Trailer cruiser revisited... as a trimaran

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.

 

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.

 

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

 

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.

 

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?

 

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.

 

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

 

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.

 

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

 

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

 

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....

 

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.

 

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

 

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.

 

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...