catamaran and trimaran hull form

[samh]

Can someone provide me with a starting place for designing catamaran and trimaran hulls as far as Cp and LCB? Also the displacement distribution in a trimaran between the amas and vaka (? I think I have the terminology right for the main hull and the floats.)

[Skippy]

For the tri, it looks like ama size is usually 100-200% of total boat displacement. The main hull of course would be bigger and designed for low wetted surface, planing, cabin space, and other considerations. If you're also asking about actual displacement when in the water, the bottoms of the amas usually sit right at the surface when the boat is dead level, taking zero displacement.

There can be important considerations based on the size of your boat and the intended use; can you fill in a little more info?

[samh]

I think Larson and Elliason give .54 as a reasonable, general starting Cp for a monohull. I was wondering if there is a same sort of target for catamarans and trimarans? Same with the multihull equivalent of a monohull's lcb of .535.

S

[Skippy]

I think Cp is usually higher for a multi than a mono, for both better pitch stability and higher speeds. John Teale has for a mono,
V/L ........ Cp
1.15 ...... .55
1.25 ...... .6
1.5 ........ .65
A cat especially should be higher Cp for the same V/L.

[samh]

Thanks. Also found http://www.tedbrewer.com/yachtdesign.html which illuminated some concepts that I didn't really understand.

S

[Skippy]

Thanks for that link, it's a good summary. Brewer's book is nice. It really gives you a feel for hull shape, and you don't have to be an expert to read it.

[dougfrolich]

Longitudonal stability is an important concern for multihulls, consider as the apparent wind angle increases, the trimming moment produced from the rig will start to approach the heeling moment in magnitude, if the trimming moment exceeds the heeling moment to early the boat will be suseptible of pitchpole. For trimarans, by placing the LCB of the ama forward of the LCG, trimming moment can be balanced. To figure how far forward; choose the crit AWA and AWS, find the trimming moment divide that by the sailing displacement, the result is the distance forward the LCB should be from the LCG ( at the chosen AWA,AWS ). Any hydrodynamic lift generated fwd of the LCG, from canted foils or bottom shape for example would reduce the distance. So basically it is a ballancing act. For your hull shape just make them as long and narrow you can for your displacement, choose a sectional shape with minimum wetted surface, and include plenty of reserve bouyancy so you dont bury the leeward bow. For more info check out Design of a 40 foot Multihull for Offshore Racing by James Antrim availible thru SNAME.

[Ron Cook]

I have alot of experience with both having over 50,000 ocean mile on them.

If a beginer designer makes mistake with design of a monohull it may not be a good boat but if you make mistakes with mulltihulls they will bite you bad.

I don't want to put you off your project. I have participated in the design of a number of mulltihulls and I have built some. it was great rewarding fun.

Be warned that monohulls have nothing in coimmon with mullties. So done use the ratios from one to help design the other it is apples and oranges.

The cp for the average mullti is .60 to .65
mullti's don't suffer in light wind the way monos do with high cp.

On a tri the float should be at 150% or higher with its center of boyancy around 12 1/2% forward of the center of B for for the main hull which is commonly around 56% aft.

Good luck with your project, I love mulltihulls But be warned that if you screw them up they will bite you.

Ron
P.S. Is there aspell checker in this posting system?

[DaveB]

Hi,

Interesting to hear the numbers... wasn't sure how they'd compare... I'm workin' on a powercat right now with a Cp of 0.63... Lucky I guess...

What do you mean when you say:

Originally Posted by Ron Cook

On a tri the float should be at 150% or higher....

?

Cheers,

Dave

[Ron Cook]

The float on a tri should be designed to carry 150 % or higher of the total crafts displacement without putting the deck under. The shape of the float on a cruising tri shuold be more v;d then on a racer. The reason is for motion comfort and less pounding which can rattle the the boat inclulding your teeth.

Ron