Overal beam of Tris vs Cats

[patrik111]

Hello All,
Apart from structural issues, why is the beam on catamarans generally about 1/2 LOA but on trimaran up to 1/1? What would be the difference of a tri sailing on one hull compared to a cat sailing on one hull?

Or is the difference obvious in other conditions? ( tacking, gybing,high seas, a s o )

Would a cat/tri be more prone to dive/get lee helm or something else?

What is the main reason for this difference?

( I'm thinking of making a Cat/Tri with a very modest center hull ( really a central beam for mast support.) or a down-scaled ORMA 60 at about 6 m LOA)

Please tell me what the catch is in this, because I suppose there might very well be a reason why this sort of design is not around.

Would be most grateful for input in this matter.

BR

[Stephen Ditmore]

I think the wake interferance of two light and narrow hulls half their length apart is insignifigant. It becomes a factor, however, if they are less that half their length apart. Where there is interferance the speed/drag curve will be wavy, meaning at some speeds there's a benefit, at others there's a drag increase. Resistance humps, however, are exactly what displacement multihulls are designed to avoid. If your boat's speed / drag curve has humps you're likely to find yourself trying to "climb up a hump" much of the time without making it to the next sweet spot, where you'd like to be.

There may be other factors as well, like how far the boat has to heel to get the windward hull, or hulls, clear of the water.

Patrik, in my opinion , I think the more beam the better. But the problem gets to be structure real quickly.
On a tri the length you mention it would be hard to get the beam and still have the boat light enough to compete with narrower lighter boats: hard but not impossible.
If you can engineer the structure light enough and strong enough you will be able to have a lot more power than a conventional 8 or 10 foot wide cat.
But then the problem of controlling that power begins to dominate.The wider the boat the more likely the boats primary means of ending a days sailing will be to pitchpole not capsize. There are things that can be done to help minimize the pitchpole risk like the use of a T-foil on the main rudder and with a trimarn you can design the center of buoyancy of the outside hull(ama) to be slightly FORWARD of the CG of the boat so that as the tri lifts onto the outside hull the weight of the boat acts to prevent pitchpole: carefull though -too much separation can cause a dramatic pitch up as you tack!
Some of the open 60 trimarans use curved daggerboards(curved so that they are retractable in light air and so the bottom approaches horizontal when deployed). These foils were designed to be an asset to the ama in supporting the weight of the boat as the mainhull began to fly; they are in essence, hydrofoils. But they are foils without an altitude control system so the faster the boat goes the higher it flies.This has proved to be dangerously unstable in some cases particularly where the boat did not have a rudder t-foil.This is a primary reason why Nigel Irens new 75 footer for Ellen Mac Arthur does not use this type of foil. But fully submerged altitude controlled foils may be a consideration on small and large tris-not necessarilly for flying like a true hydrofoil but for use to help manage the power such a boat can develop-and to do so more controllaby than the curved foils- especially in relatively smooth water. Foils can allow an outside hull to be designed with lower total buoyancy thus saving weight but they also require a lot of attention to structural design in making the whole boat torsionally very stiff.
So , on your boat, depending on how high performance you want it to be I'd say go for the maximum beam you can structurally attain within a light weight budget.
On my radio contolled models I've found that beam has zero effect on tacking: I have a 71" wide 56" long hydrofoil model that will tack on a dime when on foils(or off with no foils).The same thing is borne out in the 16' Rave where the lateral resistance for the boat(vertical fins) are separated by about 14 feet and yet she tacks well. In my opinion, beam will not cost you any tacking performance at the size you mentioned but can help to power the boat up if you want it to.

[tspeer]

Originally posted by Lorsail
... Foils can allow an outside hull to be designed with lower total buoyancy thus saving weight ...

I'm not convinced that foils allow one to reduce the size of the amas very much. Maybe for a model or a 6m day-sailer, but not if any kind of a seaway is expected. In extreme conditions, even a fully-flying hydrofoil will probably be operated hull-borne. And one still needs the stability to get up to take-off speed.

Dave Keiper built Williwaw with small amas - using the "don't need 'em 'cause I got foils" philosophy - and he capsized the boat three times; each time while sailing slowly and hit by a gust. In his autobiographical book, Hydrofoil Voyager,, each succeeding picture of the boat shows the amas getting longer, and longer...

But foils do allow the amas to be optimized for rougher conditions, with more buoyancy forward and less concern about performance under moderate conditions.

[ron17571]

Try looking at the trikini web site,they have a tri youd like to see.

[Skippy]

Originally Posted by Patrik111

I'm thinking of making a Cat/Tri with a very modest center hull ( really a central beam for mast support.)

Of course, there's a difference. The main hull in a tri is slightly below the amas, whereas the central beam in a cat is above the hulls. I think that's called an Auckland cat?

Long time and sudden bursts of interest...

I meant when flying all but one hull, 1 of two or 1 of three in the water, what would be the difference?

Tris are best in light wind (fly the amas, low wetted surface)
Heavy breeze (fly 2 of 3 hulls, more righting moment than a "normal" cat.

how come catamarans are not mor square than they are (apart from the 18m2) expect for the obvious reason of trailering??

BR

[Skippy]

Patrik, I think this is a reason:
http://boatdesign.net/forums/showthr...6393#post36393