45 deg when they take off.
My one friends boat is also like that, and I very much dislikes it.

[Willallison]

Matt,
I don't like the 'displaning' stern, you lose booyancy and the nose tends to lift up. The bass guys like it though. Some of them cannot see over their bows when they're seated. I guess it makes them feel much faster and must launch up at about 45 deg when they take off.
My one friends boat is also like that, and I very much dislikes it.

Not sure we could possibly be talking about the same shape Fanie - the arrangement to which I refer has exactly the opposite traits to the one's that you suggest!
Having said that, I wouldn't expect it to be as efficient under sail as a more 'traditional' shape - at least not going slowly, though the difference would probably be minor and well worth the trade-off in terms of better under-power performance.

[tom28571]

Ok, I'm not a multihull expert but have built and sailed them. One question I have is why the wide beam and high buoyancy amas on a powerboat? Also, why the big and strong akas?

Are such high-volume amas needed for stability on a powerboat? Making them smaller results in easier folding and launch/retrieve operations as well as lighter weight and less expense. 2.5 tons also seems like it can be reduced to, say, 1.5 ton displacement without loss of much accommodation niceties. If the sailboat vs powerboat option is still in the air, I suspect all this discussion is moot.

I also thought the monohull was going in a good direction.

OK, I'm an agitator:P

[Fanie]

Hi Will,

Most of the weight is at the transoms, the motors, battries and fuel is usually there and crew usually sits more towards the stern. The hull step makes it worse, so the boat's butt seems to want to hang a bit. Imo to get the boat to cruise evenly (flat) you have to give more trim to keep the nose pressed down, otherwise you get a bouncy ride. My experience.

The handling (steering) would be better on a stepped hull if if you have one of those monstrous motors on a relative small boat, the distance between the planing transom and the motor is longer.

Tom, personally I prefer the wider boats. I'm considering getting myself a wide catamaran setup for fishing instead of the mono... The advantages are numerous. The wives also like to go with, but not on a crampy boat. You can do so much more if there is space.

[marshmat]

Hi Tom,

The monohull is still a possibility, and I may yet continue that design, but in the last year or so I've taken a liking to multis. This boat is as much for experimentation and messing around as it is for practical use, although I do want to be able to use it for local cruising.

Also, I'd like to be able to tip up the engine and hoist a sail. I have an idea for a form of aft-mast rig (a la Brian Eiland and Fanie) that would use an A-frame mast and could be dropped easily to get through the canals. Hence the big amas. This is not pure powerboat, although it is more on the motor side of motorsailer.

There's a trade-off in ama size: I want them low enough for swimmers, etc. to climb on and for them to fold in to trailering beam, while being large enough to keep from going submarine on me when we're under sail. Right now it takes 1800 kg to sink an ama, and they're not too tall to fold comfortably, so I think they're about right. The akas are indeed large, partly for structural efficiency and partly so that I can comfortably fit a durable folding linkage inside there.

2.5 tonnes is the loaded displacement, including crew, food, some gear, etc. I am aiming for about half that on the trailer.


Currently debating rudder and engine placement. There's not enough room on the vaka for an outboard and twin rudders. So it's either an off-centre O/B and off-centre rudder, or an O/B on the vaka and rudders on the amas. Ideas?

[tom28571]

Nicest small tri I've been aboard lately was a Telstar 28 (the new version). Reasonable interior and good speed with outboard. 25hp I think it was. Don't understand the need for twin rudders and a single and an outboard wil fit on the vaca transom. Still, it's primarily a sailboat. Worse thing I can imagine owning is a motor sailer that does not sail well.

While I'm being negative, the big jib, aft mast sail thing is IMO a bad deal. All that I've seen suffer from one big ill or another. Lack of CE/CLR balance options and high hull and rig stresses. Easiest sail rig for cruising with wonderful balance options may be an unstayed cat-ketch. At least, it's my favorite.

[Rick Willoughby]

...
The monohull is still a possibility, and I may yet continue that design, but in the last year or so I've taken a liking to multis. This boat is as much for experimentation and messing around as it is for practical use, although I do want to be able to use it for local cruising.

...

Matt
It would be interesting to do a comparison of the tri and the planing mono over a range of conditions of speed and load.

From a comfort perspecive I think the tri will win. For outright speed the mono would win but with much more power.

In open water a small planing hull is often reduced to speeds around 15kts just to make it comfortable. This should be an economic speed for the tri. In fact you need a very large boat to comfortably sustain better than 15kts in 2 to 3m seas unless you are running. You can go faster of course but everything starts to take a pounding (my wife never liked bouncing across wavetops like I did)

A boat that can sustain 15kts covers a lot of ground so I do not see much point in going higher than this unless you are racing.

Rick W

[Willallison]

Hi Will,

Most of the weight is at the transoms, the motors, battries and fuel is usually there and crew usually sits more towards the stern. The hull step makes it worse, so the boat's butt seems to want to hang a bit. Imo to get the boat to cruise evenly (flat) you have to give more trim to keep the nose pressed down, otherwise you get a bouncy ride. My experience.



Ok - now I know we're talking about different beasts. The hull shape that I'm talking about is definitely not a stepped hull. Good pics are hard to come by and unfortunately I'm not near my own computer models, but see the pic below. Imagine a full displacement cat hull which tapers in beam to zero towards the stern. As the bottom tapers, so a horizonatl chine is formed, which in turn widens towards the stern. This flat plane continues beyond the canoe body some distance and is normally immersed at rest but acts somewhat like a planing hull in that the water breaks clear of it once underway - hence the displaning term. The bottom surface of the chine remains in contact with the water at all times, unlike a step, and acts in some ways like a trim tab - counteracting the natural tendency to squat.

I agree with Rick - a comparison would be interesting. I suspect that the cat would come out on top at the low to mid-range speeds, though the difference would be colser if the mono were kept light. But any comparison also needs to take into account the amount of usable space on board.

[Fanie]

If the hull tapers to narrower towards the stern it will have the same effect wrt the payload it will be capable of. The widening towards the stern that gets involved when the stern goes down will probably cause more drag than a proper shaped sailing hull, which is wider towards the stern and it should also be more stable (less tendancy to rocker). Vertical hull in horizontal out.

The mono will only win wrt speed if it can get on a plane, and it can carry a bigger load. There is no other advantage.

Ahemmm... Unless of course you see it as an advantage to get pounded by waves :D

[Fanie]

Will, Sean just posted pics of a cat with that shape stern.

http://www.boatdesign.net/forums/open-discussion/all-off-topic-posts-26910-7.html

I wonder if Tom or one of the ex perts could give us a comparison in characteristics on this shape. Doesn't mean I like it, would just be interesting to see what comes out

[marshmat]

Agreed, Fanie. Some commentary from someone who has spent time aboard such a hull would be most welcome. From a theoretical perspective, I can see how it could be very beneficial at high speed, but I don't know if there are other, potentially undesirable, traits that Tennant's stern shape would introduce. It is often hard to separate informed commentary from marketing-informed commentary in the press and on the web....

[Fanie]

One thing to notice is the part that tapers is the hight size of the prop, probably so the prop doesn't sit under the hull, but are now behind it. Protection feature.

If the step is going to be at the surface it is going to be very noisy and splashy, especially if you plan to sleep on the boat and there's a bit of a wake.

[Rick Willoughby]

There are pictures of a cat hull like you describe at post #94 here:
http://www.boatdesign.net/forums/open-discussion/all-off-topic-posts-26910-7.html

This hull would be a heavy boat designed for high speed going by the immersed shape.

I looked at drag for a hull like this for Brian Ballard. He built a model with this idea. There are photos and video in his gallery:
http://www.boatdesign.net/gallery/showgallery.php/cat/500/ppuser/22903

Rick W

[Ad Hoc]

marshmat

Boats of this size are even more about the functions and its practicality. All these are the "design drivers". No need to focus on hull forms and other aspects for now, since your SOR is quiet varied and may possibly change.

Therefore just keep focusing on the the "issues" you really want from your boat. Then once that has gone around the houses several times, then see what type of hull form and layout will best provide you with the 'design intent' that satisfyies all and allowing for "extras" later on too.

[Willallison]

I've spent a little time aboard a couple of such cats. Never found any 'undesirable traits' - apart from those that are exhibited by all cats: snappy motion, limited space in the hulls, etc
They are not heavy for their size - at least not the one's I've been aboard. In fact one that springs to mind err's quite the other way and I have fears for its longevity.
The transom is immersed at rest - though not much, so I have noticed on occasion a little bit of slap when stationary in a following sea. I have to say though that it was the exception rather than the rule. The transom is generally immersed on most powercats that are designed to travel at 'higher' speeds. Those that don't tend to suffer from squatting.
If I was pushed to find a downside, I'd say that it adds complexity to the hullshape and hence the build time / cost.
I have to say that Ad Hoc is on the money with that last comment - really the design goals should set the hull form, not the other way around...

[Fanie]

Imo the hull is shaped only so water can flow to the prop. If the narrowing down towards the stern then the whole area aft above the draft would be shaped like that.

That shape increase the draft, it reduces space in the hulls and I think it's going to increase the wetted area as well so create more drag on a displacement hull. I don't see any positive property in that shape for using with outboards on a smallish 8.5m trailable cat.

[Willallison]

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull. Yes, increasing the fairbody draft can increase the wetted surface, but that is usually more than offset by the improvement due to slenderness. If that were not the case, then displacement cats would rarely be more efficient than planing cats at the sorts of SL ratio's that we are talking about.
Many displacement cats are (or at least were) a development of sailboat cat hulls. The CS hull, as Tennant called it, is a development that reduces that tendency without resulting in a completely immersed tansom as would the case be with a more conventional displacement hull.
There was an article kicking around that graphed the efficiency of the CS hull vs 2 others - I think a traditional displacement hull and an equivalent monohull. The CS was better on almost all fronts. Unfortunately a quick google failed to turn it up.
If anything, this shape would reduce overall draft - though probably not in the o/b powered form that Matt is looking at. A displacement boat, by its very nature remains immersed at virtually the DWL. A conventionall shaped boat, if it is to resist squatting in any way, will have fairly stright and level buttock lines running aft. This mean that fairbody draft remains essentiall constant thoughout. Shafts protruding out the bottom of the boat will therefore increase that draft. They will also, as you say Fanie, be exposed to damage.
There are other attributes of the shape that are worth bearing in mind to. The boats can dry out without fear of damage to appendages for the reasons above. They also tend to exhibit lower wash - I suspect mainly due to the lower trim angles.

Sorry, but based on my research and experience, I would have to be presented with a very godd reason for using anything other than this shape for a cat hull

[Willallison]

Still can't find that graph. Tennant refers to it in this piece off his website, though the grapgh itself isn't there...:?:
Frequently in magazines, and on web sites, you will find largely unsubstantiated claims of “greater speeds”, “lower fuel consumption”, “longer range” etc for a particular brand, or type, of boat. However, when the results of fuel consumption tests are published it allows everybody to look beyond the advertising and estimated performance figures and get down to what a boat is really achieving. We pride ourselves at producing very fuel-efficient powerboat designs that give high speeds on displacement hulls with as low as possible horsepower requirements. We also claim that our efficient “CS” hulls allow for longer ranges and higher speeds with a given amount of fuel. To make sure we can substantiate these claims, when possible, we compare our fuel consumption data with other similar boats. This serves as a continual validation of our design ethos.



We recently had a 13.6m [44.6’] “Escape” displacement power cat launched here in New Zealand and an independent fuel test completed on her. We managed to find published, professionally conducted, controlled fuel tests for three other types of vessels of a similar size with comparable parameters. These were all professionally built and had been designed by reputable naval architects. These were used for a comparison.



The fuel consumption curves are for: a round bottomed, with an aft chine, “semi-displacement” powercat, a hard chine planing powercat and a “fast” cruising monohull. Our vessel, the “Escape” has the full displacement “CS” hull form that was developed by this office. The fuel consumption was matched to the displacements of the vessels so that the slightly heavier planing cat and monohulled vessel would not be penalized. ie: calculated on the basis of fuel consumption at various speeds per tonne of displacement.



The results are shown in the accompanying graph. Right across the speed range the “Escape” was using less fuel than any of the other vessels. Her closest rival below 19knots was the semi-displacement powercat. However, as can be seen from the graph above 10 knots the “Escape” is using, on average only 65% of the fuel per tonne of the semi-displacement vessel! Even below 10knots the “Escape” is only burning 60% of the semi’s fuel. This means more than 40% more range for the “Escape” at this speed.



At all speeds compared, the planing cat was using more fuel than the “Escape”. At the planing cat’s drag hump at around 11 knots the Escape was using only 43% of the horsepower of the other boat! Above this speed the planning boat gets onto the plane and her fuel consumption begins to drop until 18 knots where it begins to rise once more. At the top measured speed of 23 knots that the Escape reached with her 200Hp motors she is still burning only 90% of the fuel of the planning cat. The planing cat can go faster than the displacement cat ie: it has a higher top speed. But it uses an additional 480HP (total) to get another 6.5 knots of speed! This clearly shows the high price exacted by higher speeds. So unless very high speeds are required in a boat of this length the displacement cat is clearly superior. However having said that I should point out that we have produced displacement power catamaran designs that easily achieve more than 30 knots from very modest horsepower. But of course they have to be longer.



This graph also shows the second advantage of the displacement cat. If the speed can be reduced to a slower cruise speed then the result is a greatly increased range. If we look at 15 knots for example the Escape is only using 50% of the fuel of the planing vessel. For a given amount of fuel this equates to twice the range at that speed! To increase her range the planning cat could increase her speed to 18knots, at this point she would still only have 55% the range of the Escape. The other option for the planing cat would be to reduce her speed to 8 knots to achieve the same range as the Escape achieves at 15 knots. At 15 knots the Escape has a range of 616 nm (with 10 % reserve) and she would cover this distance in 41 hours. The planing cat would take 77hours to complete this trip. This means she would arrive 1.5 days later! On a return trip cruise you would lose three days out of your holiday just to passagemaking!

[Willallison]

Nearly.... but this one doesn't have the conventional displacement cat hull on it, which is frustrating as it's the the one I'm hoping to show the comparison against

[Fanie]

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull.
Where did you get this ?? Because as far as I know this is not true.

[Ad Hoc]

Fanie

Look at a typical Series hull form that is long and slender. These are have low displacement length ratios, and most high speed ferries, in one form or another, are basically like these too. That is, long slender high-speed displacement catamarans. Some more efficient than others, but that is down to their L/B and hence its displacement length ratio.

[marshmat]

Keep the good discussion coming, everyone :)

I'd like to address a few of the points raised that directly relate to my particular project:


Boats of this size are even more about the functions and its practicality. All these are the "design drivers". No need to focus on hull forms and other aspects for now, since your SOR is quiet varied and may possibly change.

Therefore just keep focusing on the the "issues" you really want from your boat. Then once that has gone around the houses several times, then see what type of hull form and layout will best provide you with the 'design intent' that satisfyies all and allowing for "extras" later on too.
I agree completely. Perhaps I neglected to mention earlier in the thread: the design brief for the trimaran I'm working on now has been two years in the making. My original solution was a planing monohull that, after much analysis and some model testing, was deemed feasible and likely a great boat- but not efficient enough, and too rough in choppy weather, for my planned operating conditions. The tri evolved as a more efficient, likely more comfortable design that, although somewhat more complex, would maintain the necessary people-carrying space and cargo-carrying capacity.
To summarize:
- Inland lake, canal, and some Great Lakes use in conditions up to and including small-craft warning
- Overnight for 4 or one week for 2, comfort level a step up from tent camping but no luxuries
- Carry at least a tonne of lumber, building materials, etc. which may be in awkward sizes
- Minimal wake, minimal fuel consumption
- Trailerable with a normal minivan or light pickup
- Would be nice to be able to carry a small sail rig, but is primarily a power design
- Quick to launch, recover, or prepare for lockage

Displacement cats work on the principle of high length:beam ratio's. To a point, the higher the ratio, the more efficient the hull.... (etc)
Agreed. In my proposed tri design, the vaka has L/B = 11.2, a value decided on (along with the form coefficients, sectional area curve, etc.) as the result of a long string of Michlet/Godzilla simulations. The form I've drawn works best at around 16 knots, and I believe it to be capable of about 20. There may be trim issues, of course- I am planning to try some proper CFD simulations and eventually a model test to make sure that the boat will trim properly.

I like Tennant's stern form, and would probably use something similar if I were doing an inboard-engine powercat. It shares a lot of similarities with the Sea Bright skiff (the traditional one, not the Atkins tunnel-stern thing) and is undoubtedly a clever design. For various reasons, I don't think it's the way to go for this boat (outboard instead of inboard power, somewhat slower operation than that hull is intended for, would like to be efficient under sail with the engine tipped up, and would like a fair bit of rocker for quick turning in tight canals and when under sail).

The upward angle of the buttock lines in my current hull is about four degrees, which seems for the moment to be a good compromise between low drag and ability to maintain proper trim. Note that the amas are very far aft, and could be equipped with trim adjusting foils.

Keep the discussion coming, friends.... I'll let you know how I'm progressing soon.

[Fanie]

Aaarrrgghh... I apologise. I completely misunderstood what was said there... yes correct. A 1:1 ratio is a planing hull :D and a 12:1 ratio is a good displacement hull. I'm like the English say here but not all there :D

[Ad Hoc]

marshmat

The point I'm making is that, in your original post, you ahve stated having some 9 iterations of your 'design' to date. This proves my point coupled with your comment above.

You need to sit down and draw up a list of 'wants'....what you want your boat to do. Think it through a few times then FIX IT.

Until you have fixed your design intent or SOR, you shall be constantly changing/evolving your SOR because each iteration does not converge on one solution that satisfies your SOR.

Your "wants" may well have to be compromised in order to converge on a solution, only later on once the vessel has been designed to meet your SOR will you know what, if any; ie cost, size etc. But until you have a fixed and finite list of your wants, for your vessel, you shall be chasing your own tail until the cows come home with "design solutions".

[marshmat]

Hi Ad Hoc,

I agree completely. What you say above is absolutely true. And it is essentially what I have been doing. (See post #52.)

Perhaps you and I simply use slightly different design methodologies or terminology. The approach I've been taking here is the same approach I use for most design problems:
- Define the initial requirements and use profile
- Consider all feasible design directions, and decide on one or two general concepts that are likely to result in something that meets the design goals
- Rough out that concept to form the basis of a feasible design
- Check back against the design goals and see if they can be met with this design
- Refine the design goals in more detail
- Build on the previous iteration of the design to better approach the design goals
- Repeat from point 4 until satisfied

I don't believe in firm, fixed goals that never change. As a concept progresses towards a completed design, one gains a better understanding of the design requirements, and ought to evolve them accordingly. Note that my use profile and the core design requirements have not changed substantially since I started this project ( http://www.boatdesign.net/forums/projects-proposals/playing-around-10-m-trailer-cruiser-17597.html ) although some tweaking has been done (I've cut back on the need for luxury and thus weight/size, and relaxed my desire for high speed, among other things).

My design "iterations" are not complete re-workings; they are incremental evolutions of the concept, with a few bugs being worked out each time. Most yacht designers would likely include eight or ten of my 'iterations' in each loop around the design spiral. As a matter of personal preference, I like to break this out into smaller bits, evaluating the design against the goals at each stage.

And, AdHoc, I very much appreciate your comments. You're forcing me to analyze and explain the rationale for my own design process, which I normally do not focus on in this way. It is proving to be a very useful and enlightening exercise.

[Ad Hoc]

marshmat

Only too happy to assist in anyway, having someone prodding you mentally (with real questions not just smart arse one liners) is always a good thing, whether we like the questions or not. As you modestly put, forcing you to reanalyse, as that is surely the purpose of critiquing a design and its SOR to ensure everything has been thought of before money is spent! My comments in general are not always welcomed by many of the "arm chair" designers that seem to exits here, as they don't 'understand' what design really is! (Which don't bother me one bit, i dont feel like i need pats on the back or have anything to prove). My questions are exactly the same i ask myself as does my colleague to me and me to him. Not everyone can do a "self-risk" independent assessment...

The reason why is say fix the SOR, is that it helps to focus the mind and converge on one solution at a time. It does not mean it is the only solution. Having too many variables, and constantly change ones at that, will not allow for a solution nor establish where compromises shall need to be made to meet the SOR (if not met), certainly not in a timely manner. (Hence my reference to your previous comments and 9 other iterations....see my point?)

My design spiral is not that different to yours. However, I have just 2 issues, first, I always start with the weight estimate and then the sketch based upon the weight estaimte (from previous data), then rechecking the weights based upon the sketch, before going into any kind of 'design' iteration. If the SOR is not met, i start again, just adding or slightly modifying the first 'design'.

FYI, i design in the commercial field, not leisure, so my 'design spiral' may be different to yours? not knowing your field.

[marshmat]

My field is actually engineering physics, although I have done some work in high-efficiency building design as well as a lot of time with solar cars. Boats are more of a hobby- I love doing this kind of work and am well-versed in the theory involved, but am not doing it professionally or to sell plans- just to gain experience and have fun.

I do see your point regarding a clear, definite SOR. I am at that point now with this boat. Locking down the performance and other clear, numerical requirements is fairly straightforward- the difficulty with a boat like this (as with most projects I've been on) is that, once a design begins to emerge, you get things like "well, if that's how it's likely to evolve, it would be SO cool to also have......." And so the SOR and the resulting design gradually adapt, on an ever-tightening spiral, until at some point they are close enough that only minor details are being tweaked. I like to do this in ever-finer steps: start by getting the weight, speed and seaworthiness dealt with in a preliminary design, then progressively refine the result to account for comfort, living space, build cost and time, etc.



At present, I am happy with the hull and the simulations I've done on it so far, although there is still more analysis to come that may result in small changes to the hull. There is still a sail rig to come- for a sailboat I would have started the rig early on, at the same stage as the hull, but this is primarily a powerboat. The motorsailing ability here is more of an add-on, something to play with when conditions are appropriate- for about 60-70% of the proposed use profile for this project, sails and masts are not practical (low bridges, narrow canals, inland lakes with no consistent wind).

[Ad Hoc]

marshmat

"....well, if that's how it's likely to evolve, it would be SO cool to also have...."

and there in lies the problem. Every design and i mean every design i have ever worked on (in the past 20 years) have an element of this. You can't change the SOR once fixed. Minor changes like, oh, i would like blue seats and not red, or tinted windows not clear etc are easy and not really worth worrying about. The way to think of is this, if a modification or change is wanted/needed it falls into one of 2 categories:

1)cost
2)performance.

More often than not both need to be addressed. But when it is just a cost issue, that is for the client (you in this case) to justify. But these 'costs' shouldn't affect the design concept, just the overall price.

Whereas hmmm...an extra genny for air/con or, id like a few more 24DC batteries just in case i'm out at sea too long etc etc, all add weight which affects performance (and cost). When a change like this affects your performance, the original SOR is no longer valid. So you either have to do some serious compromises, or start again.

In the commercial fields I am in, starting again is not acceptable, for both, designer and client, so a compromise is found.

This is what you need to do. Fix your SOR and any "hmmm...that would be nice"..revue against the cost and performancee goals, (not to mention delivery time). Reviewing the "cost" of your implications will answer the question of whether you really want/need it.

In 'designing vessels' this comes from the Specification and General Arrangement, drawn in complement with the Spec. These two documents are the bible. Change them at your peril. Since this is the 'fixed' SOR.

[marshmat]

Ad Hoc: I agree. The specifications and requirements cannot continually change- that causes an endless circle of redesign, which is definitely not good.

The thing is, when I started this thread, I had a pretty good SOR- but it wasn't perfect. There were gaps. I had a start on a design that would meet the requirements I had. But since I don't yet have experience with this type of boat, I decided to seek feedback from my friends on here rather early in the process. In the last few weeks, the gaps and contradictions in the SOR have been fixed up, thanks in large part to the advice and commentary of everyone on here. Simultaneously, I've been manipulating the preliminary design to see how it might all fit together.

To summarize the peformance requirements, as they stand now, which is pretty much final:
- Inland lake, canal, and some Great Lakes use in conditions up to and including small-craft warning
- Overnight for 4 or one week for 2, comfort level a step up from tent camping but no luxuries
- Carry at least 1000 kg of lumber, building materials, etc. which may be in awkward sizes
- Easy access for swimmers, or to low docks / shore
- Minimal wake, minimal fuel consumption, quiet under power
- Trailerable with a normal minivan or light pickup
- Would be nice to be able to carry a small sail rig, but is primarily a power design for cruising at 20 knots or less
- Quick to launch, recover, or prepare for lockage- should not take significantly longer than my current boat (~5min to prep and launch from trailer)
- Weight target of 1.5 t or less in trailering condition (achievable, based on structural estimates to date)
- Simple, relatively inexpensive build from commonly available materials (developable ply/epoxy)

What I have now could best be described as an outline for a design that meets all of these requirements. And from now on, the performance requirements are not going to change substantially. Styling is still open, of course. So is the interior layout (I have a pretty good idea how everything will fit, but there may yet be a more efficient way).

Just to give everyone an idea of the level of fitout we're thinking of for this one: Electronics will consist of engine instrumentation, bilge pumps, a basic GPS, a class D VHF, running lights, a few cabin LEDs, and a couple of charging ports for things like an iPod. The stove, if any, would be a 1-burner propane unit. The head would be either a portapotty or a simple pumpout tank. Water for the sink and head will be pumped by hand. Drinking water will be carried in refillable bottles. Refrigeration will consist of a portable cooler and ice block (or Peltier module). There will be a single berth in the bow and another in the stern, one of which will share a compartment with the head. The cockpit settee/dinette will fold down to a double bed. There will be a fixed windshield, but the top will be fold-down canvas like on an 18' bowrider.

Oh, and there will be no less than six bilge pumps (at least two in each hull), a full set of lifejackets, flares and man-overboard gear, two beefy anchors, the usual complement of sound-signalling gear and portable spotlights, etc. And my fiancee is insistent that we have a proper wooden ship's wheel at the helm :)

My current focus, as far as this design is concerned, is now on the interior arrangement, the styling, and the structure. More to come in a day or two, unless it's too sunny around here to stay indoors :)

[Fanie]

Carry at least 1000 kg of lumber, building materials, etc. which may be in awkward sizes
Does the wife know about this :D

What on earth for ? If it's firewood then I'm sure youll be surprised to see how long a bottle or two with gas lasts...

[Ad Hoc]

marshmat

Sounds like a fun project.

So really what you need to do, is ignore the "hull" aspects for now and just consider the envelope of space required to fulfill your requirements. The reason why i say this is because it shall give you an idea of the true space (it shall assist in hull form selection), plus, you will be be able to get to grips with a weight estimate too. Weight estimate being the most critical and you cannot proceed without it being done in some detail, with about a 10% margin added.

So, if after you have drawn the layout, and got a very good handle on the weights (can't do anything without this), you can 'balance' your boat. You're not going for top speed nor are you attempting to break records. As such the location of the LCG is going critical, BUT you need to ascertain whether it is 20% aft or 20 fwd% of midships! This is light, full and half load.

Once you have done this, you will be in a much better position to select a hull form and its ability to satisfy your whole SOR.

But hey, everyone is a critic (including me). The problem with asking for advise on a forum is to know how and where to separate the wheat from the chaff. Since invariably there is a lot of chaff and its speaks louder than the wheat, owing to 'numbers'.

Will you have a shore supply outlet?..if so, have you considered a portable IH cooker, safer!

[Fanie]

Perfect boat ?

Extremely easy to build
Cost next to nothing
Free to run
Awesome performance
Capable of transporting any load
Prefect for unlimited live aboard
Weighs almost nothing
Trailers very easy even by a very small car
Almost launches itself
Extremely stable in any weather or water conditions
Absolutely quiet even full throttle
No wake

ok, now the compromises... :D

[marshmat]

I wish, Fanie ;) I think I'm a little more pragmatic, but taking my goals to the extreme, that's what you'd end up with! But we're happy to do without any luxury, without most of the modern conveniences, etc. and are not planning to live aboard or go offshore.

Cargo - this boat might get pressed into service for hauling supplies in at remote lakes now and then.

Shorepower - Not available in most of the areas I cruise. Genset is out of the question, too expensive / too heavy / excess complexity in what is, mechanically and electrically, an exceedingly simple boat.

AdHoc- I'll post the weights spreadsheet next time I'm on here. We've actually been measuring out simulated hull dimensions in the living room to get a feel for how much space we need. It's worth noting that a lot of the work regarding interior requirements was done back when this project was a monohull; we have a pretty good idea of how we need things to be laid out. I'll post more on this once I get it into CAD; my scanner is on the fritz right now and I got laughed at the last time I put the drawing on the floor and shot it with the webcam.

[marshmat]

As promised. Sorry for the delay.

Here's the summary page of the weights and moments sheet. Obviously it's a preliminary one, but it'll give you guys some idea of just how much boat I'm talking about here.

The vaka and ama CBs were chosen based on Godzilla optimizations for a minimum-drag shape in the speed range I'm looking at (16 to 20 knots top end). The accommodations are of course concentrated near the CB to keep the crew near the centre of motion, where pitch and roll accelerations are lower. I prefer to adjust the general layout to bring the CG in line with the CB of the hull form I'm using, rather than distort the hull to suit a pre-determined layout, although I'm sure some folks do it the other way around.

[Ad Hoc]

Marshmat

No need to apologise for any delay, it is your time, your project!

The weight sheet looks 'light'. I take in what you say about being just a summary, but the interior of 120kg's, seems very light, despite not having a GA and/or spec to collaborate the figures with, ie what the duty is and what eqpt is actually going in.
Also, you need to have a design margin. For starters i would recommend having a 10% design margin added. Also, if you intend to "carry cargo" more often than not, add a cargo margin too. The weight of a boat never gets lighter, always heavier!

Just long slender and light as possible, is as good as any "hull optimisation" you need for this. Endless computer iterations for optimisation will be cancelled out during manufacturing. You wont get the exact shape, nor exact weight, the laminate may be too thick in places, or not 100% etc. So don't bust a gut over it, pointless.

I'm not 100% up on the seakeeping of Tri's (its been a while since i looked into this), but on a cat, the centre of pitch (for motions) is generally at the transom. I can't imagine that the tri is that much different without doing some further research (we did some more than 10 years ago, built a 7m manned model, was fun). Clearly for better residuary resistance the ama's placed aft. As such, it would appear on first inspection that this is not unlike that of a cat, hence the further aft the better for your accommodation, not the CoB. Roll motions on a Tri are not that good in beam seas!

If you want to "optimise" you ama locations, i can send you an interesting paper on the subject.

Forgot to add...in your weights, get ride of the 0.01 decimals....round them up to the nearest 1kg not 100th!

[Fanie]

Hi Ad Hock,

Can I have a copy of the paper as well please ?

fanie at faze dot co dot za

Would be interesting to see. Thanks

[marshmat]

The weight sheet looks 'light'. I take in what you say about being just a summary, but the interior of 120kg's, seems very light, despite not having a GA and/or spec to collaborate the figures with, ie what the duty is and what eqpt is actually going in.
I do have a rough GA and rough spec, I'll get some form of that stuff up here soon. There really isn't much in the way of interior stuff: two simple settees flanking a fold-down table, a couple of light bunks down below, a portable head, and a very simple galley counter. Steering, throttle, etc. systems are part of "driveline". I have this same calculation done for 2500 kg (full load) as well.

Apart from trying to get the damned CFD to work (those of you who know CFD know what I mean...), I'm pretty much done with the hull design.

on a cat, the centre of pitch (for motions) is generally at the transom. I can't imagine that the tri is that much different without doing some further research (we did some more than 10 years ago, built a 7m manned model, was fun). Clearly for better residuary resistance the ama's placed aft. As such, it would appear on first inspection that this is not unlike that of a cat, hence the further aft the better for your accommodation, not the CoB. Roll motions on a Tri are not that good in beam seas!
I'd heard this too, about high-speed cats. CB and cockpit centre are both about 40% of LOA forward of the transom here, which seems about right to me. The snappy roll problem is also one I've heard of- but that's a tradeoff I'm OK with for this boat.

If you want to "optimise" you ama locations, i can send you an interesting paper on the subject.

Forgot to add...in your weights, get ride of the 0.01 decimals....round them up to the nearest 1kg not 100th! Any and all technical papers are always welcome here, relevant or otherwise :) (OK, well, there are limits to non-relevant stuff, but.... feel free to pass it on if you think it's useful.) Not sure what you mean about 0.01 decimals.... all my weights are to 1 kg precision; I generally don't bother changing the display precision on intermediate values that are only for Excel's benefit.

[Ad Hoc]

see attached paper.
The book was too thick for my scanner and was hard to 'press it down', so it is not very clear, sorry about that. But the graphs are all clear.

I was confsued when i saw 79078.33, hence the ref to 100th, sorry about that.

[marshmat]

OK, so here's the latest on my (not-so-) little trailer cruiser project.

Interior / general arrangement

The interior is extremely simple, there really isn't much to it. Bow and stern berths are twin length (190 cm) but the clearances are a bit tight; kids would love them but full-grown adults would be a bit cramped. That's OK- on trips with more than two adults, we would likely be bringing tents and camping on shore. The dinette folds down into an (almost) queen-size berth. A simple head (probably a composting unit) shares the forward cabin; the aft end of the bow berth will hinge up when not in use to make the head easier to access.

Galley-wise, our main concern is having a dry place to keep food. There won't be any refrigeration; a portable cooler will suffice. Whether there will be onboard cooking facilities or just a portable Coleman stove to take ashore is again up for debate. The galley will occupy the area to port of the helmsman.

The helm is offset slightly to starboard, with wheel steering for the outboard and a nav desk to starboard. You can see over the windscreen when standing, and an adjustable helm seat will be added so the captain can look through the windscreen when seated.

A canvas (sunbrella) top will of course be necessary... we are in a cold, often wet climate, after all.

Structure

For reasons of cost and construction simplicity, I'm designing for ply/glass/epoxy construction. I'm not done calculating all the structural details yet, and may still revise a few things. To give some idea of how heavily she'd be built:
Vaka skin- 9mm marine plywood, 3mm fibreglass
Ama skins- 6mm marine plywood, 2mm fibreglass
Bulkheads / ring frames- 12mm plywood, some have added stiffeners
Total structural weight approx. 800 kg

Although rather lightly built in comparison to craft intended for offshore use, this structure is still quite substantial compared to many other small multihulls. There is a good deal of extra beefiness around the aka attachment points, and in the vaka bottom at the bow and stern. (There's also a vertical stem/bulkhead in the forward two metres that's not shown below.) The completed vessel, with all tanks empty, is in the range of 1200-1300 kg.

Systems

Power - Outboard, 40 hp or so. Although we haven't completely ruled out the possibility of a small sail rig, and this boat can stand up to one, it is looking more and more like it would not be practical for the style of cruising we would be doing in this boat. So the design is progressing on a power-only basis from here on. We'll stick with things like Sunfish and Lasers for a sailing fix, for the moment.

Electrical - Not much, really. Engine instruments, nav lights, a few LED reading lamps, bilge pumps, and a class-D VHF will be about all that's initially fitted. There will of course be some extra space on the panel to handle add-ons later, should any prove to be needed.


Any thoughts are welcome :)

[Rick Willoughby]

Matt
The windage probably needs to be considered in the overall drag calculation given some of the poorly shaped exposures you have.

Rick W

[Ad Hoc]

marshmat

Looking good...just like the patio i am currently laying (21tonne of stone and some 25tonne of stand..ugh!)..but getting there!

Structure, have you made a few test samples and then tested the configuration, just to be sure?...also, don't forget the connection with the ama's, this will have high loads too. Load transfer is essential, and may require more structure than you first envisaged!

The ama's, look rather large...any reason why this is so? Have You considered making them a "Y" shape? That is to say the leg of the Y is in the water, for minimal resistance, and the buoyancy you need in the haunch of the Y is above water and used as and when needed?

Nav Lights, i assume this is some kind of foldable mast with the lights as per colregs, since you're in the shipping lanes.

What about place for the windlass/anchor and wire/warp?

How much fuel will you carry?

Have you gotten around to doing and LCG check yet..since this is essential in the prelim design phase.

As for windage, unless you're going to be doing some 40knots..forget it..waste of time and negligible in the whole scheme of the design.

[marshmat]

Rick,

Yes, I agree that windage could be a significant contributor to the overall resistance at higher speeds.

Head-on, the projected area of the vaka, amas, akas and the superstructure (except for the windshield) is roughly 2.5 m^2, of which 1.1 m^2 is flat surfaces (CdA=~1.2) and the remaining 1.4 m^2 is streamlined (say CdA=0.4). The windshield shape is a long way from done, but a good guess would be 1.0 m^2 at CdA=0.9. So we're looking at about 150 N of air drag at 10 m/s, or an extra 1.5 kW of indicated power- so about 3 kW, or 4 hp, more than the hull resistance alone would suggest.

Since there's no chance we'll ever go above 20 kt (10 m/s) in this boat, and will typically cruise at more like 10 kt, I don't think the extra effort to streamline the akas would be worthwhile.

On the trailer, of course, this becomes more of a problem. With everything folded in for towing, there's a total of about 1 m^2 extra frontal area to worry about (the parts of the akas that are hidden by superstructure must stick up) and, towing at 25 m/s, we have about 1400 N of air drag to contend with- 34 kW, or a bit over 40 hp, not accounting for the slipstream of the truck.


Ad Hoc,

Structure- I'm trying to get my hands on a good multihull scantling guide; I'm told ISO 12215 includes some information I might find useful. I don't have a shop right now (little apartment on the 2nd floor) and so can't do test samples... yet.... although the materials are essentially the same as I used on my current boat, so I have some idea how strong they are. The akas and their attachment points (there's a bit more linkage in there, but I'm still working on it) are sized so that any one aka can handle ~28 kN at the ama in any direction (ie. ama completely submerged with s.f.=3, this kind of loading could also be conceivable in a collision with a wharf or other boat).

Large amas- why not? Their shape and size are based on a whole string of optimizations in Godzilla that led me to conclude this configuration would have lower drag than many others I looked at, including many with narrower or shorter amas. They're also meant to be swimming/diving/fishing/etc. platforms (there will be a folding, slatted deck to bridge the gap to the vaka wings) and so lots of space out there is welcome.

Foldable mast for lights, antenna, etc- Good idea, I think this might work well integrated into the centre pillar of the windshield.

Anchor gear- There's a compartment up in the bow where this will live. A windlass may be a later addition, but to start out, we'll be hauling anchor the old-fashioned way.

Fuel- Looking at about 100 L capacity, maybe a bit more, the intent is to be able to do the Rideau (~200km) without having to fill up with uber-expensive marina gas along the way.

LCG is somewhere between 350-360 cm forward of the transom including various combinations of crew, gear, etc. with an overall CB that varies from 348-358 cm depending on total displacement.

[Rick Willoughby]

Matt
I also consider the situation where you are at design cruise and also need to push into a stiff breeze. So at 10kts cruise and 20kts wind you have 30kts apparent. The wind is now costing you 10HP. Probably as much as the water drag.

Rick W

[Ad Hoc]

I guess you could just remove the whole deckhouse, oh and then just remove the hull sides poking above the waterline to really minimise windage. Just look at all the boats on the sea today that have no deckhouse/superstructure to minimise their windage....

When you deisgn a boat a real boat, everything is a compromise.

[Ad Hoc]

Marshmat

Not too sure about ISO 12215, but if you want a quick rough and ready solution DNVs multihull rules are very quick and simple for global loads. If you would like some advice on the connections and load paths and loadings in general, just ask.

Ama's. when i say larger, sorry wasn't referring to the 'above water', since yes, i can see a larger space above ideal for swimming from cargo etc. I just meant they look large for what they are "doing" hydrodynamically and stability wise.

Will you be allowed, with local reg's to just throw the anchor and haul up by hand?

LCG..did you do an lcg chase for your powering estimates?

[marshmat]

Matt
I also consider the situation where you are at design cruise and also need to push into a stiff breeze. So at 10kts cruise and 20kts wind you have 30kts apparent. The wind is now costing you 10HP. Probably as much as the water drag.

Rick W
A good point. But I think if I were beating into a stiff headwind, I would not be doing the 10 m/s (20 kt) speed at which wind costs me ~4hp. More like half that, probably less. Doing 10 knots into a 30 knot headwind (insane skipper?) costs ~16 hp in windage, but the boat only needs about 10 hp to overcome water and wake at that speed... add a bit for bashing through the waves.... there's still plenty of thrust on tap to handle the headwind.


Ad Hoc:
I'm not aware of any regulation around here that a pleasure boat must have a power windlass. I don't think I've ever seen a boat under 8 m that does.
"lcg chase for your powering estimates" - not sure on your terminology here, "lcg chase" means....?
I think I've got a good grip on the load paths, but any advice on load conditions and paths will of course be welcomed :)

[Ad Hoc]

lcg chase...this is what you do when doing tank testing.

Basically you have your model at the DWL and/or predicted displacement., level trim...run the tests. You then perform the same by moving the lcg fwd (of current location) 0.5%, 1.0%, 1.5% and 2% and the same aft. This then produces a resistance curve showing how the EHP is affected by the lcg position. You then design you boat for the "optimal" location, which may not necessarily be where you want the lcg to be, since it means moving many eqpt around, again!

Once you ahve drawn up a basic scantling, I'd be happy to have a look over them for you?..or if you have some sketches of what you're proposing?

[marshmat]

Ahh, OK. Yes, model testing in various weight distribution configurations is definitely a good idea. I can sort-of simulate that with different trim settings in Michlet, but I would like to put together a test model before building full-scale. Right now I have no shop, and no access to a shop, so a model will have to wait for the time being. It's worth noting, though, that there really isn't much equipment on this boat that can be moved around- there's an outboard engine, its fuel tank and batteries, the anchor, bilge pumps... the rest is basically all loose gear. The crew weigh more than the sum of all the equipment; the LCG shifts by up to 5% of LWL just by the crew moving around the cockpit area. It's not nearly as big a boat as its LOA would suggest.

I'll post some more structural details when I get a chance. An extra pair of eyes to look things over is always good :)

[Ad Hoc]

matt

Having done the lcg chase you will then at least know when or why your speed has increased/decrease owing to minor changes. Most designs/designers say put a trim tab or fix a wedge at the back, that'll make her go faster!...the only reason why they say this is because of ignorance of their hulls resistance data. If no lcg chase is performed how else do you know trim tabs will work, you don't. But on sea trails when the boat is a bit heavier than predicted and she isn't quite doing her speed, lots of head scratching....then someone suggest, look the trim is a bit high lets fit a trim tab....low and behold she runs better. Magic, lets do all our boats this way!...nope, just basic poor design.

When you do your lcg chase, you will at least know how flat or curved your EHP curve is. A good design will have the EHP being flat or near flat (at your designed lcg) or as the lcg goes aft, well inside the 1~3% 'zone'. Since all boats from conception to finish, their lcg's move aft, never fwd! So if your hull has a relatively flat EHP with change in lcg, you know it is not onerous. BUT if yours varies widely, you may have to sit further fwd all the time!

We always build models and tank test them. I'm shortly going to build a 25m test tank in my back garden. A CFD prog is ok for just doing some basic parametrics, but i personally would never trust the results. I know others do as that is their only frame of reference; the numbers!

I'll be interested to see how she comes together..

[marshmat]

Without a few experienced experts and some solid real-world benchmarking, I generally consider CFD results to be about as reliable as my government's ten-year budget forecast.


Scantlings

There's no requirement, in my case, for the boat to be built to any particular standards body's code. The applicable standard (Transport Canada TP1332E) requires that "Structural strength shall be commensurate with the intended service of the small vessel, taking into account the maximum anticipated loads." Any applicable code- NBS, ISO, ABS, Lloyd's, BV, DNV, or common engineering practice- is allowable, based on the designer's engineering judgement. (There's also 150 pages of somewhat more detailed, performance-based requirements for all sorts of systems, but no prescriptive methods.)

I'm using the following design loads for the hulls, loosely based on ISO 12215 for class B service (even though this boat would actually be class C):
Vaka bottom skin: 14 kPa
Vaka topside/deck skin: 10 kPa
Ama skin: 10 kPa
Watertight bulkheads: 7.7 kPa
For a marine ply / epoxy / fibreglass skin with 20 MPa tensile strength, and using the largest unsupported span of each case, suitable dimensions would be:
Vaka bottom skin: 12 mm (9mm ply + 3mm glass)
Vaka topside/deck skin: 12 mm (9mm ply + 3mm glass)
Ama skin: 9 mm (6mm ply + 3mm glass)
Watertight bulkheads: 12mm ply
Non-watertight bulkheads / ring frames: 12 mm ply, minimum 100 mm in moulded dimension

The structure is essentially taped-seam plywood construction, plus a few longitudinals where appropriate (keel, chines- not shown in the sketches below).

Each of the four akas is designed to handle 28 kN in any direction, the force being applied where the aka joins the ama. (This figure is arrived at by simply finding the maximum buoyancy the ama can exert if fully submerged, splitting it between the two akas, and applying a safety factor of 3.0.) They are 240 x 240 mm box sections, laminated of wood (species TBA, but most likely fir). These loads are transferred to the vaka through a pair of closely spaced ring frames. The akas are pinned together at midships, and when extended, are also pinned to both ring frames at this point. There is also a link on each side, running from near the bend in the aka to near the step-like flare in the vaka (this link is not drawn in yet).

The total structural weight works out to roughly 800 kg. Many structural elements do double duty as part of the interior, so they're included in this as well.

Suggestions to make this structure more efficient would be very much appreciated. For our purposes, a bit of extra weight is OK if it makes the build faster and simpler. This is a lake/river cruiser, not a racer, so saving every possible kilogram is really not a priority. Being able to bash into the occasional submerged log, or to survive sideswiping a dock at three or four knots, is much more important.

I've attached a few screenshots and a very rough sketch of how the aka attachment structure will work. I'm not sure at this point whether to go with independent 4-bar linkages on each side (like a Corsair) or to permanently pin the akas in the centre, forcing the two sides to move together (the direction I'm leaning towards right now).

[Ad Hoc]

She's looking good.

"...I generally consider CFD results to be about as reliable as my government's ten-year budget forecast..." can't argue with that ;)


So the structure has been based upon the ama's total submerged buoyancy times the lever from its CL to the deck edge of the main hull? Then a FoS of 3, is that correct?

Have you investigated the rotation/torsional moment of the ama relative to the main hull; depending upon the actual geometry, this can dominate a classic transverse bending moment as you have described. Easy way to do this is the basic assumption of the ama rotating about its midships. This leads to the classic pitch connecting moment.

The important part of the ama box beam to main hull connection is the shear/load path.
Is there adequate shear path capacity..both long.t and transverse?
Is there sufficient stiffness?...in other words, yes the stress may be calculated as ok, but what about the overall deflections?

Under the splitting loads, what deflections have you calculated using the 240x240 box? Since all your calculations shall be deflection driven. Very rarely will they be stress driven in composites.

If you wish to email me more details, i'd be happy to have a look at the details of the structural connections.

[Ad Hoc]

opps...forgot to add

Have you also considered berthing loads, such as coming along side, then a sudden wave passing (or accident/error in seamaship) and you hit the jetty/another boat/some other immovable object beam on. How will the box beams perform? A nice conservative 0.25g I've found is good in these scenarios, for small boats.

[marshmat]

Thanks for the tips, AH. The torsional moments you mention are being investigated. I will post more when I have time to work through those calculations. As for accidental impacts, etc.- the approximations I've been using so far result in any one aka being able to take roughly the entire loaded weight of the boat in any direction. They probably don't need to be nearly that strong, but it's a starting point until I become convinced that it's safe to get away with less. And I can see them being overstressed like this on occasion- hitting a dock or log, or propping them up for access to the underside of the hulls, or.....

[Ad Hoc]

mm
"...They probably don't need to be nearly that strong, but it's a starting point until I become convinced that it's safe to get away with less.."

Absolutely. Couldn't agree more. Until you have more supportive evidence, always err on the side of caution. Sounds like you have a 1g sideways load case then, ideal!

[marshmat]

Hi everyone,

My sincere thanks for the various pieces of advice that have been offered on this thread. Although I'm trained as an engineer, boat design (ie. real design, not just sketching) is a relatively new field to me. While I like to think I have a good grip on the overall process, there are numerous points where it's possible to slip up and waste a lot of time- it's nice to have more experienced folks around to point out these potential pitfalls.

So, some updates, now that I'm back in town and have had a few hours to sit down and work some things out:

Layout

We've worked out more details of how the interior will be laid out. (See attached images.) The aft dinette folds into a berth that is about 8 cm short of queen-size width. With the help of a tape measure, the largest and the smallest of the prospective crew and an adjustable office chair, we've found seat and windshield heights that work for both Katy (~160 cm) and me (~185 cm), both standing and sitting. The helm seat back folds forward to become a leaning post, and the second person can sit where they can converse with the captain. The captain can spread out charts to starboard- in the waters we cruise, you really want these right beside you (parts of the Rideau might have 10 bends, 15 possible wrong turns, and a dozen buoys, all within a mile or two).

This isn't a passagemaker; when we stay on the boat overnight, it will almost always be docked or beached. Think "boat camping". So the onboard galley isn't going to consist of much- a sink to port, the dinette table for food preparation, and some space to store dishes at the aft end of the chart table to starboard. Cooking will most likely be done on shore with a campstove or barbecue. We won't be fitting refrigeration- a couple of portable coolers will do nicely.

The addition of a small plank and a cushion makes the dinette into a nice nearly-queen-size bed, and a canvas enclosure will provide weather protection and privacy. The bow and stern compartments can be used as berths, albeit slightly cramped by powerboat standards (they're still larger than the "single" berths in many production sailboats). Still, with three or four people aboard, the odds are that a few of them will pitch a tent on shore- its 8.5 m length notwithstanding, this boat has less weight and similar volume to an average 23' cuddy.

Structure

There are a few more calculations to go before I can settle on the remaining structural details. Commentary on the validity of the following load cases that I've seen in various standards and references (and/or appropriate safety factors to use) would be appreciated:

1) Fully loaded weight supported on two points: stern of port ama and bow of starboard ama. (Roughly akin to being caught in a high quartering sea with a wavelength a bit less than the boat length.) I think this is about the most severe load case a trimaran can see?

2) One ama fully submerged in a wave, pushing upward on both its akas equally with its maximum submerged displacement.

3) Pushing longitudinally on the bow of one ama and the stern of the other (a "racking" motion that could be conceivable in the event of an impact with a dock or log)

4) Pushing laterally inward on both amas (similar to hitting a dock broadside, or having another boat raft against you)

A sketch of the aka folding linkage is attached below. The linkage consists of a simple pin joint connecting the two akas, through which a pin is placed when they are extended, locking them to the ring frames on either side of the aka assembly. A single-bar stabilizing linkage on either side connects each aka to a bracket on the side of the vaka. You'll notice that this system appears to have one too many degrees of freedom: left to its own devices, it is effectively rigid when down, but free to roll left and right when retracted. This is intentional- it allows a simple cable system (one cable per aka, running through turning blocks and joining into a single cable with a single winch) to retract the amas at the dockside, before trailering. The amas carry a greater portion of the boat's weight when retracted than when extended, and so will tend to extend themselves when the winch is let out.

I had considered a classic 4-bar linkage (a la Corsair), and that could certainly be done. However, I do like the idea of being able to retract or extend the amas with a single control (and maybe a bit of kicking) while in the water- many of our ramps are awfully narrow to launch a 4-metre-wide boat. And linkages are expensive (aluminum, with bearings, brackets, etc.) Whether or not the akas should also have a connection to the vaka "wing" structure is debatable. The akas themselves, by the way, will likely be of laminated fir or another easily available wood.

Weight & Balance

The on-trailer weight currently rings in at 1330 kg, including a substantial heap of movable gear that will probably never leave the boat. That comes up to 1645 kg by the time you top off her tanks (288 L of gasoline- probably too much, 83 L of fresh water, and 15 L of sewage). Six adult crew, with full tanks, bring the total to 2125 kg. So, pretty close to the design targets (1500 kg in normal trailering condition, 2500 kg fully loaded).

The crew are pretty much centred over the CB/CF location, and although they can change the trim a degree or two by moving around the cockpit, adding more crew does not change the overall balance noticeably. Depending on whether that heap of loose gear is kept in the bow or stern compartment, the CG can be moved between about +3% to -3% of LOA from the CB location. So we have the flexibility to adjust the trim as necessary, depending on how she actually behaves.

Outfitting

And a start on the equipment list:

Power:
50 hp outboard & associated systems

Electronics:
Dual batteries (capacity TBD)
Class D VHF
Engine instruments and fuel meter
Nav lights
LED reading/cabin/courtesy lights
Bilge pumps (2 x 1000 gph in each ama, 3 x 1200-1500 gph in vaka)
120+ dB horn
12 V distribution panel (note there is no 120/240 VAC onboard)

Ground tackle:
ROCNA 15 (I wish... they're rather expensive, might have to settle for a Bruce or CQR)
20-30 ft chain, ~100 ft nylon rode
Stabilizing bridle

Interior:
Galley sink w/ manual pump
Simple manual pump-out head
Foam or air mattresses for bow/stern berths
Dishes and other galley bits

Bosun's locker:
Fenders (lots of big ones)
Dock lines- six 20' half-inch or 5/8" nylon
Big lines- As much as can fit? Probably 80-100 feet each of 3/4" or 1" nylon or polyester, seems like a start.
Chafe protection gear and snubbers
Spare parts collection for outboard
Paddles (4) - yes, four people should be able to paddle this thing quite well if need be
Lots of lifejackets (duh!)
Ring buoy on 50' line
First aid kit



Commentary is welcome :)

[Willallison]

Don't get me wrong here... I have nothing against your tri project... but....
Is it worth suggesting that you go back and revisit the requirements for this vessel..? I wonder still whether the monohull would be a better solution. It would probably be lighter, have more useable interior volume, most certainly be less complex and expensive to build and I would venture to suggest that it may also be just as economical to operate.
Now, if you have your heart set on a tri (and let's face it, we must all be guilty of letting the heart rule the head, or none of us would have boats!!) then you can simply ignore this post and carry on!

[marshmat]

Hi Will, good to see you around here again :)

I wonder too whether the monohull would be a better solution. Hence the reason why both ideas are out here on this forum for more experienced friends to critique. (Recall - http://www.boatdesign.net/forums/projects-proposals/playing-around-10-m-trailer-cruiser-17597-11.html ) The two boats are, after all, based on the same list of requirements.

Let's compare the two a bit more directly:

Size: Both are 8.5 m LOA, ~8 m LWL. Both weigh approx. 2500 kg fully loaded. The mono is about 300 kg heavier when empty (1700 vs 1400 kg with empty tanks but including all leave-aboard gear) due to its larger engine (140-190 hp sterndrive or jet, vs. 50-60 hp outboard) and a slightly beefier structure (the tri, having some wave-piercing ability and a more structurally efficient main hull shape, doesn't have to contend with the large slamming loads a low-deadrise planing mono can experience). I think the mono can stand to slim down a little, though... will work on that.

Speed: About 15 knots cruise / 20 max for the tri, 18-20 knots cruise / 25 max for the mono. Score one for the planing monohull.

Range: Depends on engine choice, but for the same fuel capacity, the tri should have substantially greater range. At 6 knots (many of our favourite canals have a 5 knot or 10 km/h limit), the drag on the tri is about 0.45 kN, versus about 0.8-0.9 kN for the mono. The tri's drag at 12 knots is 1.7 kN, versus 2.8 kN for the mono at the same speed. Of course, the mono gets slightly better once it's fully on plane, but even up to 20 knots, the tri still has a noticeable advantage. Lower resistance, of course, translates to lower power requirements and longer range. Score one for the tri (as long as we stay below 20 knots).

Life Aboard: The main seating/helm/galley area is about the same size in both boats. The aft cockpit area of the mono is about equal to the side decks of the tri. Either could be fitted with a simple windshield or a pilothouse.

Construction complexity and time: The amas of the trimaran are really quite simple stitch-and-glue hulls (a few plywood bulkheads, plywood skins, and a bit of fibreglass- maybe a week each to build). What's left is of comparable build complexity in both cases, except for the trimaran's folding linkage, which involves machining a bit of aluminum. The two boats are built from similar quantities of similar materials, I wouldn't expect much of a cost differential.

Trailering: Undoubtedly simpler with a monohull than with a folding multi.

In other words: the two are pretty much tied for "next boat" status, and there will be much debating over the relative merits of each before we move on to construction drawings. I'm still planning to start cutting plywood, one way or the other, sometime next summer. These being personal projects, they obviously take a back seat to "real work", so I'm only working on one at a time, and then only for a few hours here and there. An update on the mono, similar to what I posted on this thread yesterday, should be coming in a few days.

[yipster]

gee i keepmissing threads, read this one only today
tri is about half the power for the same speed rite?

[marshmat]

Hi Yipster,

At low speeds, yes, the tri requires about half (or less) the power of the mono.

By the time we get to 20 knots, the tri's 50-60 hp outboard would be maxed out, while the planing mono would be using about 70 of its available 140-190 hp. (In both cases, this is at 2.5 tonnes disp., and including gearbox/prop inefficiencies.) This is the tri's top end, and a comfortable cruise speed for the mono.

(Frankly, I don't see much point in going with a planing hull unless speeds of 20+ kt are going to be used regularly- by the time you get to 25 kt, that planing mono is using more like 110 hp, so its engine would be sized accordingly.)

[yipster]

20 knots is a fine speed Matt
maybe in your neighborhood it may not be the case
but i have that funny feeling in denser populated waterway's
police will point speed guns at the planing boat first

[Ad Hoc]

matt

you seem to be plugging away rather nicely :)

As for mono-tri...it is your project and you must live with it. Forget which is "most" efficient" or "better at lower speeds" etc. All nonsense.

Focus on what YOU want. If your boat in either configuration can deliver the SOR of your choosing, then fine. So it comes down to costs. If both ahve similar costs, not just capitol costs but running costs and maintenance costs, fine, if not go with the cheaper long term one. Finally safety. Which would you feel most safe in following an accident/collision.

That is the bottom line now you've come this far.

[Willallison]

That's what I was getting at when I suggested that Matt re-visit the mono, which I think would be a much easier boat to live with. Having said that, efficiency was one of his earliest stated design objectives.

Matt,
Pitty you can't build them both... it'd be interesting to see just how close to your predictions are to the reality.
Must say I'm surprised that the mono works out heavier and that the (theoretical) efficiency of the two is so far apart.
For 'typical' production boats, I agree with your comments regarding the use of planing hulls for lower speeds. A solid case can be made for the planing hull, however, as lonmg as bottom loadings are kept low

[Mat-C]

What's SOR short for?
I gather it's an acronym for the design objectives....

[Ad Hoc]

Statement Of Requirements = SOR

..in other words, what you want the design to achieve. This is then condensed into a specification and General Arrangement.

[marshmat]

Thanks for the insights, Will / AdHoc / yipster :)

Just in the interest of putting all this in perspective (and, of course, just for the hell of getting out on the water), Katy and I have been taking Sunset Chaser (a 5-metre Bolger runabout) out into some of the cruising grounds we'd be frequenting in the new boat. Some observations:

- At our cruise speed of 16-17 knots, we literally fly past the car ferry and virtually all the other powerboat traffic.

- 22 knots, in these waters (Lake Ontario, 1000 Islands and the Rideau), is just screaming along, fast enough that it becomes hard to keep track of the 40 other boats around you, and puts about fifty interesting destinations within less than an hour's trip.

- There are a lot of heavy, deep-V boats in this area that like to plow around at 15 knots, bowsprit pointing at the clouds. The resulting confused mess of large, steep wakes makes it hard to sustain high speeds for long.

- Canadians pay A LOT for gas, currently about a buck a litre (= $3.25USD / USgal), it was up to $1.40 during the spike last summer ($4.55USD / USgal, still cheaper than English petrol, though). Although there's no "official" evidence of collusion, it's well known that Canadian fuel prices will, at all fuel outlets, simultaneously skyrocket in response to any glitch in the market, and take weeks to settle down again. So a low fuel demand is a bigger priority than speed.

[Willallison]

There's no doubt that speeds on the high side of 20 knots are the exception rather than the rule for most cruising folk. That's why I was determined to produce a craft that was comfortable throughout the lower end. It's nice to have it on tap, of course.

They don't exactly give fuel away here either. Petrol runs at about $1.20 / litre. Diesel uo to $1.50 / litre
To be honest, I don't think it makes much difference. If it costs you $100 to go away for a weekend, as opposed $80, does it really matter? Of far more importance is how easy the boat is to operate.
Additional range is an obvious advantage for lower fuel consumption - and indeed that's why I went for the diesel in Graphite. The additional cost of the unit (about 20K) will never be recovered.

[marshmat]

Hi Will,

Being able to do the whole Rideau on one tank, with a fair bit to spare, is the absolute minimum range we need. (The Rideau is about 100 naut.miles if you go straight through, but with a bit of exploring it's easy to do 150 in a week.)

I'm currently looking at carrying about 250 L (66 gal) of fuel. Now, as we all know, outboard manufacturers are not keen on publishing detailed fuel consumption charts. So taking the old assumption of 10 hp/gal/h for a small gas engine, and using the boat's estimated drag (2.5 t loaded displacement) of 1.5 kN at 5 m/s (10 kt), propeller efficiency 60%, we find ~17 hp being used at 10 knots. So about 6.5 L/h (1.7 gph) to go 10 knots, ie. a mile and a half per litre (3 km/l, 32 L/100km, or about six nautical miles to the gallon).

That would suggest a range at 10 knots of about 375 nautical miles. Gunning the throttle to 18-20 knots drops that to about 200 miles; dropping to 7 knots gives about 11 nautical miles to the gallon for a 750 mile range.


Also, some structural updates, now that I've had a chance to sit down and run some numbers:

I've been looking at combinations of broaching, torquing and racking loads suggested in various standards (oftentimes they refer to catamarans only, so there's some engineering discretion involved in translating the load cases to a tri). So, for the engineering types here, a couple of the loads we're looking at are:

Wave torque (crest under port stern, crest under starboard bow, trough under vaka): Approx. 41 kNm

Broaching / bow burying: Approx. 11 kN up or down, approx. 2 kN lateral, acting on the bow of the ama cantilevered out ahead of the crossbeam

Now, this boat is likely going to get beat up on occasion- hitting rocks and logs, running up on beaches, bumping docks rather hard, landing incorrectly on the trailer (or hitting potholes while on the road), etc. I don't want to hit this stuff, but I know it will happen now and then. So I'm designing the outriggers, crossbeams and their attachments such that it will take a 28 kN load up, down or inward on the end of any one crossbeam to break anything.

Overkill? Definitely. It means, among other things, that the lower folding strut will have a breaking strength of 73 kN, about seven tonnes. And, importantly, it means that if everything does go to hell, things will fail in the order I want them to fail: damage to the outrigger will be contained to the outrigger, without affecting the integrity of the crossbeams or their mounting points on the main hull. But, lo and behold, the level of strength I'm aiming for in the crossbeams is achievable with a box-beam of oak or other decent-quality wood, 2.5 cm thick walls, 24 x 24 cm. And, under that 28 kN load, such a beam only deflects about seven millimetres.

The ring frames in the vaka at these points will of course have to be strong enough to safely pass these forces into the hull. But the crossbeams still end up being a substantially lighter structure than I had been planning on.

[Ad Hoc]

Matt

One Q...have you included any FoS in your load cases, or just used the derived values as absolutes?

Good to see you're doing a deflection check. The low modulus materials you're using means that deflection is the 'driver', not stress.

One load case, perhaps?
Have you considered you chugging along full chat, when the port or stbd ama hits a floating abject head on?...this collision slows down and stops the boat...using your values, this gives around 40kN load applied on the ama. So the cross beams will be in shear and tension (depending upon the stiffness possibly direct in-plane bending too), as it tries to pull away from the main hull.

In your fuel consumption figures, allow 10% unusable at the bottom of the tank. And and 10% for genny use too.

BTW, always wondered, what is vaka?

[marshmat]

Vaka = largest hull of a multihull, ie. middle hull of a tri.

Safety factor = yes, the 28 kN up/down/inward load is after the FoS is applied. Torquing is the worst-case situation on the water (ie, no hard objects involved) and I want an FoS of about 3 in this case.

10% unusable fuel capacity has already been deducted when I talk about 250 L capacity. No generator is fitted as the electrical system will be minimal, only as much as can be charged by the outboard's 25 amp or so alternator.

The case of hitting something is one that I'm not entirely sure how to handle. It could be treated as a simple racking motion (ama stays parallel to main hull, but is forced directly aft). Or one could consider that the boat will yaw sharply on impact, thus the ama would tend to pull out and up.

(Each ama has at least two, possibly as many as four watertight compartments, and their skin is simple and easily repaired. The vaka has three separate watertight compartments in the first three metres. Thus, a hull breach- undesirable as it would be- is not a major threat to the safety of the vessel. As a result, in a major impact, it would be preferable for the ama or the forepeak to be damaged- much like the engine bay of a modern car- rather than breaking something more critical.)

[Ad Hoc]

Vaka, i suspected, but not used to the nomenclature.

I would take the momentary impact. The ama is forced aft, relative to the main hull. So you have a racking moment & shear (depending upon the stiffness) and possibly bending too, since it cannot perform as a truss. (ie classic truss theory m=2j, j=no. of joints).

Once yawing occurs, the load transfer is slightly more complex and somewhat difficult to fully ascertain without spending a lot of detailed analysis, since how much yaw will occur and will the yaw of the 2 hulls be the same relative to each other?..which ever way, the load would be less in a yaw case, owing to the damping.

[Willallison]

No - it's frustrating the way diesel engine manufacturers happily provide power / consumption curves, but outboard manufacturers do not. I guess all you can do is look at some of the published test data from reoutable magazines... though not definitive, I'd think they'd give a more accurate indication than the old 10 hp/gal/hr....
I installed diesel in my current build for similar reasons - there are few places that provide petrol refuelling and the increased range was one of my objectives.

[marshmat]

OK, I'm back. And no, the boat hasn't died. I'm getting married in a couple of weeks, though, and have just started a new job- so, obviously, the boat has had to wait.

HULL

Two shots of the underwater surface are attached- one at 1.3 tonnes displacement (ie, empty) and one at 2.5 tonnes. This may help to clarify what I'm really after with this hull shape. The aim is for something that's quite efficient at low to moderate speeds, and will cut cleanly through the short, heavy chop we get on Lake Ontario. (Pounding into this stuff on plane just doesn't work- but we have a few Farrier tris and various midsize cats around here that can knife right through it without much pitching or spray, and that's the behaviour I want here.)

At the same time, it needs to be a hull that is stable, can carry a fair bit of cargo, and offers enough deck area to be a suitable "party boat" for day cruises. This way, the central cockpit can be kept sheltered and separate from all the busy and/or wet activity out on the amas.

As I've mentioned earlier, in 2.5 tonne loaded condition, the total resistance at 20 knots should be roughly 3.0 kN, ie. 30 kW indicated power before propeller inefficiencies are considered. We would likely be considering a "big foot" style 50 or 60 hp outboard as a suitable match; such an engine would be quite comfortable loafing along at 1/2 to 2/3 throttle which would give 12-16 knots or so in typical loading conditions.

The boat could also be built with an inboard diesel, if desired. However, it is unlikely we will go this route, as neither the range advantage nor the fuel price advantage is substantial enough to warrant the extra cost in this particular case.

COCKPIT

I don't want a pontoon boat- they aren't pretty, IMHO, and they are no fun once the waves are higher than deck level. But they do have those wonderful, ranch-style gates that let you walk on, level with the dock- no gunwales to clamber over while carrying a 60 lb cooler. Those same gates will be found on this boat.

Whether it's swimming, boarding, fishing, carrying lumber or just lounging around, we've decided that fold-down rigid decks to bridge between centre hull and outriggers would be preferable to the trampoline nets used on sailing tris.

There's no need for much of a galley, since we'll usually be docked or beached when spending the night on board. But a small sink will likely prove useful.

A comfortable helm station is a must, along with a convenient place for charts within reach of the helmsman. On the Rideau, one will typically have several sheets at 1:20,000 that are being referenced every three minutes or so; I want them easily visible at a glance.

Whether it's for dinner, for cards, or for prepping fishing tackle, there's something to be said for a decent size table- so a table for 4 is included; it'll drop down to form a double berth or, with the addition of a filler cushion at the boarding gates, a queen-size.

STRUCTURE
I'm plugging away on structural calculations as time permits. The vaka 'wings' are non-structural, supporting only their own weight plus whatever people sit on them. A beefed-up stringer at the wing-to-vaka joint will handle the longitudinal hull girder loads in this area. Outrigger strut (aka) loads will be resolved via a pair of ring frames at each aka, distributing these stresses into the main hull.

The folding linkage- I don't have a good drawing to post yet, but I'll put it up when I do- bears a slight resemblance to that of a Farrier tri, but with somewhat different geometry. The lower (diagonal) strut, running from the bend in the aka down to the main hull side, is the most highly stressed component of the assembly; it will have to have a breaking strength of about 70 kN in pure tension or compression. I'm thinking stainless steel or aluminum box section with a simple bolted pin joint at each end, suitably sized.... but would proper bearings be preferable?

[Ad Hoc]

Matt

Nice to see you're still plugging away.
So, with your 1.3 tonne, i assume you have now done a detailed weight estimate, with margins?

I'm confused when you say a breaking strength of 70kN. If you haven't selected the material, nor the section, how can you say a breaking strength of 70kN? Or do you mean, with the current geometry that you have drawn and thicknesses etc, the load imparted to this structural member, you ahve calculated as 70kN?

[Willallison]

I'm getting married in a couple of weeks

Haven't read the rest of it, but congrats (or comiserations...):D
This being a boat design site, I feel it encumbent upon me to point out that this move will SIGNIFICANTLY alter ALL design decisions you will make in regards to your own boat.
Shower is the 1st word that come to mind
Closely followed by sunpad.
Galley, sadly, will probably change from one of 'her' priorities to one of yours.

The list is endless... but fear not... we are here to support you....;)

[marshmat]

Clarification: I mean that, given the geometry of the linkage and the loads applied to the outrigger, with suitable safety factors, the load on that strut when it fails in pure tension or compression should be 70 kN. There is also a small bending load on that strut under some conditions, that I have yet to fully analyze.

Yes, ~1300 kg empty is the result of a full weights-and-moments calculation, including a healthy complement of loose equipment. The bare hull is closer to 830 kg; the rest is driveline, tankage, interior fittings, equipment, etc.

And, Will, this may come as a surprise to many.... but the lovely lady has been involved in this one pretty much since the beginning, and has been guiding many of these decisions. (And she's just as good with fibreglass as I am, if not better....) This is going to be our "general purpose local" boat- camping, weekend cruising, carting lumber around, fishing, partying with friends. We're fully expecting to build something bigger and more elaborate once we're experienced enough to be comfortable with some more serious cruising.

[Willallison]

And, Will, this may come as a surprise to many.... but the lovely lady has been involved in this one pretty much since the beginning, and has been guiding many of these decisions. (And she's just as good with fibreglass as I am, if not better....) This is going to be our "general purpose local" boat- camping, weekend cruising, carting lumber around, fishing, partying with friends. We're fully expecting to build something bigger and more elaborate once we're experienced enough to be comfortable with some more serious cruising.

Yes - I know... she sounds like quite a find...
But... let me tell you a little tale....
A friend of mine was have an argument with his wife. She was quite rude to him and when he asked what gave her the right to talk to him that way, she simply pointed to her wedding ring and said, "this".....

In all seriousness, I sincerely do congratulate you.... 16 years of wedded boating bliss myself and wouldn't have it any other way (no she's not looking over my shoulder!)

[Fanie]

And she's just as good with fibreglass as I am, if not better....)
Good choice there Mat :D Luck of the Irish in Ontario finding a gal like that !
If you are already looking for something for mure serious cruising then you ARE getting married ;) Good luck, enjoy and come back dead tired or else we'll have to send you back to try again...

Er... if you make excuses to sneak away and come post on the forum, she's doing it right ;)

[Ad Hoc]

Matt

I missed that one too...congratulations. Oddly enough my wife is quiet happy for me to take up half the back garden with my new 25m test tank...i think she sees it as me just being an odd Englishman in Japan!!!

If you have a load/geometry/force diagram of your linkage, be happy to cast my eye over for you...?

[baeckmo]

Remember lads, that among almost all mammals, including Homo Sapiens, it is the female who selects her partner on the basis of maximum genetic variation...........! The clever ladies just let us believe that we have any saying in the process.

[Willallison]

Matt

...Oddly enough my wife is quiet happy for me to take up half the back garden with my new 25m test tank...i think she sees it as me just being an odd Englishman in Japan!!!



You think it's a test tank... she KNOWS it's a reflection pond....;)

[Ad Hoc]

hahaha...either that or she'll surreptitiously put some Koi in it, and say we now have a nice water feature!!! :P

[marshmat]

Koi shouldn't mind the occasional test model flying through their pond, as long as you can keep the raccoons out.... ;)

I will post some of my structure/force sketches as soon as I get a chance.... most certainly they're not up to the standards you 'pros' are used to, but should convey the general idea. (They don't bother teaching engineering physicists how to draw or do structural calculations anymore.... apparently quantum mechanics and mathematical modelling is the 'way of the future'?)

Thanks for the kind words, everyone; 8 days to go so I might be a bit hard to find for the next week or so.

[Ad Hoc]

Matt
"...They don't bother teaching engineering physicists how to draw or do structural calculations anymore.."
That's because you're a physicist, by training, not a naval architect. We did 2 years of structural theory and calculations. Mind you was a bit abstract at times! My first job after graduation, was upon reflection, a very very simple calculation, almost child's play. But, the abstract nature of the theory and designing structures was so complex when we finished the "basics" it was far removed from real structures, it took a while to "tune" my brain into the real practical world to bridge the link with theory. Once i did that, its a piece of cake...mind you, i can't remember how to do complex matrix calculations any more...i just do basic EId^2y/dx^2= bending moment calc's now, even then i get a bit lazy these days and break it down into the very simple bending moments, (taken from Roark's) saves doing lots of calculus.

Good luck on the nuptials...if you ever 'sail' your way around to Japan, we have a spare room..maybe some fresh Koi sashimi too..hahhaa

[marshmat]

I actually have time to work on boat stuff this weekend.... wtf?! (well, :) )

There are a few more structural load calculations to do before I'll be posting anything more on that front.... yes, they are coming, I just want to make sure I'm at least somewhat confident that I understand what's going on first.

I recently realized that I hadn't posted a decent linesplan yet.... since I generally work directly in 3D (and, in previous work, have often manufactured straight from 3D as well), it took a few minutes to brush up on my rarely-used AutoCAD. But here it is, lines and main particulars for the most recent revision to the hull.

The main hull is designed for taped-seam ply/epoxy and its lines should be accurate; the amas, however, are intended for compounded plywood- which, as we all know, can introduce some slight variations compared to the expected shape.

One of the power tri's big downsides- the added wetted surface of its amas- is evident here. In the condition shown (2500 kg disp.), 39% of the boat's weight is on the amas, yet they cause 47% of the drag at 20 knots (most of this being skin friction). In the end, a trimaran configuration is going to have slightly more drag (as well as being more complex) than a similarly slender displacement mono. And if I were doing, say, a commuter boat- or an ordinary cruiser- that's the route I would take. (Going to tiny amas, like Nigel Irens' power tris, would partially defeat the reasons I want amas to begin with, and the drag penalty for somewhat larger amas is really not all that large.)

BUT:
- At locks, docks and such, we are billed by length, while beam is rarely penalized.
- We need a stable platform with room for activities near the water; a tri's "wings" serve this function nicely and give it pontoon-boat-like stability (and ease of access) at the dock.
- This boat will likely be pressed into working service on occasion. We have to be able to carry lumber, etc. and work off one side of the boat; this tri can handle lopsided loads very well (up to a tonne and a half out on one wing before you're likely to get in trouble)

So despite the added complexity of construction, the trimaran form is looking like the preferred option for our next boat. She'll be built a fair bit sturdier than many trailerable tris (~830kg of structural mass for an 8 m waterline boat- the main hull skin is a hair over 8 kg per square metre, about twice what you'd find in a lightly built foam-core composite craft of similar size), as I'm expecting to run aground on Shield granite, beach her on gravel, load her down with concrete and 2x4s, and have newer helmsmen ram her into the dock at four knots (ie, the sorts of things you really try not to do to an F27 or Telstar on a regular basis.)

A test model, to verify that the hull performs as anticipated, is in the works for the coming summer. No launch date has been set, of course, but I expect to start building smaller bits and components next summer, with hull assembly likely to start as soon as I (1) can afford to, and (2) am reasonably sure of staying in one house for long enough to get something done.

[Ad Hoc]

Matt
Looking good. Your autocad skills would put many CAD draughtsman i know to shame! I would ahve expected the aft lines of the buttocks to run near straight and/or parallel; since you're running at higher Fn's. (forgot to add...rounding off the hard chines aft will reduce the drag too).

Your amas are 6.8m @ 20knots that a Fn of 1.26...that is residuary territory. These are creating a lot of waves, ie more residuary drag than frictional. The L/D ratio of the amas is 5.9, so this confirms a bit on the low side, hence, wave making is dominant.

However, as you have pointed out your SOR is not trying to produce the most ideal design/hull. The SOR is to suit what you want/need and works...and that is what design is about. Collating all the variables, often conflicting, and making it work. Well done :)

[marshmat]

Ad Hoc -
Thanks for the compliment :)

The amas are indeed operating at a Froude number where, for most forms, wavemaking would be dominant.... however, these are only about 40 cm wide on the waterline when she's fully loaded. (there's a mistake in the file I posted earlier today, where I copied from the wrong data table: L/B for the ama is actually 17 and each of them is carrying 275 kg of the total, and L/B for the main hull is 10 while it displaces 1950 kg. Brain fart :rolleyes:, not the best day today.... ) My estimates indicate about 0.4-0.5 kN of skin friction and less than 0.1 kN of wave drag on the ama at 20 kt, loaded to 2500 kg displacement. A smaller ama would reduce the friction, but would sacrifice the extremely high at-rest stability we need.

Overall, we are looking at just a hair under 3 kN total drag at 20 knots.... or about 30 kW plus prop losses (say 50 kW, 65 hp rated power) to do this speed fully loaded, about 2/3 of these figures when lightly loaded. The likely choice will be a 50-60 hp outboard which would give 20+ knots running light while remaining small and cheap enough.

The reason for the slightly upswept run aft is to cut down on volume in the aft sections, necessary to balance the fine bow and reduce the tendency of deep, straight sterns to get caught by following waves.... a sacrifice in efficiency, yes, but not a huge one. Although we want to be able to sprint at 20 knots now and then, I'll take a slight bow-up attitude at top speed as the trade-off for a better motion in choppy seas at lower speeds. The bottom is nearly flat, transversely, from about station 6 aft.

[Willallison]

I was just looking at the lines as Ad Hoc made his post. I came to the same conclusion - I'm surprised at the amount of rocker you've designed into the hull. I too would have expected to see straighter buttocks to avoid excessive squatting

[marshmat]

The model testing may yet result in some changes.... but that has to wait until things thaw out here. Is a 4.5 degree quarter-beam buttock angle really that high? 3 to 6 degrees seems about typical, at least among the fleet around here, for boats that operate between the displacement and full-planing ranges....

[Ad Hoc]

In a word, yes.

[marshmat]

G'morning guys.... a bit more rested now and taking another look at a few things....

I should note that there aren't many power multis around here- we have a couple of F27s, a couple of condomarans, and a Hobie or three that are commonly seen on the lake here, that's about it. I have a heap of books and theoretical papers, but frankly, I don't put much faith in them without some real-world feedback. So I don't have as much data to go on as I would like- the feedback on this board is proving quite helpful.

I'm now looking at a few variations on the bottom shape that might alleviate the high-speed squatting issue that Will and AH have suggested could pose a difficulty for this hull- more to come tonight or tomorrow. This would involve reducing the midships draught slightly and reshaping the midships section to maintain a similar volume distribution- I don't want to add any more volume than necessary in the aft sections, but after thinking about it for a while, I think you guys are right, they can stand to be straightened out a bit.

[Ad Hoc]

Matt

So long as you are happy with the end result. Our advice is just that, since at the end of the day, it is your boat and have to live with it and all its compromises.

Just because our advice is "technically" the better solution, doesn't always make it the most practical for your application!

PS..don't forget to round the chine from about 1/3rd aft length to the transom :)

[marshmat]

My latest tweaking and attempts to keep the bottom panels developable have led me to conclude that just switching to strip plank for the main hull underside would allow a shape I'm happier with, at no real penalty in total build time. It's essentially the same main parameters and curve of areas as before, just smoothed out a bit.

I know none of us trust CFD (myself included)- but until the Warm Season begins in another seven months, it's all I have. The calculations for the shape shown in the attached screenshots look promising. (Total drag on the order of 2.9 kN at 20 knots, I estimate an uncertainty of about +- 0.3 kN in that figure).

Fully loaded (~2500 kg):

Vaka Ama
LWL 8.00 6.73 m
BWL 0.86 0.41 m
T 0.46 0.26 m
Disp 1920 290 kg
L/B 9.3 16.4
Cp 0.74 0.71

[marshmat]

Updated linesplan to reflect the last few days of reworking.... trying to unfold the developable hull into something that looked easy enough to build to justify the performance compromises, simply didn't look right. This is essentially the same shape with a little less rocker and soft bilges. Quarter-beam buttock angle is now 2 to 2.5 degrees. Cp for the main hull is 0.75 (!), I know that sounds high- but for reaching 20 knots in displacement mode, ie. Fn = 1.13, that's how things go....

[Ad Hoc]

Matt
wow, that was quick!
Just a few comments, if I may.
The rounding of the chine, may be i was not clear in my explanation, sorry. If, in the original, you had a small bilge radius instead of a chine, say some 100~200mm rad, from stn 7~10. Not a large one for the entire length as now drawn. Since some of the compromise is the ease of build. We have found on higher Fn boats a small rad in the chine, for 1/3 of the length aft reduces drag considerably.

The waterline, in plan view, the second outer one, from CL, this is near circular shaped, as it returns to the hull CL. These WLs would be better if the return is "straight" (ie perpendicular to the CL), as on the transom. This means as you go aft, the rise of keel, and the hull bottom, is a flat panel. If you had this "flat/straight" out to the small bilge rad, would make construction easier than the shaped you have. But i don't know your skills and ability. Only you can comment on that.

[Ad Hoc]

Matt
Attached is an aft view of a catamaran. The WLs shown are straight as one goes aft. This enables the fitting of a water and duct so much easier, as well as 'encouraging' smooth flow into the duct. This is what I mean for yours too.

It is a 50m catamaran, round bilge, just for info.

[Willallison]

AH - what you describe is what I would refer to as a 'traditional' semi-displacement cat hullform. I take it that you retain the hard chine forward in an attempt to achieve water separation and hence lower wetted surface....
Clearly the flatter aft sections will go some way to increasing lift and thus reducing squatting further.
I'd be interested to know your experience of this shape as opposed to what is often referred to as the displaning hullform - where generally parabolic sections end in a canoe stern underneath and just short of a flat transom (as in the attached pic)
ps. Sorry Matt - won't nick your thread for long!

[Ad Hoc]

Willallison

The lines i posted are what 'we' describe as a displacement hull, not a semi-displacement hull. These hulls are not trying to plane, nor are they seeking any "lift". Any lift is a 'nice feature' if obtained (from endless sea trails almost no lift is observed). They simply "plough through" the water at high Fn's.

The 50m cat posted, her Fn is around 1, and hp/tonne roughly 40, with an L/D ratio around 9, and L/B of 16~18

A "typical" semi-displacement hull form falls outside such parameters. Their L/D ratios are generally 5~6, their L/B ratios around 5~8. So whilst the Fn's may be similar, their hull forms are very different.

On our Cat's, we generally don't have hard chines at all, the larger boats, these are all round bilge full on displacement type hulls. But on the smaller cats we do, for resistance, ie these are usually higher Fns. and are more "semi-planning", with their associated lower form factors, and consequently, are easier to build, but not the sole reason for the chine.

Typical form factors for such would be:
L/D = 7
L/B = 10

These, have a more pronounced hump in their resistance too. Hulls with high L/D ratios have almost no hump and very little trim, because they are displacement hulls and not semi-D. Think plank on edge.

Does this answer your Q?

Sorry Matt, for slight Hi-jacking..

[Willallison]

Does this answer your Q?


Partly.... given the size of the vessel and her operating speed, she is clearly, as you say, operating fully in a displacement condition. I take it therefore that the flat aft sections are more to do with intake flows than anything else.
But, onto the part of my post that you didn't address... do you have any experience with the type of hullform that I posted and how it compares to 'your own'?
Clearly, it is not one suited to typical commercial applications where waterjets are the norm, so you may not have had reason to analyse the shape before...

[Ad Hoc]

W.
"..therefore that the flat aft sections are more to do with intake flows than anything else.."
Correct. The flat sections aft help fitting waterjet ducts. But, in Matt's case, will ahve the benefit of being flat, ie easy to build, even though no waterjets fitted.

It is difficult to see the shape of the hull, from that small pic, especially the transom, also where is the waterline on the hull, high up or low down? But it doesn't 'appear' to be any different from what I've done or seen before.

[marshmat]

For a waterjet installation, it is quite obvious that the flat aft sections are a highly desirable shape. Jets seem to like a smooth, straight stream of water at the intake, nothing rotating or swirling.

I don't think ease of construction is going to change very much whether the bilges are slightly rounded or significantly rounded, not in such a small, slender hull. The only construction options on the table are developed plywood or strip plank wood/epoxy; I prefer the shapes that can be done in strip plank and I doubt it will add much if any build time/complexity compared to developable shapes.

When fully loaded, I'm trying to pack a fair bit more weight in this thing than an 8-metre tri would normally carry, so she is necessarily somewhat fuller-bodied than would be 'ideal' for a typical, lighter boat. Also, I keep reminding myself that 20 kt / Fn=1.13 is a sprint speed, and normal cruising will be somewhat below that- 10 to 15 kt (Fn approx 0.5-0.8) will be where most of her underway time is spent, with the ability to handle Force 5 seas and high chop being more important than the 20 kt sprint capability.

Will- I've seen that "displaning" shape around, I think it was a Malcom Tennant idea? I have a few old sketches and preliminary calcs for hulls similar to that, but haven't been able to find enough published literature to really understand it. My understanding of it is that it's basically a canoe stern from baseline to a few inches below LWL, and a flat, sometimes even concave or down-angled, lifting surface above that for the aft few feet. Not unlike the box garboard of a Sea Bright skiff, really. The prop's in a protected spot, and the flat part is supposed to generate a bit of lift at higher speeds to counteract the tendency to squat. I've heard it works fairly well if it's usually run above Fn = 1.0 or so. You'd certainly have a hard time putting a waterjet in it. Never actually seen one, though- as I've mentioned, multihulls in general are a rarity in these parts, except for Hobies and the occasional F27.

[Willallison]

Apologies for the delay - it took me some time to track down a pic that adequately shows the transom....
Waterline on this type of hull is typically 100-200mm above "transom flat" - ie the transom is immersed at rest. The slight prop tunnells seen on this particular boat are not common to them all.
The shape is, as Matt suggests, generally attributed to Malcom Tennant and has proved to be an effective choice for many shaft drive (and more recently, outboard) installations. Clearly it has advantages in terms of driveline protection and in allowing a horizontal shaft, but there's little data out there as to whether it is any better / worse than a 'traditional' displacement cat hull - hence the questions as to whether AH has had any experience with it....

MM - I would expect the flatter aft sections to actually be more difficult to build, as they would require the strip-planks to be twisted quite a bit as they run aft. Still should be doable though - have you any early indication on the required core thickness yet? That may influence things....

[marshmat]

Will - My original plan for the hull skin was 9 mm (3/8") wood, with 2 mm epoxy/glass outside and 1 mm inside. That was calculated using a ply core and may change slightly with the revision to strip plank below the spray chine. Pending model tests, I think the shape as it is now should meet my performance objectives, and it is certainly buildable.

The thing that gets me about that 'displaning' stern is that it doesn't seem like something that's easy to design- there are several aspects (curvature and fore/aft slope of the flat surface, for instance) that will be hard to analyze with CFD, and hard to get right the first time without a lot of model tests. From what I've heard, it took a number of mediocre boats before Tennant and the other designers who use it really figured out the right balance of parameters to get it to work as intended. The knowledge of how to get the shape just right doesn't seem to be readily available, for obvious competitive reasons....

[Ad Hoc]

W
if that waterline is above the transom flat....it is just adding parasitic drag!

[Willallison]

MM - Yes - I've seen a number of variations on the theme. Many of them rather more complex with no obvious benefit. This, of course is the problem with any 'out of the ordinary' hullform - real analysis can only really be done by building / testing.

With strips approx that thickness, I agree you shouldn't have too much trouble getting pretty much any shape you want.

AH - The term displaning was coined to describe the way the two different parts of the hull behave at speed. As I said, at rest, the transom is immersed (as I assume would be the transom on the hull that you posted...) At speed the 'canoe body' of course remains fully submerged, creates no discernable lift and operates entirely in displacement mode. At speed, the 'transom flat' that extends beyond the canoe body appears to operate in more of a planing state: it creates measurable lift and the water breaks cleanly away from its trailing edge. In doing so, it also prevents the hull(s) from squatting.

[Ad Hoc]

W.
The stern on that hull is no different to that found on a SWATH, for clear and obvious reasons. However, if the hull shape is orientated to be like a canoe, and with the corresponding high L/D ratio, there will be no squatting or very little to actually write home about!

If the L/D ratio is low, then flat portion will drag terribly, most noticeably creating eddies on the chines. The flat portion may give the appearance of "providing lift" (or even 'some' minor lift), but at a significant increase in drag, owing to its shape and angle of incidence to the oncoming flow.

Hulls generally can't do both, no matter what the literature says, without a quid pro quo elsewhere. It is all swings and round abouts :)

[marshmat]

There certainly must be an increase in drag, with the "displaning" stern, compared to a simple canoe stern. The wetted surface must necessarily increase and, if there is enough pressure under the flat to generate some lift, there will certainly be chine eddies to contend with- just like a planing hull when the water isn't quite breaking away cleanly. However, I suspect (but cannot confirm) that in some of the boats that use this shape, the added drag of the transom flat is considerably smaller than the added drag you'd get from squatting. Not incidentally, this shape only seems to appear on cats that are relatively fast and relatively heavy for their length, ie. are not as long as would be optimal for their speed/weight. Going longer is the obvious solution, but if you really have to fit too much weight into too short a hull, something like this might work fairly well.

***

Update on my little project.... resistance and powering calculations for the new hull indicate that, at 2500 kg displacement, 10 m/s (~20 kt, Fn = 1.13) requires 3.2 kN thrust (I estimate +- 0.2 kN uncertainty in this figure). That is to say, indicated power is 32 kW / 43 hp. The propulsive coefficient for a well-chosen outboard prop is likely going to be in the range 0.6-0.7, so 61-72 hp to reach 20 knots fully loaded.... more likely we will fit a 50, as the sprint speed will not be needed when in "workboat mode" and the power requirements decrease at lighter displacements.

A sedate 5 knots, as will often be the case on the canals, demands a mere 1.5 kW indicated (approx. 3-4 hp at the propshaft).... a nice efficient cruise speed will be around 12 knots using 10 kW indicated (~22 hp at the propshaft).

[Ad Hoc]

Matt

Without seeing the shaft line and where the prop actually is in relation to the hull etc, i would best aim for a PC of 0.5~0.55. Aiming for 0.6~0.7 from the start is very very optimistic.

You will also need to take into account (or at least be aware of) weight and surface growth and wear...hence the power requirement in some 5 years time would be shy by some 10~15%, without continual maintenance, at best.

[marshmat]

Without seeing the shaft line and where the prop actually is in relation to the hull etc, i would best aim for a PC of 0.5~0.55. Aiming for 0.6~0.7 from the start is very very optimistic.
Preliminary Bp-delta calculations, using a Honda BF50 as a representative motor, suggest a PC of 0.62 should be achievable with a stock 11.5" dia. prop on this boat, and that's with a rather conservative estimate of the advance velocity (Taylor Wf = 0.72). A "big foot" gearcase with steeper reduction, swinging a larger diameter prop, would likely do even better. I wouldn't call it overly optimistic to shoot for >0.6, although 0.7 is probably going to be an unreachable upper bound due to the diameter constraint of an outboard's aperture.

You will also need to take into account weight and surface growth and wear...hence the power requirement in some 5 years time would be shy by some 10~15%, without continual maintenance, at best.
True for something that lives in salt water all the time.... this is a trailer boat used mainly in cool, fresh lakes and so fouling is not nearly so much of an issue. Most of the time we will be running at 1800-2000 kg displacement, which reduces the power requirements to 70-80% of what they are at full load (2500 kg).

[Ad Hoc]

Matt

maybe this is a definitions misunderstanding..????

When you refer to PC...are you talking about the efficiency of the prop (eta), from the line of efficiency on the prop charts, or are you referring to the overall propulsive efficiency, ie effective power/shaft power (EHP/SHP).

Your boat will slowly, over time gain weight, not matter what you do. But for other growths, in FW you will ahve much less, especially of cleaned regularly. But wear and tear will slowly degrade the overall efficiency too. These are not concerns, just things to be aware of.

We have just had an operator come back to us asking why the ferry we designed 10 years ago has somewhat different deadweight today than when delivered. Same reasons.

[marshmat]

Outboard engines, at least in N. America, are supposed to be rated at the propeller shaft.... so apart from the drag of the gearcase/skeg, the propeller efficiency (eta) should account for virtually all of PC..... unless I'm reading the textbooks wrong....
Of course, for the present purposes there's no point in splitting hairs that finely, since "50 hp" on an outboard engine really means "Probably between 46 and 55 hp, most of the time". A 3% difference in PC means nothing when you have a 10% uncertainty in your actual engine rating.

[Ad Hoc]

Matt

There is a big difference between the efficiency of what one can get from a prop...reading the charts..and what the overall "total system efficiency" will be.

Ive scanned in a typical chat i use. The efficiencies shown, are for the prop in idea conditions. But these are used only for establishing the size and what is the 'best' or most suitable, to get the maximum performance, given the data to hand...ie delta(opt).

Once the prop is in the water, many other factors come into to play which affects the overall efficiency of the whole system.

This is why naval architects refer to PCs as the "power in versus power out", basically. In other words, it represents how efficient the whole "package" is, engine, shaft, gear box, prop, hull etc. Of course the overall efficiency is effected by water flow into the prop, shaft angle, etc etc....this is already assuming one has selected/designed a prop with the from the charts with delta(opt) at the maximum value.

So, going back to my original point. depending upon how the shaft line is, the location of the prop etc...your PC...ie what will eventually be used to push your boat through the water as required power, may be different to what you "theoretically" can obtain by the prop alone, in ideal conditions, ie no hull, open water etc etc etc.

So, when on sea trails, we use strain gauges to measure the power output. That gives us one part of the equation. Laying over the EHP from tank testing, we get the overall PC for the boat.

Does this clarify?

[Rick Willoughby]

Outboard engines, at least in N. America, are supposed to be rated at the propeller shaft.... so apart from the drag of the gearcase/skeg, the propeller efficiency (eta) should account for virtually all of PC..... unless I'm reading the textbooks wrong....
Of course, for the present purposes there's no point in splitting hairs that finely, since "50 hp" on an outboard engine really means "Probably between 46 and 55 hp, most of the time". A 3% difference in PC means nothing when you have a 10% uncertainty in your actual engine rating.

Mat
You will have to consider a high thrust prop for your application. Moving a 2.5t boat with 50HP is not in their typical range of application. For example Yamaha make a high thrust 14 X 11 prop (pontoon series) that will do a bit better than the smaller diameter standard range under the heavy loading. Even with this the prop will be in partial cavitation.

You would need 60HP on the prop to get your 10m/s with 3.2kN drag.

Rick

[marshmat]

There is a big difference between the efficiency of what one can get from a prop...reading the charts..and what the overall "total system efficiency" will be.
.....

Does this clarify?
Yes, it does. I have already accounted for the wake fraction, the change in 'delta' for the prop not being in open water, etc. in the calculation above. Leaving all those factors out suggests eta ~ 0.7 to 0.72 for the prop alone in open water; with all those various losses considered, I calculate closer to 0.62 for the overall PC. So I think we are talking about the same thing.

Rick- yes, it does look like a "hi-thrust" outboard with a steeper than normal reduction is called for. Recall that 20 knots is not required at max. load, only for short sprints at partial load (Rt is closer to 2.6 kN at 2000 kg displacement, 20 knots, ie. about 80% of what is required to hit this speed fully loaded).

[Willallison]

I shan't hijack this thread any further re the displaning cat hullform other than to say that it's originator, Tennant applied it succesfuly to a range of vessels - many of them with quite low D/L. It has been adopted by many in the recreational cat arena since. There's no doubt that whilst submerged, the flat transom would be les eficient than a canoe stern. However, at speed the transom is effectively operating in the planing mode.
Now - back to the topic at hand. I'd concur fully with AH. Aiming for an overall propulsive efficiency of anything over 60% is certainly optomistic. You may initially achieve a bit more, but the cost of slightly overpowering is far les than the cost of underpowering...

[marshmat]

the cost of slightly overpowering is far les than the cost of underpowering... Generally speaking, this is true.
I must point out again, though, that in my case, the ability to hit 20 knots is a sprint speed for when we're relatively lightly loaded.... most of the time, we will be operating at 10-15 knots or less, weighing 1500 to 2000 kg. With a tonne of cargo on board (2500 kg disp), we won't need as much speed.
I would rather not spend extra money on a larger engine than necessary, since even a 50 is going to be run at half throttle most of the time.

[Willallison]

I know you know this, but one of the advantages that 4-stroke outboards have - in particular over diesels - is that they are happiest when lightly loaded. Unfortunately, if you stick within the Honda stable, jumping up to anything bigger immediately incurrs a weight penalty. Beyond that I would think that going bigger only offers advantages...
Yes - we (I) understand that your top speed is a low priority. However, there's no point in having a design goal if you don't strive to achieve it. Ok, other factors may come into play that will force you to compromise - they invariably do - especially on a lower priority items. But to include an optomistic variable in your calculations and then justify its inclusion by saying that the result is of lower importance, seems like a somewhat illogical way of going about it to me....

[marshmat]

I agree with you, Will, on most of that.

From the calculations I have done so far, though, I do not think a PC of a bit over 0.6 is overly optimistic. It's a figure I calculated using Bp-delta charts for several propeller patterns common on boats this size, along with fairly conservative estimates of the wake fraction, the loss in prop efficiency caused by being behind a hull and strut instead of open water, a resistance value from a calculation set up to err on the conservative side, etcetera.... the math has me fairly convinced that it's realistic.

Please feel free, though, to point me in the direction of a set of equations that you have found to yield more accurate PC estimates, should you suspect I'm off base with these numbers. Not trying to offend anyone- it's just that, as an engineer, I'm more easily convinced my own calculations are off if I can see more accurate ones, than if I'm just told they look a bit optimistic.

[Willallison]

After I re-read my own post, I rather thought you might (quite reasonably) say that!:D
I know you're not one to pluck a number out of the air... I agree that something around 0.6 is going to be close, but I'm yet to come across an installation that has managed 0.7.
Again - not saying it can't be done... I just haven't seen one myself and I always like to base my numbers on existing craft, where possible.

[Ad Hoc]

Matt

One can only say with a higher degree of certainty, a such, with a drawing of the installation and its relative location/position to the underwater portion of the hull. This is where the "numbers" obtain their source, not just from charts. The location is the all important key factor and a visual image aids this immensely and is the only way to say for certain what overall PC to use..

[marshmat]

Thanks, W and AH :)

See attached.... this is for a 14" dia. prop on a "big foot" style outboard gearcase.
I have a suspicion that the wake fraction I'm using (1-Wt = 1.11 - (0.6 Cb) for Taylor wake fraction Wt, which gives 1-Wt = 0.72) is rather on the conservative side for this boat... but the other formula I have here (1-Wt = 0.83 * V(knots)^0.047) suggests that 1-Wt is closer to 0.95. That, I think, would yield an optimistically high advance velocity, so I've been using the block coefficient formula even though it probably errs on the conservative side in this case.
I'm not stuck on Honda, they're just a typical mid-range engine in a suitable size for which spec sheets are handy.... outboards are (supposed to be) rated at the propeller shaft, so gear losses etc. are accounted for by a reduction in "rated" horsepower.

Most likely, it will come down to a choice of a 50 or 60 hp outboard, either 4-stroke or DI, as I don't think I can justify the added weight/cost of 70 hp when the peak power would be used so rarely. But some 50s and 60s share the same block and are the same weight, in which case the extra ten horses would be easier to justify. We'll see what the market has available when the time comes.

And there is a good chance I will be able to do full-speed, full-scale resistance measurements on the actual hull before buying an engine for it; computer simulations can only tell you so much.

[Ad Hoc]

Matt
Looks fine. Do you know what your static waterline trim is, in light and fully loaded?

[marshmat]

AH,

Included in the "light ship" weight estimate (currently sitting at 1300 kg) is a 100 kg allowance for all that miscellaneous gear that builds up over time.... moving the "junk pile" between bow and stern compartments gives us +300 / -200 kg.m of fully controllable trimming moment.

The moment to trim 1 cm is estimated to be approx. 3.8 kg.m/cm (or 70 ft.lb/inch). (This is by an older formula for displacement mono shapes, corrected for a tri-hull configuration; it may not be quite right here as this boat is relatively full in the ends.) The cockpit is directly over the CF/CB, and depending where the crew place themselves, the trimming moment (normally ~0 at any displacement) can vary up to about +- 100 kg.m.

So to summarize, static waterline trim will typically vary by a maximum of about +- 10 cm at the stern and perhaps closer to +- 16 cm at the bow, and is highly dependent on where the passengers decide to sit.... I know of no accurate way to simulate trim changes with speed in a hull of this type, so we will have to wait until sea trials to find the trim conditions at which she runs best (the model, planned for next summer, will also yield some insights into this).

[Fanie]

Hello Matt,

Haven't chatted to you in a while. How's married life treating you so far :D


Wrt your boat. You still set on going for the tri... a trailable folding cat may have a lot of advantages. For one the hulls can be made much wider, more deck space and I can think of no advantage a tri will have over a cat...

Just a thought though.

[Ad Hoc]

Matt

I assume from your post, you have estimated trims etc, via hand calculations? If so, if you have a dxf output, i can dump it into my stability program and run a set of hydro's if you like?

Assuming your figures to be "roughly" correct, the amount of trim over that length is not much. So your drive wont be affected really, so 'should' be good across the speed range. I don't have a lot of experience with out drives like that (other than my small old outdrive on my small fishing boat i had before i left the UK, and skiing boat when i was a teenager), so can't comment much more....since the boats i design are commercial, and not small add-ons installations like yours. Other than, are you selecting the prop size, or is the manufacturer suggesting one to you?

Unless you have run model tests, there is no other accurate of estimating or establishing trim, with speed.

[Willallison]

:) Hah.... great minds as they say..... I just sent Matt a PM suggesting the same thing!

[Ad Hoc]

"Im gonna have to get up pretty early in the morning to catch you out..!!"
:)

[Willallison]

Well - maybe we could both do it and see if we get the same answer!!

Re the prop size and location, one of the great advantages in going for an outboard is that everything can be changed easily and generally at no cost. Trim and height are simple adjustments and most reputable dealers will allow you to test a number of props in order to make sure you get the best one. It'll be interesting to see how the performance from light to heavy ship varies with different props - if you are going to be travelling any great distances, you might find it worthwhile carrying a couple of different sizes - one for each condition. A spare is always a good idea anyway....

[Ad Hoc]

good idea.
Have one matched for the 'top end' speed...and then increased blade area for the lower end speed when 'you're pulling'.

[Willallison]

A good example - though perhaps a bit extreme for Mat - is the 'high-thrust' range from Mercury. I assume they still make them - we've had ours for quite a few years.
Very low pitch / high blade area, but the interesting thing is that the hub is shorter than the spline onto which it is attached. This allows the prop to slide back and fwd. In fwd the prop slips fwd and allows the exhaust to exit as normal. In reverse, however, the prop slides back against a 'cup' that covers the aft end of the hub. The exhaust gases that would otherwise flow aft then back across the blades, now exit between the prop and the gearcase, allowing the prop to swing in clean water. Simple - but very effective - we've used it a number of time to tow 20+ ton boats off the hard (with a 10hp outboard btw)

[marshmat]

Fanie - Good to hear from you :) We're getting along quite well (have survived 4 weeks so far....)
Re. cat vs. tri- I'm not sure if I mentioned this on here; if I did, it certainly wasn't in much detail. About a year ago I spent a fair bit of time doing preliminary sketches and calculations to help figure out what the "next boat" was going to be- and most of that work was based around a displacement cat design. The cost, complexity and size/weight were similar to the tri I'm working on now. The downside? Well, every time I tried to give it some sort of protected space on the bridgedeck, it ended up with the high windage and low clearance of a pontoon boat. If we were in a warmer area, I'd more likely go that way, with a more open bridgedeck. As it is, in the cool Canadian climate, we decided that the trimaran's lower, more protected central cockpit would be preferable. We didn't really look at proas, due to our length constraint, although I would seriously consider them in a case where excessive length was not penalized. (Some of you may recall a planing monohull proposal that I did discuss on here; I may yet finish the design of that one but will likely not build it myself.)

AH, Will - Yes, I am doing many of these calculations the "old fashioned" way, albeit with areas and volumes from the 3D model rather than from planimeter & Simpson's rule. This project is a learning exercise for me, the perpetual student. And, having grown up with computers and 3D CAD (and now programming the darn things as part of my job), I simply refuse to trust a computer's word on any calculation that I don't know how to derive and understand for myself.

That said, I have been using the hydrostatics features in Freeship (via an ACIS transfer from Rhino4), once I've done sufficient research to know how each parameter affects the boat's behaviour. W, AH - I'll PM you regarding your generous offers of assistance.

I hadn't thought about carrying multiple props, but now that you mention it, the idea makes a lot of sense. One higher-pitch, low-DAR wheel for lighter load cases, and a shallower, high-blade-area one for when we're called into "lumber barge" duty and need more thrust at lower speeds. 14" outboard props are not excessively costly and, for a trailer boat, are very easy to change.

[Fanie]

Hi Matt,

The first consideration is of course safety, I'm sure this won't be a problem with either a cat or a tri. Then secondly would be comfort. Imo this is important, especially if you and someone else is going to spend some time in it. When it gets to too little space it is something that can begin to irritate quite quickly. A mono hull as in a tri does not have space for one to move around on.

If you think in terms of a cat, how about a lobsided set of hulls (similar to a proa), the one the primary hull with liveabord stuff, berthing etc while the second hull is for storage ie batteries, water etc. The primary hull can extend over the secondary hull and obtain full trailable width. When you fold it open the extended part can rest on the beams.

I like the boat's size though, 8m is ok, 9m would be better of course. One should keep in mind while trailable it must be possible. Trailing is the secondary in line, I would rather trailer a bit more difficult but have more comfort on the water, unless you plan to spend more time trailing than sailing :D

Anyway, just thinking out loud. I'm not trying to convince you to go to a cat, a tri's center hull can be made very much the same in width, I've seen some very nice tri's insides. Personally I just favour the cat a bit more.

On your tri I would lift the ama's a bit, and make them a bit deeper. Both them ama's in so deep is going to slow you down quite a bit. Doesn't have to be the tri falls over from side to side, but on the little tri one hardly noticed if it slanted from one side to the other. Your's is larger and would be much more stable.

I'm not sure what your tri's BOA is, but an 8m tri can comfortably be 6m wide... Have you thought about a hard deck between the main hull and the ama's ? When weather allows it would be nice to loaf around on it.

[Fanie]

I've been thinking about your tri...

You can have a full trailable width center hull if you fold the beams and ama's like in the picture. I know it's rough, but you can work the details out yourself.

The sizes is roughly 2m400 wide center hull and 1m800 headroom. Folded out the tri is 6m wide in total, and folded in the total trailing height on the trailer is about 2m700.

To lower the ama's into the cavities you could use a gin pole with a winch. They would hinge up and when they reach the upper position, you can winch down and they will drop down into their sleeves. To fold out you would winch the ama's up, put a stopper pin in below the beam (red) and push them over so they hinge to the water as showed. They can then have another locking pin to keep them from hinging up or a single bold to bolt each beam down.

The beams can be located in line with bulkheads, so the bulkheads would serve a duel purpose, be bulkheads as well as be a housing for the beams.
It may look complicated but it should be fairly easy to make.

You would score the space in the center hull that the ama's would take up on the trailer.

[marshmat]

Hi Fanie,
You've raised some good points, I'll try to address a few of them:

Proas
I like the idea.... there are some proas that work very, very well indeed. I wouldn't rule one out as a future boat. For now, though, we aren't ready for something quite so radical- even a cat/tri is a rare sight around here.

Size
We're trying to keep this thing down to dimensions and weights that will allow it to be towed by a properly equipped minivan or similar. Length is penalized on the canals and can cause problems at smaller docks; 8 m on the waterline seems to be about the right compromise for how much we want to carry and do. (Even so, this is a metre or two longer than most boats of similar usable space.) Think boat camping and day-tripping, rather than cruising.

Layout
We are 4.6 m BOA with the amas out, and yes, there is a folding hard deck bridging the gap. (A good platform for fishing, diving, etc. especially with walk-through side gates, no gunwale to climb over.) At the passenger seating level, the main hull is a full 8' wide.

Amas
The folding scheme you describe is one that I looked at early on; once the main dimensions of the hulls were figured out, it became evident that the amas could tuck away in otherwise unused space below the vaka 'wings' in a similar manner to a Farrier sailing tri (although the linkage is somewhat different).
The amas barely skim the surface at light-ship displacement (1300 kg), yet they are quite large- almost sailing tri size- compared to the tiny "training wheel" outriggers of many power tris. Because we will often be working out on the wings, or carrying loads (sometimes asymmetric), the added buoyancy of relatively big amas outweighs the slight nuisance they pose when trailering. The added drag of the larger amas is more than offset by the corresponding reduction in L/B and D/L of the vaka. Obviously, this would play out differently if we could simply make the vaka longer, an option we have already exercised to the maximum extent feasible for our requirements.