Scientific Definition Of Sailing Terms

May 17, 2004
2,099
Other Catalina 30 Tucson, AZ
Ship mates: I talk to a lot of sailors (phone, letters, Email) and in 99% of the cases I can provide a simple answer. I have no scientific background in anything -- I'm just a country boy with some basic common seaman sail trim skills, which I try to convey to sailors in a simple, easy to understand fashion. In fact, the forward of my book states the following "If you wanted a scientific presentation that includes intricate details, together with complicated formulae for hull speed, wave lenght and endless verbiage then this book is not for you" . I know there are sailors on this forum that could easily help me answer this letter. Any help you can give me will be appreciated.

Here's the letter:
Don,
I dropped a hint to my wife and she got me your book for Christmas. With the sailing season well under way here in Missouri, I have been going over the material in the book again. I have some questions and a comment.
Question: Can you define the terms “power” and “overpowered” in terms of forces and vectors? You use the terms without definition, (initially pg. 9, Overview). I have encountered these terms hundreds of times from expert sailors and others and have never been able to get a good definition of these terms. I have a strong background in math and engineering and understand forces and the basic physics of motion in terms of two dimensional and three dimensional vectors. Technically, power is the ability to do some amount of work in a unit of time. Work is defined as the product of a force applied over some distance. While I’m sure the use of the term “power” as applied to sailing does not follow this technical definition, it is confusing to read that a flat sail produces less power and more speed. In physics, f=ma and air resistance (drag) equals velocity x k; equilibrium is reached when the force is equal to the drag. Thus the confusion: more power means more force applied over the same distance, which should result in more speed. Apparently “power” means something different in sailing.

Comment: I have encountered the airplane wing (air foil) analogy of a sail, many, many times. I think this is a very badly flawed analogy. A sail in not an air foil, it does not have thickness, a sail is a membrane. Most importantly, a sail does not produce useful force in the same manner as an air foil. An air foil produces force, (lift), by increasing the velocity of the air over the top of the wing compared to the velocity under the wing. By Bernoulli’s law, increased velocity produces lower pressure. Lower pressure above the wing results in lift. Conversely, a sail produces useful force by deflecting the air, thus causing angular acceleration, and by f=ma a force is produced at a tangent to the acceleration curve. The ability of a sail to produce force is relative the mass of the deflected air and the angular acceleration. An airplane wing can also produce force due to deflection when the angle of attack is increased, as when the flaps are down, but this has little to do with the air foil shape of the wing. I do wish that instructors would not use the air plane wing – air foil analogy and focus on the fact that the force produced is relative to the mass of the deflected air and the angle by which it is deflected.


Question: Is there a three dimensional illustration, or preferably an animation, of the flow of air across a sail and the resulting forces? I would like to see this particularly for sail settings that produce different amounts of “twist”, and the resulting relationship to forward force compared to heeling force. I think this has a lot to do with the question above on power.


Your book has been helpful in understanding the mechanics of sailing. Thank you for your insights.
 
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Likes: jon hansen
Feb 20, 2011
7,990
Island Packet 35 Tucson, AZ/San Carlos, MX
Jeebus. Get a life... :rolleyes:

But seriously, if the airplane wing analogy won't work for this guy (which should, I believe), I'd hope that draft position and depth effects would suffice.

Outside of your book, let him do the self-perceived necessary research.

Does that sound too harsh? o_O
 
Jul 7, 2004
8,402
Hunter 30T Cheney, KS
I think his issues with "lift" are misguided. A sail does produce lift if apparent wind is on the entry point of the sail. His "deflected air" explanation is more appropriate for running downwind. That's how I understand it. YMMV

For apparent wind angles aligned with the entry point of the sail, the sail acts as an airfoil and lift is the predominant component of propulsion. For apparent wind angles behind the sail, lift diminishes and drag increases as the predominant component of propulsion. For a given true wind velocity over the surface, a sail can propel a craft to a higher speed, on points of sail when the entry point of the sail is aligned with the apparent wind, than it can with the entry point not aligned, because of a combination of the diminished force from airflow around the sail and the diminished apparent wind from the velocity of the craft. Because of limitations on speed through the water, displacement sailboats generally derive power from sails generating lift on points of sail that include close-hauled through broad reach (approximately 40° to 135° off the wind). Because of low friction over the surface and high speeds over the ice that create high apparent wind speeds for most points of sail, iceboats can derive power from lift further off the wind than displacement boats.

Various mathematical models address lift and drag by taking into account the density of air, coefficients of lift and drag that result from the shape and area of the sail, and the speed and direction of the apparent wind, among other factors. This knowledge is applied to the design of sails in such a manner that sailors can adjust sails to the strength and direction of the apparent wind in order to provide motive power to sailing craft.
 
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Likes: Johann
Jan 30, 2012
1,123
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
Don

He is going to love this one:

The Physics of Sailing Ryan M. Wilson JILA and Department of Physics, University of Colorado, Boulder, Colorado 80309-0440, USA (Dated: February 7, 2010)

Lots of obscure math including Navier-Stokes equation analysis. Also reliance on Arvel Gentry's work.

Charles
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
yes , there is a simple, clear, easy to understand place to read all the answers to all his questions.
 
Jul 22, 2011
146
Mariner Yacht Co.(NH) Mariner 28 Atlantic Highlands, NJ
The Symmetry of Sailing by Ross Garrett is also good.
A sail is most definitely an airfoil. The Bernoulli pressure vs. Newton's opposite reaction debate has been going on since Wilbur and Orville disagreed. Sailing and flying are very closely related. I've heard sailors say pilots are always good sailors and pilots say sailors are always good pilots.
I suppose if you want to get technical The Aero-hydrodynamics of Sailing By Marchaj.
 
May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
why and when does the sailor want twist? thats easy! in lighter winds the apparent wind is at different angles the father off the the earths surface. as the wind increases the less twist you want. when ground effects washes out all together a sailor wants no twist at all. :)
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Sails are always in three modes: Unpowered, Fully powered up, Overpowered. In sailing the word 'power' is used a verb, and not as a noun, which would have a technical definition..

All boats and sail plans will be slightly different in terms of true and apparent wind speed ranges

Underpowered 1- (say) 7 knots TWS: The sail plan is underpowered and you are doing every you can to add power to the rig. Easing outhaul, softening the rig and halyards, cracking off a degree or two.

Full Powered up: Joy, 8-14 knots. Sail shaping set for max lift vs drag, Crew high, optimal heel angle.

Overpowered: 14+ knots. Trying to deal with excess pressure, flattening sails and hardening rig and halyard tensions. Easing (reduce AOA) in puffs. Finally reducing sail.
 
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Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Any fluid dynamics expert will tell you that thickness has LITTLE bearing on the lift generating capabilities. Its the SHAPE that matters.
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
then there is the fokker effect that is fun to discuss with the multiple mast boys. thats what made the tri wing fighter work.

sails are air foils that create lift. keels are foils that create lift
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
There are plenty of resources that will talk about the relationship between lift and sailplan. Twist factors into this, but only because the effects of the changes in velocity and angle (friction and coriolis) as elevation increases. The underlying concepts do not change.Google 'CFD sailplan' for more insights.
cfd sail.jpg
 
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May 25, 2012
4,333
john alden caravelle 42 sturgeon bay, wis
nope, frank died. i'm a master sailor. read lots and lots about sailing over the last 50 years. frank's book is the bible of sailing. easy to understand format.

goggle bethwaite. the rest is obvious

in order to understand the trimming of sails one must understand how the air crosses and interacts with the surface of a given body of water. franks book is easy to understand. it has all the answers. gold medals baby. lots of gold medals.
 
Nov 26, 2012
1,653
Hunter 34 Berkeley
This guy does not know what he is talking about. Can we get a definition of douche bag?
 
Nov 8, 2010
11,386
Beneteau First 36.7 & 260 Minneapolis MN & Bayfield WI
Maybe my last post (doubtful) in this:

You cannot directly compare the lift generating capability of airplane wind vs sails.

Why? Because airlines use motors to generate THRUST! Sail have a totally different relationship to the airflow because of this, from a AOA, lift and drag perspective. Comparing them directly is silly.