# sail thrust formula

Discussion in 'Sailboats' started by metin_mehel, Feb 22, 2011.

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### metin_mehelmech.eng.

Hello,
Does anyone know how to calculate the sail thrust force?

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### Perm StressSenior Member

For what?
Sail thrust is depend on:
*Plan form
*gap between foot and hull
*profile
*true wind speed
*angle of attack
*boat speed
*heel angle
*rig type /interaction between sails or other boats sailing nearby
*wind gustyness
*...

Basic formula :

F=C*A*(ro/2)(v^2)

where :
C -force coefficient
A -sail area
ro -air density
v -air speed across the sail

except for sail area, all the other variables are function of above-mentioned (and mentioned) factors.

3. Joined: Jul 2009
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### metin_mehelmech.eng.

...

I am trying to design a 7 meters sailing yatch. My referance book is Principle of yatch design.
I have some question marks:
1- What is the max wind speed to calculete heeling moment?
2-What are the formulas and constants?
3-What are the formula of sailing thrust force and constants?
4-What are the sail dimensions formula?
5-What is the balance equation between sail and keel

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### Perm StressSenior Member

Read "Principles ..." carefully. It is all explained there.
It is too early to ask specific questions.

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### metin_mehelmech.eng.

what is the difference between driving force coefficient and Cl
and also side force coefficient and Cd
Thanks

6. Joined: Sep 2009
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### Perm StressSenior Member

They refer to different total sail force splitting methods:

Cl and Cd describe total sail force split relative to apparent wind direction: Cl for lift, or cross-wind force, Cd, for drag, or downwind force.

driving force coefficient and side force coefficient describe total sail force splitting relative to direction of boat's motion -along the direction of motion -drive force, across the direction of motion -side force.

The same money distributed to two pockets of your pants or to two pockets of your jacket...

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### RepelsteeltjeJunior Member

I do not know a quick answer to your question but if you are willing to do some calculations your answer could be in the ORC VVP documentation.

Go to www.orc.org and download the latest VPP documentation. This contains a calculation model for the sail forces.

8. Joined: May 2009
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wind pressure rule of thumb

A simple method I have used in the past is to design a monohull keel boat for 1lb per sq.ft wind pressure at the CE of the rig. You can approach it from the standpoint of your maximum RM at, say, 20 degrees angle of heel. Then with your biggest upwind sails design the rig as above.
For small fast multihulls it is interesting to note that they are designed for approx. 1.8lb. per sq.ft. wind pressure unreefed and with the rig fully powered up( and two guys on a trapeze). Larger multis and mono's can be designed for higher wind pressures(Orma 60 tri designed to fly the main hull @ 2lb. per sq.ft. wind pressure) but the "rule of thumb" was always 1lb. per sq.ft. at the CE.
This is an area where your own experience sailing can allow you leeway with the "rule of thumb" depending on what you're trying to achieve.

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### Mikko BrummerSenior Member

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### metin_mehelmech.eng.

Orc Is Not Free

11. Joined: Jul 2009
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### metin_mehelmech.eng.

SECOND QUESTION:
I think there should be some I,E,J,P combinations for the best efficient values of high lift and low drag. Is it right?

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### Perm StressSenior Member

VPP documentation is free.
Designers VPP software is 500EUR per year.

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### Tim BSenior Member

I'm going to start simplifying my answers if people don't provide at least vague contextual information. Hence:

Yes, I do. Thanks for asking.

Tim B.

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### Perm StressSenior Member

:!:

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