# How can we calculate the driving and drag force?

Discussion in 'Hydrodynamics and Aerodynamics' started by metin_mehel, Aug 27, 2011.

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### metin_mehelmech.eng.

Hello Guys.
Unfortunately I still cant calculate the driving and drag force.
I know there is a known formula F=1/2*Cl.... but,
What will be the Cl (lift coef.) and Cd (Drag Coef.)?
What will we consider the velocity in the formula? Is it true wind speed or apparent wind speed?
Thanks

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### metin_mehelmech.eng.

sorry driving force and heeling force...

3. Joined: Oct 2005
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### ErwanSenior Member

You can calculate Implicit Driving force

Hi Metin,

You should start with what you know and what you can mesure (with more or less precision).

The righting moment
The sail area
The true wind speed
The apparent wind speed.
The angle of Incidence.
The sail aspect ratio

From these data you should be able to have an idea of the "Implicit lift coefficient" which balances the known righting moment.

Then the Induced drag, and so on.

Of course it is much easier for a C-Cat than for an East-Coast Schooner or even for an 18 feet skiff.

Don't be shy, put the figures on a paper and take a beer, you willl make it

Regards

Erwan

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### metin_mehelmech.eng.

I am designing a new catamaran.

The righting moment: I think we will determine it after calculating sail force
The sail area:7.39m2Mainsail area , 9.24m2Fore triangle area
The true wind speed= It should be maximum according to standards
The apparent wind speed: it will be calculated
The angle of Incidence.: It will be determined to have maximum load at mast.
The sail aspect ratio:6.05 Mainsail aspect ratio, 3.10 Fore triangle aspect ratio

I have to unknown.
1- which speed I will use in formula.
2- what are the coefficients

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### tspeerSenior Member

Since the heeling moment of the rig is limited by the righting moment of the hull (when on the verge of capsize), it's common to estimate the aerodynamic loads by working backwards from the allowable heeling moment rather than by trying to work the problem from first principles. Unfortunately, while this will give you some maximums for the global loads, it does not provide details as to how those loads are distributed. You need to make other assumptions in that regard.

Take a look at Shuittleworth's stability indices to help match the size of the rig to the dimensions of the hull.

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### metin_mehelmech.eng.

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### daiquiriEngineering and Design

Very nice and informative article.

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### Mikko BrummerSenior Member

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### metin_mehelmech.eng.

Thanks but I want to make my own excel spreadsheet.

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### Mikko BrummerSenior Member

You can look at the source code and copy it to excel...

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### metin_mehelmech.eng.

Is it possible? How?

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### daiquiriEngineering and Design

That's very generous of you, Mikko.

By viewing the HTML source of the page with your browser.

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### metin_mehelmech.eng.

Great

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