Trailer cruiser revisited... as a trimaran

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!

 

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...

 

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.

 

Weights estimate

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.

 

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!

 

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

 

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.

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.

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.

 

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.

 

Updates

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

 

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

Rick W

 

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.

 

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.

 

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

 

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.

 

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?