Different hull shapes for Trimaran

[waynemarlow]

Its becoming very apparent to me that some mono's hull shapes are particularly " slippery in lighter airs in even probably up to 10 knots. I base that on the fact that in mixed handicap racing that I participate in, the 49er will leave our 16ft cats behind relatively easily. I would guess that the two hulls in the water are more drag than the 1.

Now if we could mimic that hull shape ( 49er ) for the centre hull of a Tri then we get huge gains in the accomadation department for no real loss in speed until things get real busy with stronger winds, but then that hull will almost certainly be at point where its almost flying so will have minimum hull in the water.

Any thoughts on the concept. Anybody been there and done it before ?

[Guest625101138]

For a given displacement an optimum monohull will have about 40% less drag than an optimum catamaran. So if the trimaran is balanced on one hull in light air it will travel faster than a catamaran, operating on two equally loaded hulls, for the same sail plan.

Rick W

[CTMD]

What sort of 16' cats are you comparing? I'd be very surprised if a 49er could beat an F16 in any conditions except maybe extreme light (Drifter).

[waynemarlow]

No in real life practice the 49er seems to get away until about 10 knots of wind, largely due to its much larger sail area ( 21 to 15sqm but the 49er is much heavier overall ) but I do suspect a lot to the drag on the hulls. In under 5-6 knots it is faster. I also suspect there is some sailor skill involved but even so the hull water line shape is very very slippery.

My query however is that the hull shape lends itself to a small cabin ie its much wider foward than a conventional tri hull and yet narrows back towards the rear, sort of getting all the right bits in the right places if it was scaled abit to say 23ft. Have any designers tried these sort of hull shapes and was it a success or disaster.

[CTMD]

Interesting,

We used to race F18 vs 18' skiff quite often in NZ and the 18's couldn't come close, however, they were more competitive in the light as they could switch to bigger rigs.

The 49er is far from an ideal light weather shape. I don't believe its hull shape that is making the difference, more likely sail area and skill.

[rapscallion]

tri hulls are narrow for a couple of reasons: so the main hull wake doesn't interfere with ama wakes, and max displacement with min wetted surface.... Rocker and prismatic coefficient also come into play....

there is a great discussion about this very topic in "multihulls for cruising and racing."

[Capn Mud]

Wayne,

Interesting point - one I've been considering myself. I was planning to test the idea myself by combining a Sportsboat style hull (smaller size - centreboard instead of keel) with tornado style amas with the design concept being that for good sailors that can keep things in balance it sails with amas out of the water removing the hull wake ama interaction mentioned by Rapscallion. For less accomplished sailors the amas act as "training wheels".

There are several reasons for trying this design that dont relate performance but rather comfort of crew etc but I was thinking like you in terms of performance.

Will post some design pics soon for comments.

However I havent seen the discussion mentioned by rapscallion. Thanks for that note rap man - I will look into it - perhaps I am on wrong track here.

Cheers,
Andrew