Tri performace

I am new to the world of multhulls. I understand the idea of displacement/planing hull types in mono hulled craft, but how does a trimaran get the speeds that it does? Does the hull plane? What kind of factors/equations are used to predict the speed potential of a given trimaran.

It appears that more of the disigns are being build with amas that are flush with the bow of the main hull, and they are full length. Does the length of the amas affect the speed of the boat? (as in, longer waterline length gives you a higher hull speed?)

I'm reading the "Good reasons NOT to design" thread with interest. As one of those guys who can't help but designing my own boat, I'll admint I'm starting my research on a 18'-20' minimalist weekending/cruising tri. Still alive after one kayak and one 13.5' day sailer. <grin>

Marvin

 

Welcome aboard, Marvin.
The simple answer is that a multi's hulls can be thin enough (length/beam in the 10:1 to 15:1 range is common) that wave resistance is not nearly as big an issue as it is with monohulls, which must be fatter in order to stay upright. The full answer would take several hundred pages, but the general idea is that multis are thin and so don't have the same 'hump speed' problem that wider hulls do.

 

Can anyone recomend readings on multihull design and sailing theory?

 

In order to understand multihull design, you have to understand general sailboat design, hydrodynamic and stability

I have found many books on sailboat design but almost nothing on multihull design.

An older book is Skeenes Element of yacht design but it is very old.

An other book is Principles of Yacht design by Larsson and Eliasson, more modern but doesn't talk about multihull design.

The best reference that I found so far is:

James K Antrim "Design of a 40ft Multihull Sailboat for Offshore Racing", Marine Technology, vol 27, no 5 Sept 1990 pp,285-299

It is a technical article and you will need a good understanding of naval architecture to read it.

It is available for purchase from www.sname.org eighter as an separate article or with the small craft paper compilation CD

You can go and review many multihull designers web site, they will have articles that will explain some design option.

Finally, one of the area with the most information is right here, at boatdesign.net.

 

Some trimaran main hulls plane - the Farrier designs are a good example. Many do not, and very few amas do not. A Farrier ama, having very "V" shaped sections does not plane.

In general, all sailing multihulls employ very slender hulls so that the wave drag is low. So while they also experience a drag rise near hull speed, it is not so much as to exceed the thrust available, and therefore does not limit the speed. That's why they are multihulls - a monohull that was just as slender would require an excessive amount of ballast to keep from falling over.

Of course. The longer the ama, the lower the wave drag at a given speed, and the smaller the maximum cross section is for the same displacement.

You might find Michlet useful for calculating the drag of a multihull. You still need to account for the drag of the foils. And, of course the aerodynamics of rig and topsides.

John Shuttleworth has a wealth of information on his site. I particularly recommend the Dogstar 50 article.

 

Hi Marvin, one of the questions you asked in your original posting was about the amas being full length and flush with bow of the main hull.
If you trace thru the the last 35 years or more of trimaran design from the prominent designers of their time, eg. Piver, one of the pioneers to Farrier, for instance, (but it doesnt necessarily have to be these two names), you will notice a clear trend.
Hulls and amas became relatively narrower in relation to length, and amas grew in length, relative to the length of the main hull.
The effect of the first change was to increase performance (speed) throughout the wind range, and the effect of the second trend was for the ama to increase in buoyancy so that it could readily support the entire displacement of the boat when forced to do so by wind pressure.
There are a number of reasons why this happened and I will not attempt a full explanation here, except to show a few reasons in point form.
(a) In the 60's tris were not generally accepted as seaworthy offshore, and the leeward ama gradually being submerged was a signal for the skipper to ease off and reduce sail.
(b) performance in the early days was much less a priority than safety.
(c) By increasing ama buoyancy to 150% of total displacement and beyond to perhaps 200%, righting moment was increased enormously enabling taller, more powerful rigs.
(c) To accomodate around 200% buoyancy in a low drag ama, required high length/beam ratio, so lengthening the ama as much as possible was seen as desirable.
More reading, even on the internet will elaborate a lot more, and give further reasons why.

 

Not this again

Hi Tom,

You wouldn't happen to have any photos (actually a long video clip would be better for the purposes of this discussion) showing a trimaran actually planing on its main hull without the amas touching the surface, would you?

I'm sure your realize that with a photo being but a fraction of a second, it wouldn't really provide much in the way of empirical support for the argument that a tri will plane without the outside assistance of its amas.

I'm of the opinion, that if a boat needs to be "helped" with one wheel dragging, so to speak, it isn't really planing at all. It would seem to me that a planing vessel is completely supported on the surface by its principal hull due to having climbed its bow wave through power applied. No ama contact whatsoever during the period of the plane or the period of the actual getting up on plane. I don't see the same scenario if most of the boat's weight is actually being supported by one of the amas.

Please clarify the issue for me.

Having a lengthy clip of this claim would go miles towards supporting the statement.

 

An introduction to multihull design issues and more is "Multihulls for cruising and racing" by Derek Harvey. A easy to read book with a lot of nuggets for the buyer/designer/sailer of multies. Too limited for the designer but a good startingpoint I would think.

Anders M

 

Not to get off topic, but has anyone here thought about writing a book on the subject? Could be some good money in it...

 

good argument but i think of tunnelhulls, ASD sytem, trimtabs, even wonder if you can say a stepped hull having liniar ama's

 

I've been curous why I've never seen any stepped hulls on multihull boats. Now I see they are too narrow to benefit.

In looking around at different boats. Some seem to have symetrical amas and some have asemetrical amas. How much benfit is there in a asymetrical ama? I read a sailors review of a Tremilino (In Dave Kerr's Backyard cruising designs) that were fitted with the half-moon amas and sounded like they were a significant improvment. With the number of modern boats that use symetrical amas, has the benefit of asymetrical been replaced by length or something else?

 

http://www.powermultihulls.com/magazine/articles/cathulls/cat%20hulls.htm has some cat hulltypes but more is possible

 

The ASD hull (see ypster's pic) has nothing to do with multihull or tunnel boats.

It is a vee hull with 2 annexes in the after part in order to increase the lift at all speeds range. The resistance is a little higher if compared to traditional hulls but the propulsion device works in a parallel fluid and so doing the OPC (Overall Propulsive Coefficient) is higher to that of a hull running with less wetted surface.

Have a look on the lateral small waves system at 42 knots....

 

good picture and explanation and knew you were going to say this Otto, sorry
still other than strakes it are annexes and only point i was trying to make little boats use only the pricipal hull to plane

 

No problem yip, really no problem....