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 Boat Design Forums force from a sail

#1
02-19-2004, 03:58 PM
 man308 Junior Member Join Date: Feb 2004 Rep: 10 Posts: 2 Location: USA
Force from a sail

I am working with a group of college students designing a sail car. We were wondering how we could calculate the total force from a sail and how it affects the mast. I figure it forms some kind of parabola. Any suggestions of books, websites, or if anyone has an idea of the formulas to use would be a great help.

P.s. I am a beginner with sailing slang

Thanks
Ash
#2
02-19-2004, 09:43 PM
 waterman Boat Geek Join Date: Feb 2004 Rep: 10 Posts: 24 Location: Nofolk, VA
Look in some "good" texts on yacht design

The key here is "good". what you need is a great library or bookstore that has lots of marine titles. A good book on yacht design is "principles of yacht design" by lars larsson and rolf eliasson,ISBN # 0-07-135393-3. your guess is right, an unstayed mast will look approximately like a parabola, however once you start to stay it, its shape will change considerably. My guess would be to model it as an cantilevered beam and go from there. Another option would be to build a unstayed mast or unstayed wing mast. Being interested in rigging, I have yet to find a good book specifically on the design/engineering of unstayed masts. A possible book to look at on this subject would be blondie haslar's book "junk rig". I have read parts of it and it has pretty good vector diagrams of the forces involved.
#3
02-20-2004, 02:19 AM
 grob Senior Member Join Date: Oct 2002 Rep: 53 Posts: 211 Location: Hove, Sussex, UK
The simplest method is to work out the maximum righting moment of your vehicle, which in your case will be the combined weight of vehicle and passenger multiplied by half the distance betweeen the width at the wheels.

Since the righting moment must equal the heeling moment you can then work out the height from the ground to the center of area of the sail, and divide RM by height to give you the force on the sail.

RM = width x weight
HM = height x Force from sail

There are a whole bunch of assumptions in this, the principle one being that you have a vehicle with the passenger strapped into the vehicle on the centrline.

How this maximum load affects the mast depends upon how you plan to support the mast. Buckling formulaes for a stayed mast, bending forumlaes for an unstayed mast.

All the best

Gareth
www.fourhulls.com
#4
02-20-2004, 06:17 AM
 Raggi_Thor Nav.arch/Designer/Builder Join Date: Jan 2004 Rep: 711 Posts: 2,457 Location: Trondheim, NORWAY
0.5 x U ^ 2

Wind preassure increase with the speed (U) squared.
If you use the metric system it's very easy.

0.5 x Density of air x U squared
where Density is kg/m3 and speed is m/s.

Drag may be Area x Preassure x 0.3 for a car or x 0.4 for a Volvo.
Lift may be approx Sail Area x Preassure.
#5
02-20-2004, 02:32 PM
 gonzo Senior Member Join Date: Aug 2002 Rep: 1732 Posts: 8,377 Location: Milwaukee, WI
Skene's Elements of Boat Design has very easy formulae to calculate all that. It will take you about ten minutes to figure it out.
__________________
Gonzo
#6
02-20-2004, 11:44 PM
 Stephen Ditmore Senior Member Join Date: Jun 2001 Rep: 679 Posts: 1,304 Location: New York
Two things to note concerning upper limit on sail force:

1. Total sail force will be limited by the boat's maximum righting moment / the heeling arm (except downwind in a severe blow).

2. For purpose of first approximation most boats are designed on the idea that sailors will reef down once sail force gets much over one pound per square foot of sail.