# Wild Thing Modeling A Class Catamaran

Discussion in 'Multihulls' started by GONCAT, Sep 16, 2008.

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### GONCATNew Member

Hi all,
i am looking for answers related the performance of the A cats downwind.
Generally over 10 knots you try to fly the windward hull and you steer in order to keep boatspeed and AWA as constant as you can. The story here is that there are slightly diferent ways of seting up the boat to achive the best VMG possible. My aim is to have a simple matematical model (in house vpp) of the A cat sailing the "wild thing" and answer things like: optimal sail shape distribution for diferent ranges of wind, optimum mast rake, optimum boom angle, optimum TWA, centetrboard position, crew position.... lets say, a vpp.
I know the best way to do it is just try, but a bit of understanding will help on my research.
Has anyone an idea of how to simulate this properly?
GONCAT

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### ErwanSenior Member

A-Cat VPP vs KFD (Kitchen Fluid Mechanics)

Hi GONCAT,

This question is familiar to me as it is a long time I scrutinize A-Cat performances after sailing 15 years, with one World attendance at L'Estartit in 96. I am like you, very interested in deep understanding of perforance sailing but far from Ph in fluid mechanics. I can manage only basic concepts I used to call my approach "Kitchen Fluid Mechanics" KFD.
I will try to present it in the following text.

Tho Golden rule is to start with what you can mesure easily: The righting moment.
You know the weight of the boat and your weight, you know where you sit downwind or how far your center of gravity is from the hull in the water when you are hooked on the trapeze windward.
Here are the first inputs.

Then you can have slight assumptions:
About sail twist: 17° average winward, seems to be a good trade-off.
About average sail's incidence / boat centerline: 22° windward seems to bee a good compromise.

Apparent's wind / sail angle is below 15° because it will stall beyond. Windward, with the righing moment and the distance between center of daggerboard and center of sail area you can compute the IMPLICIT lift coefficient of the rig for each wind speed you want, considering the boat @ equilibrium @ constant speed.
You will understand that at low wind speed you need a high Cl above 1 and with highter winds the lift coef decrease sharply because it is a square function of wind speed.
Also the Induced drag coef of your sail (not rig) is a square function of its lift coef: Cl
It just means that with low wind requiring Cl above 1, induced drag rises sharply, and with more wind and Cl below 1 induced drag falls sharply,
because your righting moment is constant.

Windward is easy because you achieve quickly the max righting moment, and then only sail power can be adjusted decreasing lift coef and/or lowering center of effort.

Sailing the wild thing is more challenging because you have to make assumption on the maximum "diagonal" righting moment, your windward hull/bow can stand before being upside-down.
Also induced drag is probably less important than windward with regards to sail's angle, but it could be significant @ the foot-sail level.

If you can move the rig aft and windward like a wind-surf, then you can "push the enveloppe".

You will be surprised to discover the relative magnitude of the induced drag windward with wind speed at 4.7 m/s and 4.5 m/s boat speed, around 675 Newtons lateral force? or global force? (I dont remenber exactly) for the sail full trapeze with a 75 kg crew, and so Induced lift coef is around 1.2 and so on..
I do not remenber exactly the figures it is a long time, hope it could help.

You will have to address indirectly sail's optimal shape distribution via the sail's center of effort position.

Basically your optimal shape distribution is likely to be similar to the windward one, minimizing induced drag, focusing at the foot sail level.

About simulating, let's me quote the Hobie Tiger designer (Mr Jacques Valer) who provides a simple and smart philosophical approach to VPP:

" When you consider 2 Tornado crews equal level, when they go sailing F18 on very different boats, even when sailing the same race: catching up with each other all along the race, they end 1 h race with 30 seconds gap:
below 0.5 %. It is interesting to compare this figure with the "confidence interval" of VPP programs "

In other words he questions the marginal efficiency of complexity. But he does not condam VPP.

Back to the A-Cat, one can say: more generally, light modern boats with high righting moment, achieve speed above windspeed quickly and Aero drag becomes more important.

Centerboard position will depend on the wind speed and therefore the boat's speed, and for the crew position try not to be too aft, making the rear croos-beam touching the waves.

My post it longer than my other post for a simple reason, I think it is the only topic I can bring a little contribution at this forum, so you are lucky!

Good luck in your research and keep us informed

Best regards

Erwan

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### GONCATNew Member

Hi Erwan,
- R.M.->how to calculate Cb of the boat and the exact Cg of the boat aswell.
- Sail Twist: do you mean relative to centerline, to boom, wich height?
regards
Gonzalo

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### ErwanSenior Member

If Cb is center of buoyancy, it is not necessary for windward, your assumption is that the boat is at equilibrium and constant speed.

Twist is between Boom and Square top of the sail

For Wild Thing, your Cb depend on your position on the tramp, and the forces on the rig.

Taking 45° average angle of the sail plan, you will got the lateral force according to the position of your weight on the boat and wind speed.

And you will get the longitunal forces as well, and the Cb position will balance all these moments according to your assumption on Cgravity.

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