I ordered a copy after you posted about this on a thread last week. Thanks for the reference.
Scientific Definition Of Sailing Terms
- Thread starter Don Guillette
- Start date
dude, call up your buddy Rick Stearns, ask him about frank and what he taught us all.
tell Rick the boys from sturgeon bay say hi
I wrote to Don Guillette looking for answers on the terms "power" and "over powered" relative to sailing; just looking for a practical explanation.
I appreciate the reference to books and I'll do some additional reading, but frankly I'm not looking for anything that technical. I am familiar with Navier-Stokes equations and computational fluid dynamics - familiar with them, not an expert. Those are way beyond what I need. Particularly, when someone says "depower the sail" or "depower the boat", what do they mean from the standpoint of a simple high school physics level vector diagram? Don's book and others that I've read give a number of different how-to's on depowering. I would like to understand the physics of what they do in terms of air flow around the sail and the how forces are affected. I just want to understand the concepts, not calculate any values.
I included the comment on a foil vs. a sail because I've heard it from so many instructors and I don't think it explains what makes the boat go. Apparently that's an argument that's been going on long before I encountered it.
SongBirdSkipper
I appreciate the reference to books and I'll do some additional reading, but frankly I'm not looking for anything that technical. I am familiar with Navier-Stokes equations and computational fluid dynamics - familiar with them, not an expert. Those are way beyond what I need. Particularly, when someone says "depower the sail" or "depower the boat", what do they mean from the standpoint of a simple high school physics level vector diagram? Don's book and others that I've read give a number of different how-to's on depowering. I would like to understand the physics of what they do in terms of air flow around the sail and the how forces are affected. I just want to understand the concepts, not calculate any values.
I included the comment on a foil vs. a sail because I've heard it from so many instructors and I don't think it explains what makes the boat go. Apparently that's an argument that's been going on long before I encountered it.
SongBirdSkipper
@SongBirdSkipper
See my first reply first.
https://forums.sailboatowners.com/i...inition-of-sailing-terms.185252/#post-1372874
Depowering is the act of making the sail shape less that 100% efficient from its maximal. This can be doe mostly by flattening the shape (think flatter F-104 wing vs fat SPAD wing), and also lowering the AOA by lowering the traveler or easing a jib sheet.
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I think JackDaw hit it. Sailors use the word power as a description of the efficiency of their rig. So it is not directly transferable to physics. And efficiency is not well defined either. There is an element of heeling tolerance implied in the definitions.
Hey Songbird, I didn't get the harsh responses either. I also don't like the airplane-wing analogy. It certainly doesn't help me relate anything to my sails, so I generally lose interest when the talk turns that way. I was watching a sailboat racing instructional video just the other night (Sue rented it from Netflix to provide me with some entertainment while she was out of town). When explanations started referencing airplane wings, I drifted off to sleep (well, it was late).
I also like Jackdaw's explanation that the term "power" is used as a verb rather than a noun with definition. I liken the use of "power" as it relates to a sail as more like a lower gear that helps you punch your boat through difficult conditions, such as waves, rather than a speed setting at a higher gear. As your apparent wind decreases with lower speed, say, due to wave resistance, you may want a deeper draft to improve you ability to accelerate through the waves. With little or no wave resistance, once higher speed is attained (and higher apparent wind), you may want to flatten your sails for less drag and to reduce the heeling force, keep sails upright for better efficiency at higher speed.
I also like the book references from Jon and Charles, but that Navier-Stokes equation and Arvel Gentry work is way above my pay grade. I would also put the book written by Rich Stearns and Adam Cort on my reading list.
I also like Jackdaw's explanation that the term "power" is used as a verb rather than a noun with definition. I liken the use of "power" as it relates to a sail as more like a lower gear that helps you punch your boat through difficult conditions, such as waves, rather than a speed setting at a higher gear. As your apparent wind decreases with lower speed, say, due to wave resistance, you may want a deeper draft to improve you ability to accelerate through the waves. With little or no wave resistance, once higher speed is attained (and higher apparent wind), you may want to flatten your sails for less drag and to reduce the heeling force, keep sails upright for better efficiency at higher speed.
I also like the book references from Jon and Charles, but that Navier-Stokes equation and Arvel Gentry work is way above my pay grade. I would also put the book written by Rich Stearns and Adam Cort on my reading list.
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OK, I think I'm getting the picture: For a given wind speed, apparent wind angle and sail area there is a certain amount of "power" available which the sail turns into force - some of which is lift propelling the boat forward and the rest of the force causes heel, (neglecting any drag affects). The objective is to maximize the lift vector. Since any given boat can tolerate only a certain amount of heel, if the available power is too great, the sail has to be reset to reduce the heeling force vector but you may not be getting the maximum lift from the available wind "power"; thus the sail, and boat, are "depowered".
If that's at least approximately correct, then trimming for twist in the sail will depower the top of the sail where the heeling moment is greater, more than at the bottom where the heeling moment is less. Does that sound reasonable?
That would help explain why flattening the sail is said to depower, while allowing more twist - which is definitely not flattening, also is said to depower. So on a scale from light wind to high wind: In light wind the full draft produces maximum lift since there's no concern for heel. Medium wind, heel is a concern so flatten the sail to reduce heeling force, while still getting as much lift as possible. High wind, trim for more twist to dump "power" off of the top of the sail. Is that close to correct?
If that's at least approximately correct, then trimming for twist in the sail will depower the top of the sail where the heeling moment is greater, more than at the bottom where the heeling moment is less. Does that sound reasonable?
That would help explain why flattening the sail is said to depower, while allowing more twist - which is definitely not flattening, also is said to depower. So on a scale from light wind to high wind: In light wind the full draft produces maximum lift since there's no concern for heel. Medium wind, heel is a concern so flatten the sail to reduce heeling force, while still getting as much lift as possible. High wind, trim for more twist to dump "power" off of the top of the sail. Is that close to correct?
That's pretty much got it ;-)
Past the point a sail is fully powered up, the complete exercise is what to do with the excess pressure!
And yes again, twist becomes more relevant when the wind comes up because due to a combination of forces, the breeze is stronger at the top of the sail than at the bottom. When that happens, the apparent wind moves aft, (more so at the mast top then base) and you have to twist the sail to get the correct angle of attack top to bottom. After certain point you add more twist to move the angle of attack past optimal to de-power the sail.
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Maybe. I detected an air of superiority and a mocking tone towards Don because his book uses laymen's terms.
No slight to Don Guillette intended, I enjoyed his book and learned from it. The techniques he gives are practical and step-by-step. But I think in concepts and vectors, I don't remember instructions very well unless I understand why. I think that's why my parents told me "you don't pay attention!".
OK, I will bite on his one!
A sail is a wing, and all of the aerodynamic equations that describe one work for the other. On the windward side of the sail, (under side of a wing) air is decelerated, leading to higher pressures. On the lee side of the sail, (upper side of the wing) air is accelerated leading to lower pressures. In fact, this acceleration provides most of the lift. The sum of these two pressure changes is lift. The use of the lift force differs. On an airplane, the wing is angled in such a way that the lift counteracts gravity. On a sailboat, the sail (wing) is angled in such a way that part of the lift drives the boat forward. The other part of the lift, balanced by the ballast, heels the boat. Most of the lift is generated near the lead edge of the sail/wing where the curvature is the highest.
Wings and sails have different shapes due to their structures. Sails tend to be less optimal because the mast, stays, and boom define their shape. However, increased roach, flat heads, high high aspect ratios (taller sails with shorter foots) are making sails look more like wings for the wind/air speeds at which we operate.
Lastly, we do use power according to its scientific definition. We use lift to convert wind energy to forward energy to propel our boats. Power is energy over time. So a boat is overpowered when the sails convert more wind energy to lift than the boats hull shape can convert to boat speed over time. The resulting power also generates an excess of heel, which we understand means the boat is "overpowered." By the way, if the boat can plane, or has foils, the power from higher winds can be converted into speeds from 10 to 40+ knots without that overpowered heeling.
My comfort with this topic is based on learning the supporting theory from Professor Peter Wegener while running a well instrumented wing in a wind tunnel around the same time I was learning to sail.
A sail is a wing, and all of the aerodynamic equations that describe one work for the other. On the windward side of the sail, (under side of a wing) air is decelerated, leading to higher pressures. On the lee side of the sail, (upper side of the wing) air is accelerated leading to lower pressures. In fact, this acceleration provides most of the lift. The sum of these two pressure changes is lift. The use of the lift force differs. On an airplane, the wing is angled in such a way that the lift counteracts gravity. On a sailboat, the sail (wing) is angled in such a way that part of the lift drives the boat forward. The other part of the lift, balanced by the ballast, heels the boat. Most of the lift is generated near the lead edge of the sail/wing where the curvature is the highest.
Wings and sails have different shapes due to their structures. Sails tend to be less optimal because the mast, stays, and boom define their shape. However, increased roach, flat heads, high high aspect ratios (taller sails with shorter foots) are making sails look more like wings for the wind/air speeds at which we operate.
Lastly, we do use power according to its scientific definition. We use lift to convert wind energy to forward energy to propel our boats. Power is energy over time. So a boat is overpowered when the sails convert more wind energy to lift than the boats hull shape can convert to boat speed over time. The resulting power also generates an excess of heel, which we understand means the boat is "overpowered." By the way, if the boat can plane, or has foils, the power from higher winds can be converted into speeds from 10 to 40+ knots without that overpowered heeling.
My comfort with this topic is based on learning the supporting theory from Professor Peter Wegener while running a well instrumented wing in a wind tunnel around the same time I was learning to sail.
i truly apologize if my writings were seen as curt. i try to not be such. i try to be short and to the point. my life's great passion is sailing sailboats. my family's hobby as a child was sailing,and i fell in love with it. i'm 64 and still very passionate about my sailing. i raised my kids and shared my skills with them. i have well over 1,000,000 miles at sea. the only job i have ever had is being a professional sailor. 12 ft or 1,000 feet it does not mater, i have the training to drive them all with great skill. its all i've ever done. ...... but i'm not trying to teach what i know.
i am trying to share with you all that there is a book written by one of the greatest yacht sailors ever, that easily explains ALL aspects of these wonderful toys we all enjoy. lots of pictures too. it is the encyclopedia of sailing yachts. yacht : old swedish word for boat not used for work. i have lived a very charmed life. met many top sailors and sailed with them too.
that all been said, my boat is way prettier then yours! my boat is way faster than yours! LOL
jon
OK David - pretty close but consider the second force (circulation.)
Here is the famous Bath Tub experiment which accounts for the circulation force. https://northsails.com/sailing/art-and-science-of-sails/gentry. Also - thanks to someone - many of Gentry's articles are assembled here: https://arvelgentry.jimdo.com/articles/. By the way these are actually very understandable.
Don's articles are excellent because they tell you what controls to use and how each control changes sail shape. Both Gentry and Don would be quick to say tell tales (including Gentry Tufts) will erase much mystery.
Charles
Here is the famous Bath Tub experiment which accounts for the circulation force. https://northsails.com/sailing/art-and-science-of-sails/gentry. Also - thanks to someone - many of Gentry's articles are assembled here: https://arvelgentry.jimdo.com/articles/. By the way these are actually very understandable.
Don's articles are excellent because they tell you what controls to use and how each control changes sail shape. Both Gentry and Don would be quick to say tell tales (including Gentry Tufts) will erase much mystery.
Charles
'the compleat cruiser' by l. francis herreshoff is another must read. each of my kids have well over 50,000 miles of sailing with dad while growing up. they all will still come running to sail some more with me. this book taught me a lot about sharing the joy of these toys. the kids are grown and launched but the lazarette is still filled with kids toys.