Trailerable Power Trimaran Design

cheers groper, will certainly be trying to keep the l/b ratio as high as possible. unfortunately length is limited to not much over 10m to keep trailering legal, but if the weight is kept low I should be able to make the central hull very narrow.

I will be doing all construction myself, out of alloy. Hopefully design also but my knowledge in this field is currently minimal! will hopefully be able to improve this, thanks for the tips that's a great help!

 

hmmm, i havnt looked into aluminum thoroughly, but the lack of its use in performance sailing multis of around 10m length, suggests its not a feasable material to build a high efficiency displacement power multihull from... if the length was 50ft or more, id suggest that things would start to become more feasible.

In boat design, the structural engineering`s challenge is a stiffness problem, rarely a strength problem. The reason is, when a boat is stiff enough to control excessive flex so it doesnt fatigue and crack itself to peices in a few years time, its usually excessively strong enough - just the way it works when you look at the engineering.

So whilst aluminium plate is certainly strong enough, in plate form it doesnt have a good stiffness. So whats needed is lots of frames and stringers to solve the stiffness problem, which costs building time and money from the added complexity, and adds weight.

So whilst i havnt looked right into it, i would not be surprised if aluminium turned out to be a poor choice in an optimized design of this type your looking for. Sorry to say it, but keep this in mind and its worth exploring other material options...

 

Thanks Groper. I have looked into other materials too but for a fishing boat which might not always be launched in ideal conditions aluminium is just hard to beat. I understand in a multihull sail application weight is extremely important but for a powerboat might it be less so? Should a planing hull not be even more sensitive to weight yet most trailerboats are aluminium?

 

Well, if you think a typical plate alloy trailer boat with your stock standard planing hull is efficient then think again... they are atrociously inefficient.... if you wanna build the same, thats your perogative...

But just so you know, an efficient displacement powercat (or power tri) of +35ft length, can get similar fuel burn for a 20kt cruising speed as a 20ft plate allow trailerboat @ same speed, but could have 3 bedrooms, a large kitchen and saloon, huge back deck area, in general a MUCH bigger boat - for the same fuel burn and speed...

They have to built light, and they need long skinny "high speed displacement hulls"... not planing hulls... a planing hull would be a better choice if you wanted to cruise @ 30kts and your burning lots of fuel regardless... but like you realize, the sea conditions rarely permit such speeds in comfort so why design for it ?

An example of a game fishing type efficient power multi can be seen here;
http://www.supercat.co.za/supercat_38_sport_custom.htm

Id encourage you to watch the video and have a good look through some of the photos... pretty slick fishing rigs and very efficient for an almost 40ft er

At the end of the day, you need to decide what you want from this thing... do you want speed and economy, or speed at any cost, or good seakeeping and stability, or a wide beamy boat for more usable space, or.... many things to consider...

But dont assume you can just build a trimaran and it will be fast and efficient just because its a trimaran... you could end up with an expensive white elephant if you get it wrong...

 

Groper, you seem to know a bit about Tri's. I'm a newbie here, just signed up because i came across the thread looking up design info on powered tri's.
there isn't much info out there on the web about powered tri design that i've found. what can you suggest?

 

well where to start.....

You have planing hulls and displacement hulls... youll have to do some of your own research about these and what purpose your trying to meet.

When your designing a high speed displacement multihull, the hull resistance is proportional to the length to beam ratio and displacement to length ratio. So for higher speeds and higher efficiency, these ratios need to be pushed toward the more extreme end of the spectrum. The tradeoff is that you have less and less useable volume in the hulls as they become increasingly skinny and cramped inside. As a designer of a high speed displacement multihull, 1 important part of your job is to understand what the speed regime of the boat is to be and design these ratios accordingly, whilst still being able to fit the required accommodation spaces into / onto it.

Theres a whole lot more to boat design, and i couldnt possibly hope to cover it all in a web forum - this is why so many books have been written. Powered tris are however rather a rare breed and theres not much on the subject directly, however all the same principals apply to powered catamarans. The same applies to monohulls too, but the stability is a problem which is why you dont see extremely slender monohull designs...

 

Groper,

I undestand the differences between the two, planing VS displacement. what i would like to know or figure out, is how does one calculate the needed displacement of the amas? i assume the distance of center of bouyancy the ama to the main hull plays into account?

and as i have read on this forum, their placement, in regards to the lateral bouyancy of the main hull has an effect on their effeciency. from what i've observed just by looking at all the designs out there is that you would want the amas somewhere slightly forward of the main hulls CLB?

and one last question for you, given a Tri or a monohull at "high displacement speeds" which really is more effecient? i've looked at the FPB designs of Dashell which have good speed and economy for powered monohull, but they are much bigger than what we are looking at and assume their lengths have a lot to do with their effeciency. maybe i just answered my own question on that one?

 

Nobody can answer that question without knowing alot more details about the rest of the design and what your trying to achieve...

There is the consideration of "wave interference drag", which is basically the wave train of main hull and amas interacting with one another. The sum of the waves can either increase the total wave making resistance, or the individual demihull waves can help cancel each other out, thus reducing wave resistance. The ideal spacing and longitudinal location depends mostly on what speed your trying to optimize for... then of course, you need to balance this consideration with your seakeeping performance, and structural weight considerations aswell.

Great freeware has been published on this forum by leo lazouskas, called "Michlet", it allows you to model the different arrangements, make resistance predictions, and the "Godzilla" part of the software can optimize placement of the hulls in a theoretical environment which has proven reasonably accurate... its not very user friendly tho, with a very basic GUI...

You cant really say one way or the other, the design of the entire vessel is more complicated than that and again, it really depends on what your trying to achieve... but you can see, via the same principals, that long and slender displacement hulls are more efficient than short fat ones, provided the displacement length ratio is also kept favourable ie, need to be kept lightweight... You cant expect them to carry alot of weight efficiently, because the increased wetted surface area (of a very deep thin hull) and thus increased relative viscous drag, wipes out potential savings your getting from the reduced wave making drag of the narrow waterline beam hull...