Shapes For Fiberglass Trimaran Hull

[Unique Trimaran]

Hello, I'm a "long time listener, first time caller". I've had pontoons for most of my life, but I've wanted to build my own boat for roughly (40) years now and I'm not getting any younger! Ha! However, I don't know anything about how hydrodynamics work. I could give it a shot at trying to design my own hull, but I have no idea how this would turn out and actually perform. Having said this, this is what I would like to do:
I would like to build my own 27' trimaran boat with an 8'-6" beam using a one-piece fiberglass hull and use a 250 hp outboard, and would weigh 7,000 lbs. max., fully loaded with fuel, gear and people. I would like to have this boat go 35-40 mph.

I do not like the round aluminum pontoons that all tritoon manufacturers put under their boats. In my opinion, the round pontoons are somewhat dysfunctional from the start, since their basic design is more of a displacement hull rather than a planing hull. They provide floatation but don't provide much in performance if they remain in their basic round form. I understand that pontoons have used the round aluminum shapes since the "beginning of time", but now they include aggressive lifting strakes, etc. to obtain characteristics of a planing hull rather than a displacement hull. These add-ons are very helpful, but I'd still be stuck with the round aluminum toon (which looks bad in my opinion) if I purchased (3) of these toons for my floatation from a boat manufacturer. Most of the top tritoon manufacturers use a deeper and wider middle toon to emulate a v-hull (when I say V here I'm only referring to the bottom surfaces and how they relate to each other with the center float sitting deeper in the water), which makes sense as it helps in cornering as it will actually "roll" into a corner, instead of sitting flat on the water trying to rotate the boat during cornering. I like the idea of a deeper/wider middle float since it would be very helpful in my opinion and is something that not only would provide better cornering performance but also would create more buoyancy, and provide a larger area for fuel tank, storage, etc.

Instead of having (3) hulls that would all have a small V on their bottoms, wouldn't it be better to have at least the center float to have what looks like an inverted V shape on the bottom in an effort to actually hold water underneath it to get on plane quicker? My thoughts are this would not only plane quicker but this would also reduce the amount of spray/splash that would occur between each float. Maybe all three floats should have this same, or similar shape? I've also thought about having a 32" wide center float (with the inverted V on the bottom for the center float and then having the two outer floats being shaped similar to what they use on a racing cat, with the outer floats being curved on the outer side and then somewhat vertical shape that would face the inner float, shaped somewhat like a D shape. If this would work, the combination of these shapes would reduce the spay/splash effect. Again, this is a real head scratcher for me since I know nothing about hydrodynamics but if I'm going to spend the time to build this, I would like to build something that planes quickly and is very efficient in cornering and also provide a smooth ride. Would making the bottoms flat of all three floats be an option?

 

[Herreshock]

You can make a dragonfly 28 style trimaran but you should bring more people doing similar projects and use some land with tents

You just have to pick a free donated 70s-80s 28 sailboat and remove the keel and use plywood to change the shape of the hull, this has been published by eric spongberg in professional boat builder "case studies in redesign".





You just need to modify the hull and build the lower base of the trimaran main hull

and make the remaining 2 amas hulls with plywood as well (check comuzzi plywood catamaran)

For the beams you can use discarded lighting poles and bend them and use plywood to give the wing airfoil profile.

I'll publish a post about that redesign that can bring back to life plenty of abandoned sailboats

Trimarans are fast because they can use more sail area than rest of sailboats besides not bringing ballast and having thinner hulls

And besides of that they are stable and by lowering the rig height with a ketch or schooner rig the risk of wind pitchpolling or rolling capsize is avoided

 

[Unique Trimaran]

The boat I want to build will be powered with a single engine 200-250 hp and will not have sails. Imagine a 27' boat with a cabin, or a small houseboat - this is what I would like to build, but thank you for your input.

 

[Herreshock]

The boat I want to build will be powered with a single engine 200-250 hp and will not have sails. Picture a 27' boat with a cabin, or a small houseboat - this is what I would like to build, but thank you for your input.

Is rather the boat you have seen advertised, and don't forget that engine boats can burn easily with leftover fuel on deck, smell, are noisy, chop resting fishes, swimmers, divers, etc and also the engine can stop anytime and left you at the mercy of the sea without even rowing poles or a proper anchor

Anyway, here you have the idea and also some people will watch the above post and think about it

Dont forget that sailing trimarans are also trailerable

However for a lake crewed boat a 28 plywood sailing catamaran without centre cabin will do, using a custom big Bimini when anchored and seats, on deck just like these pontoon boats

Just like this Edel 28 catamaran but with plywood and only bare pole beams and net on deck so you can disassemble it for a trailer

 

[Unique Trimaran]

I'm glad you like and prefer sailing trimarans, and wish you the utmost sailing enjoyment!
We have a cottage on a lake and this is mainly where I would like to use my boat. However, it would easily be trailerable and there are hundreds of inland lakes around us, which is where I would like to use my boat. This would not be built for "the sea".

 

[bajansailor]

It sounds like you basically want a long and skinny (and fast) monohull powerboat that has stabilising outriggers / amas?

 

[Ad Hoc]

It sounds like you basically want a long and skinny (and fast) monohull powerboat that has stabilising outriggers / amas?

Yup...a stabilised monohull.

 

[Herreshock]

I'm glad you like and prefer sailing trimarans, and wish you the utmost sailing enjoyment!
We have a cottage on a lake and this is mainly where I would like to use my boat. However, it would easily be trailerable and there are hundreds of inland lakes around us, which is where I would like to use my boat. This would not be built for "the sea".

To be honest I'm posting it because who doesn't want a dragonfly trimaran type, it's just like a sort of clickbait boat and they are irresistible for any monohull or catamaran sailor that would like to swap it, and people would argue they want bigger inside area of monohulls, but the stability and performance of those boats are amazing and the looks too. They are the envy in mooring area and the reason there aren't these trimarans are the high pricetag even second hand and few builders.

You always can do coastal sailing, and for that the smaller the boat the better, any small daysailer will do just fine as coastal cruiser, so they can be climbed to sand without much effort . A sailboat that can be climbed anywhere will remove the worries associated with leaving it anchored or moored in a crowded marina

 

[CT249]

You can make a dragonfly 28 style trimaran but you should bring more people doing similar projects and use some land with tents

You just have to pick a free donated 70s-80s 28 sailboat and remove the keel and use plywood to change the shape of the hull, this has been published by eric spongberg in professional boat builder "case studies in redesign".





You just need to modify the hull and build the lower base of the trimaran main hull

and make the remaining 2 amas hulls with plywood as well (check comuzzi plywood catamaran)

For the beams you can use discarded lighting poles and bend them and use plywood to give the wing airfoil profile.

I'll publish a post about that redesign that can bring back to life plenty of abandoned sailboats

Trimarans are fast because they can use more sail area than rest of sailboats besides not bringing ballast and having thinner hulls

And besides of that they are stable and by lowering the rig height with a ketch or schooner rig the risk of wind pitchpolling or rolling capsize is avoided

Have you ever sailed a trimaran? Have you ever sailed a boat?

 

[Unique Trimaran]

I've never sailed a trimaran or a boat, but looks like it would be a cool experience. Like I've mentioned, we live on a small lake, which is where I would mainly use my boat, but plan on trailering it to other small lakes within Michigan, so having a sail on it would not be applicable. I would also need much more deck space, since I would like to put a good sized cabin on this boat. Basically a tritoon hull (I hate the look of round aluminum "toons"!) in a one piece fiberglass hull.

 

[Herreshock]

I've never sailed a trimaran or a boat, but looks like it would be a cool experience. Like I've mentioned, we live on a small lake, which is where I would mainly use my boat, but plan on trailering it to other small lakes within Michigan, so having a sail on it would not be applicable. I would also need much more deck space, since I would like to put a good sized cabin on this boat. Basically a tritoon hull (I hate the look of round aluminum "toons"!) in a one piece fiberglass hull.

Don't forget that there are free-standing masts that you can place in the bow like cat ketches do or just a mast there and a gib sail and you will have all space available.

You can sail it down Mississippi river to Louisiana and sail around mexico doing coastal cruising till brazil, that's a fine coastal cruising boat route.

The boat i would recommend for the lake would be a rog 15, it can be built and rowed easily and lifted to shore

 

[willy13]

https://www.powerandmotoryacht.com/uncategorized/sea-blade-35

 

[CT249]

I've never sailed a trimaran or a boat, but looks like it would be a cool experience. Like I've mentioned, we live on a small lake, which is where I would mainly use my boat, but plan on trailering it to other small lakes within Michigan, so having a sail on it would not be applicable. I would also need much more deck space, since I would like to put a good sized cabin on this boat. Basically a tritoon hull (I hate the look of round aluminum "toons"!) in a one piece fiberglass hull.

Sorry, Unique, that remark was aimed at Herreshock, who is making some far-fetched claims such as implying that using a monohull as the main hull of a trimaran is a good idea. I understand that you want something different.

I'm not a powerboat guy but one thing I'd be concerned about with an inverted Vee bottom is the noise and wave slap both underway and at anchor, even on lake waves. My little RIB has a section where the tube-to-bottom join makes a gentle inverted Vee and it's noticeably noisier at rest because of that. On the plus side, a gentle concave/inverted U can improve lift without making too much noise. Obviously the cost/benefits would change according to personal taste.

 

[gonzo]

Inverted vees where designed for wider hulls than a trimaran would have. Why a trimaran? It cost at least 60% more building a third hull and all the connections compared to a catamaran. I think that if you build a multihull as heavy as 7,000 lbs, 250HP may not get to 30 mph. I have an old 24 ft slickcraft and it weighs about 4,000 lbs with a heavy V8 and sterndrive. It has a forward cabin and hardtop. With 235HP I can cruise at maybe 26 mph loaded. By the way, the engine is like new and the propeller is well matched.

 

[Unique Trimaran]

Thank you Willy13, CT249 and Gonzo - much appreciated! A buddy of mine has some boat design software that we thought would be great in helping to design my hull, but the truth is, neither of us have ever designed a hull, so we're not exactly sure what we need to see in my hull design. The current hull design actually looks good in my opinion, but again, I don't know the advantages or drawbacks of what shape X, or what shape Y would result in.

Gonzo, the reasons I thought the third hull would be helpful:
1) I planned to locate the fuel tank (towards the stern) in the center float, along with other miscellaneous storage
2) Provide the additional buoyancy to properly keep 7,000 afloat, perform well and adequate freeboard.
3) Easier to build and support the transom if attached to the center float
4) This is not a reason for the third float, but more about going with a flatter bottom: I'll probably run this boat up on the the area sandbars occasionally and that a relatively flat hull design would be better since it wouldn't tend to teeter-totter, like it would tend to do with a V hull
If I have flatter bottoms on each of the (3) floats, would this plane quicker and go faster than if I had each float having a V shape on the bottom of each float? I'm hoping a 250 hp engine will get me there but it doesn't sound like it will.
Thanks again for your input!