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.
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.