I understand there is a formula for calculating mast rake.I have been unable to find it.Does anyone know what it is?I think it involes the "I" measurement.
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Terry Cox, Hunter 42, Belle-Vie
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Ken Prichard
Terry NO.I'm looking for mast rake-the angle relative to vertical that the mast should be angled forward or aft.It is adjusted by the forstay and backstay and as I understand it primarily controls weather helm.
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Alan
...is controled ONLY by the length of the headstay. The backstay has nothing whatever to do with rake. Rake is set differently for each boat. There are many factors in weather helm, not the least of which is rake. The rake is set with all shrouds loose including backstay. After rake is set the mast can be put in column and bent. On racing boats the rake is changed along with shroud tension depending on wind and wave conditions. On a cruising boat the rake is usually setup and left alone. A general rule for rake on a cruising boat would be 1 to 2% of mast height. Example: mast height 40ft, rake 4.8" to 9.6"
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Alan
More mast rake
Increasing mast rake improves the flow of air off the leech of the main. The more leech area that is perpendicular to the flow, the more efficient the sail and therefore the better the boat sails. In light to medium air you want to project as much sail area as possible to the breeze and so the mast should be as verticle as possible. But as the breeze builds, the closer the air flow can be to 90* as it exits the leech, the more efficient the sail becomes. For this reason there is no 'fixed' position. It's just another adjustment to be made just like your mainsheet.
Increasing mast rake improves the flow of air off the leech of the main. The more leech area that is perpendicular to the flow, the more efficient the sail and therefore the better the boat sails. In light to medium air you want to project as much sail area as possible to the breeze and so the mast should be as verticle as possible. But as the breeze builds, the closer the air flow can be to 90* as it exits the leech, the more efficient the sail becomes. For this reason there is no 'fixed' position. It's just another adjustment to be made just like your mainsheet.
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Mark
Rake and Bend?
Rake effects helm. Mast bend on the other hand effects sails shape thus performance. More bend flatter the sail.
Rake effects helm. Mast bend on the other hand effects sails shape thus performance. More bend flatter the sail.
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Gerry Cooper
wind angle
I read that the wind is not parallel to the sea and actually has a roughly 2 degree down angle. (sort of makes sense with wind shear). To then have the mast and sails at right angles to the wind would mean raking the mast aft by around 2 degrees. Weather helm would be boat specific but I would start with the above.
I read that the wind is not parallel to the sea and actually has a roughly 2 degree down angle. (sort of makes sense with wind shear). To then have the mast and sails at right angles to the wind would mean raking the mast aft by around 2 degrees. Weather helm would be boat specific but I would start with the above.
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Dan McGuire
Don't Understand
Don't understand how the wind can have a down angle over a large area. That would mean that over a period of time the atmosphere would migrate closer to the surface. This appears to be impossible. There must be more to what the author was saying. Just curious
Don't understand how the wind can have a down angle over a large area. That would mean that over a period of time the atmosphere would migrate closer to the surface. This appears to be impossible. There must be more to what the author was saying. Just curious
Wind shear is complicated
Even if the wind shear angle were two degrees, won't it change in gusts and with boatspeed, just like the horizontal apparent wind angle? My guess is that on anything less than a flat-out race boat, 1 to 2 percent of the mast height should be good (just a guess). When I first got my H23 I struggled with excessive weather helm and tried to dial it out by shortening the forestay to reduce mast rake (about a foot or 4% of mast height on an H23). After a couple of months of this I found that the forestay turnbuckle could only do so much. Then I bought a couple of books on sail trim and discovered faster, easier and more effective ways to deal with the the problem. Mast bend works wonders (especially if you have a fractional rig and an adjustable backstay). A traveler is also great when sailing in gusty air.
Even if the wind shear angle were two degrees, won't it change in gusts and with boatspeed, just like the horizontal apparent wind angle? My guess is that on anything less than a flat-out race boat, 1 to 2 percent of the mast height should be good (just a guess). When I first got my H23 I struggled with excessive weather helm and tried to dial it out by shortening the forestay to reduce mast rake (about a foot or 4% of mast height on an H23). After a couple of months of this I found that the forestay turnbuckle could only do so much. Then I bought a couple of books on sail trim and discovered faster, easier and more effective ways to deal with the the problem. Mast bend works wonders (especially if you have a fractional rig and an adjustable backstay). A traveler is also great when sailing in gusty air.
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Gerry Cooper
Dan, my mistake, the angle is around 4deg,
I'm quoting from 'The Boating Bible' by Jim Murrant. He say's: "Although most people believe the wind blows straight along the surface of the earth, it does not. It blows slightly downwards at an angle of about 4 degrees. A mast which is leaning back 4 to 6 degrees will actually be presenting a leading edge which is at right angles, or very nearly so to the direction of flow of the wind."
I'm quoting from 'The Boating Bible' by Jim Murrant. He say's: "Although most people believe the wind blows straight along the surface of the earth, it does not. It blows slightly downwards at an angle of about 4 degrees. A mast which is leaning back 4 to 6 degrees will actually be presenting a leading edge which is at right angles, or very nearly so to the direction of flow of the wind."
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Dan McGuire
Pretty much Impossible
I believe what the Boating Bible says is impossible, at least on a large scale basis. All of that air coming down has to go back up somewhere. I will see if I can get a copy of the book. After years of being an engineer and a meteorologist, I have come to some conclusions. One is that, if I doubt that something is wrong, it is either wrong or there is something which I don't understand. Therefore, I rarely hesitate to speak out. In either case the results are good. I either correct an error or I learn something. Frequently, I have found that a lot of experts were wrong.
I believe what the Boating Bible says is impossible, at least on a large scale basis. All of that air coming down has to go back up somewhere. I will see if I can get a copy of the book. After years of being an engineer and a meteorologist, I have come to some conclusions. One is that, if I doubt that something is wrong, it is either wrong or there is something which I don't understand. Therefore, I rarely hesitate to speak out. In either case the results are good. I either correct an error or I learn something. Frequently, I have found that a lot of experts were wrong.
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Gerry Cooper
Dan, I would be very interested to know if
it's true. Most of us at some stage have taken advice from 'experts' and in a lot of cases through books and magazines. Most of us tend to accept the advice at face value and I'm sure some of it, if not completely wrong is a bit off track.
it's true. Most of us at some stage have taken advice from 'experts' and in a lot of cases through books and magazines. Most of us tend to accept the advice at face value and I'm sure some of it, if not completely wrong is a bit off track.
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Dan McGuire
Tough to Prove or Disprove
This is a tough theory to disprove, except by logic or a very complicated computer model. Logic. Even if you somehow conclude that the air is descending over water and rising over land, that much falling and rising air would create incredible winds as the descending air tries to get to the point where it could rise. Additionally there is no reason that I can think of which would cause the air to uniformly descend over water and rise over land. This does occur for short periods during the day close to land due to sea breezes where the air rises over land and descends over water. The descending air over water flows towards land to displace the air rising over land.. What I suspect happened is that the author observed something which the descending air would explain and he may have seen some evidence of it such as ribbon which indicated descending air. The problem is that the author left himself with something much harder to explain, which he failed to do. I am also surprised that he would come up with a very precise angle such as 4 degrees. I would be interested in how the author came to the conclusion that the air was descending. Maybe the author would somehow learn that I am questioning his theory and explain himself in this forum. I realize that I may be opening myself up for extreme embarrassment when he comes up with a very good explanation.
This is a tough theory to disprove, except by logic or a very complicated computer model. Logic. Even if you somehow conclude that the air is descending over water and rising over land, that much falling and rising air would create incredible winds as the descending air tries to get to the point where it could rise. Additionally there is no reason that I can think of which would cause the air to uniformly descend over water and rise over land. This does occur for short periods during the day close to land due to sea breezes where the air rises over land and descends over water. The descending air over water flows towards land to displace the air rising over land.. What I suspect happened is that the author observed something which the descending air would explain and he may have seen some evidence of it such as ribbon which indicated descending air. The problem is that the author left himself with something much harder to explain, which he failed to do. I am also surprised that he would come up with a very precise angle such as 4 degrees. I would be interested in how the author came to the conclusion that the air was descending. Maybe the author would somehow learn that I am questioning his theory and explain himself in this forum. I realize that I may be opening myself up for extreme embarrassment when he comes up with a very good explanation.
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Gerry Cooper
Boating Bible
Dan, the angle was between 4 and 6 degrees and if you want to look at the book, the ISBN is 0-207-16166-6, published by Angus & Robertson. Page 9, Tuning the Mast is where the words of wisdom are.
Dan, the angle was between 4 and 6 degrees and if you want to look at the book, the ISBN is 0-207-16166-6, published by Angus & Robertson. Page 9, Tuning the Mast is where the words of wisdom are.
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Dan McGuire
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O_Salt
Could this be the explanation
Could this be the explanation? Dan, could this down angle in the wind possibly be caused by the friction of the earths surface, water or land? As the wind touches down on the surface of the water, it should slow down do to friction. The wind above is moving faster and should be bending because of the friction below it. This would be very similar to the way a wave hits a shore line. Just an idea.
Could this be the explanation? Dan, could this down angle in the wind possibly be caused by the friction of the earths surface, water or land? As the wind touches down on the surface of the water, it should slow down do to friction. The wind above is moving faster and should be bending because of the friction below it. This would be very similar to the way a wave hits a shore line. Just an idea.
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Dan McGuire
Can't be the Reason
O_Salt. Something like that might happen over a small area. But the implication it is occurring over the entire water surface is absurd. Imagine a permeable surface 20' feet, or whatever height you want to pick, above the surface of the water. Air is constantly moving down through this surface. As time goes on more and more air would be added to this layer between the water surface and the permeable surface. There would be either a increase in pressure or the air would have to flow out at a very high rate. What might be a possible explanation is that the sails moving through the air are causing a downward motion of the air. That I could believe.
O_Salt. Something like that might happen over a small area. But the implication it is occurring over the entire water surface is absurd. Imagine a permeable surface 20' feet, or whatever height you want to pick, above the surface of the water. Air is constantly moving down through this surface. As time goes on more and more air would be added to this layer between the water surface and the permeable surface. There would be either a increase in pressure or the air would have to flow out at a very high rate. What might be a possible explanation is that the sails moving through the air are causing a downward motion of the air. That I could believe.
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Alan
Here, here for O_Salt
...he has found the answer to todays quiz. In fact, the breeze slows down as it approaches the surface (water or land). This is what causes the bend downward at the surface. Not an unexplained impossible force, just friction.
...he has found the answer to todays quiz. In fact, the breeze slows down as it approaches the surface (water or land). This is what causes the bend downward at the surface. Not an unexplained impossible force, just friction.
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Alan
Dan
What you fail to understand is that the air is a fluid mass made up of localized events. Just because a huricane is blowing in the south Atlantic doesn't mean that it can't be calm in the north. What happens as a puff touches down at one end of a bay usually has very little to do with the breeze 1/2 mile away.
What you fail to understand is that the air is a fluid mass made up of localized events. Just because a huricane is blowing in the south Atlantic doesn't mean that it can't be calm in the north. What happens as a puff touches down at one end of a bay usually has very little to do with the breeze 1/2 mile away.
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Dan McGuire
Sorry Alan, But you don't Understand
The quote provided by Gerry is as follows: "Although most people believe the wind blows straight along the surface of the earth, it does not. It blows slightly downwards at an angle of about 4 degrees. A mast which is leaning back 4 to 6 degrees will actually be presenting a leading edge which is at right angles, or very nearly so to the direction of flow of the wind." As a retired meteorologist, I realize that air is compressible and a fluid. It is physically impossible for the air to continue to flow into a given volume forever without air flowing out. That given volume is bound by a boundary between the top of the mast and the water surface. As defined by the author, this condition occurs everywhere. The author did not say that it occurred in localized areas or for short periods. By the way, the air, as defined by the author, is flowing down at a component which is .07 of the horizontal component (Tan 4 deg). Therefore with a 10 knot wind the air must be flowing downward at a rate of almost 0.7 knot. If it continued at that rate, I believe that within very few days all of the air in the atmosphere would be compressed within this very shallow layer. Talk about global warming, that would give you true global warming. Please explain what is happening to all of that air which is blowing down towards the water/ground, etc. Hey guys, this is fun.
The quote provided by Gerry is as follows: "Although most people believe the wind blows straight along the surface of the earth, it does not. It blows slightly downwards at an angle of about 4 degrees. A mast which is leaning back 4 to 6 degrees will actually be presenting a leading edge which is at right angles, or very nearly so to the direction of flow of the wind." As a retired meteorologist, I realize that air is compressible and a fluid. It is physically impossible for the air to continue to flow into a given volume forever without air flowing out. That given volume is bound by a boundary between the top of the mast and the water surface. As defined by the author, this condition occurs everywhere. The author did not say that it occurred in localized areas or for short periods. By the way, the air, as defined by the author, is flowing down at a component which is .07 of the horizontal component (Tan 4 deg). Therefore with a 10 knot wind the air must be flowing downward at a rate of almost 0.7 knot. If it continued at that rate, I believe that within very few days all of the air in the atmosphere would be compressed within this very shallow layer. Talk about global warming, that would give you true global warming. Please explain what is happening to all of that air which is blowing down towards the water/ground, etc. Hey guys, this is fun.
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