# Leeward Ama Generates Leehelm ?

Discussion in 'Multihulls' started by mcm, Jul 26, 2011.

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### mcmSenior Member

I am unfamiliar with Trimarans, yet i'm curious if the submerged leeward ama
acts like a resistance brake-point around which the boat will automatically pivot leeward?

If the above is true, how does one calculate the leeward force imposed by the submerged ama, and what does a designer do to successfully counter that leeward tendency?

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### TeddyDiverGollywobbler

yes it does..
by resistance calcs and geometry..
adjust lead to get well balanced design..
BR Teddy

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### mcmSenior Member

Thanks Teddy,

I don't find many references to resistance calculations in monohull design books:
Just center of lateral resistance and center of effort relationship, and displacement/sail area ratios.

Can you give me an idea of the factors used in resistance calculations, such as velocity.

And by geometry do you mean center line to center line lever arm between ama and main hull?

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### SamnzSenior Member

dont forget by the time the leward ama is so low the whole boat is heeled. the lee helm generated is partly counted by the rig adding weather helm so the effect is minimal

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### mcmSenior Member

TeddyDiver,

Frictional and wave-making resistance based on length/speed ratio and expressed in tons of resistance per ton of displacement are the only calculations i know of.

Then i assume that calculated resistance has to be multiplied by the centerline to centerline lever arm distance.

If i have that right so far, then what i need to know is how to use that information to determine the best center of effort to center of lateral resistance lead to correctly compensate for the resistance.

Samnz,

Ten degree heel has the ama 75% emersed, and that adds up to a lot of resistance.

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### cavalier mk2Senior Member

Tri's often have the static CE and the CLR in line with each other with little to no lead. To counter the Lee helm the ama CLR moves forward as it is immersed negating the lee helm.

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### mcmSenior Member

Thanks cavalier mk2,

It's a complicated subject for a non-professional to grasp.
Any help is much appreciated.

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### oldsailor7Senior Member

I am not qualified enough to venture a technical opinion.
I can only say----from sailing experience---that when you bury the lee float the Tri tends to round up.
I have not experienced that in a Cat. :?:

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### TeddyDiverGollywobbler

Let's picture a tri that heels 5deg. According that heel sails generate some weather helm. How much is a question of height of rig and accordingly CE.. the leeside ama produces some more resistance, other one maybe flying and the mainhull the rest. So the center of transverse resistance (CTR5 here) lies somewhere btw the mainhull and leeward ama. Depending of how this CTR5 and CE are lined boat might develope some turning moment to lee or weather. This can be dealt with lead, just taking into account that with different heel and speed the outcome is something different.. So you take all these variables and try to find some reasonable compromise. If that doesn't work you change the hull geometry to make it better..
Does that make any sense?

BR Teddy

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### oldsailor7Senior Member

If you watch this vid of Sodebo digging it's Stbd bow, you can see how, when the float bow is deeply immersed it cranks the whole boat sharply off to windward.
Note that all the foils, except the rudder on the Port float, are in the air and Coville is cranking hard on Stbd helm. As the boat is heeling at about 40deg the horizontal component of the sail force is also helping to torque the boat up to windward.

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### DennisRBSenior Member

Looks to me as if most of the round up occurred once the boat had dug in and fell back toward windward as it recovered from the half pitch pole. That's a wild ride for sure. Looks like the helmsman didn't even bother to adjust the sheets. Just tried to keep it pointing downwind to avoid an increase in apparent wind. I wonder if the same thing would have happened if it we sailing closer to the wind? I would love to go for a ride on that thing.

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### oldsailor7Senior Member

"Looks to me as if most of the round up occurred once the boat had dug in and fell back toward windward as it recovered from the half pitch pole".

Thats exactly right.

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### ImaginaryNumberImaginary Member

Cavalier,
How are the amas shaped, which results in the CLR moving forward when they are immersed?

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### Doug LordFlight Ready

================
Finding the proper board position on a trimaran is somewhat as mysterious as doing the same thing on a mono-it is where a designers experience counts a whole lot. One of the best things you could do is look at a bunch of tri's and note their daggerboard positions( and sail as many as possible). Keeping the board more or less in line with the sail CE (Cavalier, post #6 )has worked well for me-I've actually used the 1/4 chord point of the main as a reference rather than the geometric CE of the whole rig. This is an area where the need to see what others have done coupled with your own experience is the most beneficial way to approach it.
Trying to calculate the resistance of the ama and using that to judge board position relative to the CE isn't such a good idea. Tri's are fast and the ama does tend to resist leeway to some extent and because lift varies as the square of speed the lift* generated by the ama can be much greater than its off center resistance thereby having a much greater effect on helm balance.

*"Lift" here refers to lift to windward
=====

For reference: important work was done in catamaran design by Bill Roberts who designed the Arc 21 utilizing the concept of "shared lift". In essence , he put the daggerboard forward of the forward cross beam, made it a bit smaller than "normal" and increased the size of the rudder. This can be used on trimarans and monohulls as well especially when there is some benefit in having a board more forward than usual(say, on trimarans with curved lifting foils that don't use a daggerboard in the main hull). http://www.aquarius-sail.com/catamarans/arc21/index.htm

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### cavalier mk2Senior Member

Hi Imaginary Number, This is a subject that still has art as well as science. The area forward can be adjusted by everything from ama keel profile, ama position bow shape and foils . At rest not much in the water, hard pressed more area forward. Because of the need for more buoyancy forward in a hard pressed tri you can solve two problems at once. The current wave piercing bows are also doing this to a greater extent, it would be interesting to check if this results in the weather helm -too much area forward- observed by old sailer. Getting smooth transitions of balance in a boats power curve can be a challenge if left to accident rather than design.

Last edited: Jul 29, 2011
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