Wide-hulled trimaran?

In the recent thread about hooking two monohulls together to make a catamaran, there were a couple of posts about converting a monohull to a trimaran. That got me to wondering what the downsides were to a wider-hulled trimaran? To put my question in context, I’m interested in a seaworthy, seakindly, bluewater cruising boat, about 35 feet long, which can carry a load of at least 4,000 lbs. I’m attracted to the flatter sailing and shoal draft of a trimaran, but find their minimal payload limiting. I also need the trimaran to be demountable, though not necessarily easily trailable, so the hull could not have the cabin wings that are often found on larger trimarans.

So my question is, if I’m willing to sacrifice some speed for extra displacement, what will be the undesirable consequences of making a 35 foot long trimaran with a breadth of 10 to 11 feet? Will it be able to sail on all points at least as well as a similarly sized cruising monohull? I’m assuming this trimaran would weigh less than the monohull, because even though it had the extra weight of the amas, it wouldn’t have ballast. Again, this would be a cruising trimaran, so I don’t expect it to even have the performance of a racing/cruising monohull. But I would like it to have steady and reliable performance on all points of sail even when loaded. Is such a design possible?

 

I recall a few years back someone building a tri with a Soling centre hull, which sailed ok but wasn't particularly fast. There have been some successful conversions of surf boat hulls into tris here and in Aus, but these are of course somewhat smaller than what you are looking for.

Are you thinking of building such a boat from scratch or finding an old mono to use as a center hull? You could get close to your desired payload with some of the more cruisey tri designs. The Marples Seaclipper 38 and 41 have designed payloads of 3000 and 3,500 lbs respectively and the Horstman tris have fairly wide hulls too, although not demountable as designed (in the larger sizes).

 

Richard Edlin has designed some succesful cruising trimarans with fairly low length to beam ratios I think Ladyhawke's mainhull is 8 to 1 which calculates out to 5' on the 40' waterline. Even demountable designs can provide quite a lot of volume in a hull flare up to the maximum towing width of 2.5metres with the floats mounted in a cradle underneath the flare.

 

I have more interest in building the hull rather than modifying an existing monohull, but would consider a modification if the right hull presented itself -- assuming this whole idea isn't a disaster in the making.

Marples trimarans are interesting, and I've purchased some of his study material. But the hulls are longer than is my preference, and even with that extra length they don't have the carrying capacity that I want.

So I'm still with my original question of whether a wide-bodied trimaran is just a not-so-fast trimaran, or if it would be slow even compared to a monohull? Would there be any sort of adverse hydrodynamic interaction between a wider hull and the amas? Would a wide-hulled trimaran have a wetter or a dryer ride than narrow hull trimaran? Would a wide-bodied trimaran be more or less likely to pitchpole? Could the ama design from a narrow-bodied trimaran of the same length be used with a wide-bodied trimaran, or should there be more buoyancy because of the increased displacement of the main hull? Since this is intended as a cruising boat, not a racing machine, there would be no expectation of flying the hull; just wanting to keep the mast pointing skyward, even in nasty conditions.

 

Look at the horstman designs

 

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You'll be limited to slow displacement speeds like a mono( because of weight and L/B ratio) and the only advantage of a multihull you may have would be reduced heeling. The L/B of the ama will mean nothing because of the wide main hull and weight that limits speed. It would be real easy to overpower a boat like this because it will have massive RM-but it's speed is limited to that of a mono so the extra power that is possible would quickly overpower the boat . A major advantage of a multihull is speed-speed to out run storms etc-if you give that up you have to seriously question the results.
This is a sort of hybrid that would definitely require the services of an experienced multihull designer and marine engineer-very few people would have experience designing a heavy multihull to perform poorly(intentionally)-you need to discuss this with someone that can explain the structural issues to you from an experienced standpoint-as well as the issues that would apply from a cruising standpoint. I think it is a very bad idea but that is a gut feeling from lots of experience in small, fast multihulls-mostly tri's -under 20'. However, someone experienced in designing and engineering cruising trimarans may be able to arrive at a compromise design that would give you most of what you want without really creating a hybrid design.
Consult with an expert on cruising multihulls and please don't attempt to design something like this without experienced guidance. And...good luck!
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Just as an aside: small tri's can be designed with wider main hulls than the high L/B "normal" tri's if the main hull is designed to plane-like the Weta for instance. But they must be relatively light.....and/or high powered.

 

rberrey,
I've purchased Horstman's study plans. His wide hulls come perhaps the closest to what I have in mind. However, it appears that only an 18' and 31' tri of his are demountable. These are too small for the type of cruising I wish to do, but maybe it's possible to modify one of his larger designs to be demountable.

Doug,
Yes, a big advantage (to me) is reduced heeling and related characteristics like minimal rolling when sailing downwind or in a rolly anchorage, as well as being shoal draft and the ability to stand upright when dried out.

I understand the concern of not overpowering a sluggish trimaran. It wouldn't be able to spill wind from a hard gust like a monohull, and it couldn't quickly accelerate like a typical trimaran. But this concern could be partly addressed by not putting a large sail on her in the first place -- after all, I'm not wishing to race, just to cruise (safely).

I would certainly appreciate the comments of an experienced cruising trimaran designer. Do any of them frequent this forum, or can you recommend any who don't?

 

From what I've read that you want this looks almost perfect:

http://www.dragonfly.dk/boats/dragonfly-1200/presentation.aspx

 

Imaginary number, I can see what you are getting at. Good luck with it

Doug Lord "and the only advantage of a multihull you may have would be reduced heeling"

I have no where near your experience or knowledge, a complete amatuer, so please correct me if I am wrong. I see more advantages than that, as stated by imaginary number:

reduced heeling
minimal rolling when sailing downwind
minimal rolling when at anchor
shoal draft
the ability to stand upright when dried out.

And to me the most important difference between a ballasted mono and a multi, the ability to float after a massive hole is put in the boat.

Doug Lord "It would be real easy to overpower a boat like this because it will have massive RM-but it's speed is limited to that of a mono so the extra power that is possible would quickly overpower the boat"

This is a very interesting point. Would well designed high bouyancy amas help to solve this? Scenario, boat gets a large sudden gust, boat heels massively flys windward hull and possibly even main hull. Lee hull shape bites into the water, because of it's high bouyancy and very streamlined shape/wave piercing bows it rockets along on its path.

Or how about this for an idea? The boat is designed with relatively rounded shapes on the bottoms of the hulls, and a small amount of leeway prevention is added. Scenario, boat gets a large sudden gust, boat heels slightly, possibly flying the windward hull, the force skids the boat sideways on the water surface, no "tripping" over the hulls/daggers/keels.

 

can i have one for christmas

 

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1) I think that is correct-but it has to be designed very carefully so as to retain some of the performance advantages of a multihull ,in my opinion. And if its very heavy as originally suggested it is not likely to float.

2) The problem is that IF the boat is heavy AND the main hull is wide(at the waterline) there will be no "multihull acceleration" since the boat would be limited in top speed. Flying the main hull in a boat like this is out of the question except in a really dangerous situation. The key would be not to power the boat up commensurate with its extraordinary RM as I.N. suggested.

3) Maybe on a light boat with lateral resistance retracted-not likely on a heavy boat.
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From what I understood that I.N. wanted the Dragonfly 1200 seems almost perfect without being a hybrid and retaining the performance advantages, to a large extent, of a trimaran.
But, really, the whole key to this is to not do it on your own-get help from an experienced cruising multihull designer and a marine engineer.

 

Doug,
The Dragonflys are lovely boats. I particularly like their swing wing amas. I suspect I'll have to join whitepointer in the line of kids waiting to see Santa Claus. Their payload of 1,500kg (3,300lbs) isn't as much as I'd like, and I find it odd that both their 35 footer and their 12meter models both claim to have a payload of 1500kg. Nevertheless, I would do my best to keep a stiff upper lip if I found either one of them stuffed into my Christmas stocking.

As a point of reference, Kurt Hughes has a 38' trimaran with demountable/telescoping amas. Kurt lists its payload is a little over 2000lbs. So these Dragonflys are more spacious than some comparable tris.

 

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Because it seemed so close to what you're suggesting I thought it might be a good starting point with a designer that you choose.....

 

Doesn't it have more to do with the weight than the width? I would think that a light boat with a wide hull would go plenty fast, wouldn't it? That is how a lot of fast motor boats are designed, after all.

 

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I mentioned earlier that small trimarans that are relatively light can and do plane and can have relatively wide main hulls. Farrier's main hulls are designed to plane and he claims thats why they tend to have more inside room. I.N. was talking about a wide ,heavy hull which would limit speed to displacement speeds like monohull cruising boats.
The 14' Weta has a main hull L/B ratio of 5.3/1 which is wide but it has the power and light enough weight to plane. A planing main hull is a perfect companion to a displacement ama with a high L/B ratio(skinny). On the Weta the L/B ratio of the ama is 17.6/1.