You mentioned some time ago that the wind is about 60% greater aloft due to loss of surface friction. We airplane drivers know that the wind also rotates clockwise for the same reason, at flight altitudes. BUT, is there any significant rotation at the lofty altitudes that the mainsail reaches, and if so, is it enough to be a factor in sail trim? Thanks, Don.
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You're Suspicions are Correct!
However, I'm not the person who you refer to but will provide an editorial comment never the less. When sailing it’s often referred to as wind shear. The wind on the water or over the deck can be from a different direction than 30, 40, or 50 feet aloft at the mast head, order of magnitude, say, 5, 10, 20, or maybe more degrees. Shear is probably/most likely present when the sail trim is different on different tacks. If you’re racing the wind shear is very significant because it can change tactics (one tack being faster than another)and if the trimmer doesn’t recognize what’s happening you won’t go as fast (poor boat speed).
However, I'm not the person who you refer to but will provide an editorial comment never the less. When sailing it’s often referred to as wind shear. The wind on the water or over the deck can be from a different direction than 30, 40, or 50 feet aloft at the mast head, order of magnitude, say, 5, 10, 20, or maybe more degrees. Shear is probably/most likely present when the sail trim is different on different tacks. If you’re racing the wind shear is very significant because it can change tactics (one tack being faster than another)and if the trimmer doesn’t recognize what’s happening you won’t go as fast (poor boat speed).
Twist for wind shear
The difference in wind speeds, and resulting apparent wind direction is addressed by the twist of a sail. The upper part of the sail twists more to leeward to match the apparent wind moving aft with distance above the water. Twist is usually controlled by pulling down on the clew to decrease twist, and easing up to increase it. This is done with the Mainsheet and boom vang on the main, and by moving the sheet fairlead block (forward to pull more down, aft to ease off) for the headsail. There are lot's of good discussions on this subject in the archives, along with some links to good teaching websites, David Lady Lillie David
The difference in wind speeds, and resulting apparent wind direction is addressed by the twist of a sail. The upper part of the sail twists more to leeward to match the apparent wind moving aft with distance above the water. Twist is usually controlled by pulling down on the clew to decrease twist, and easing up to increase it. This is done with the Mainsheet and boom vang on the main, and by moving the sheet fairlead block (forward to pull more down, aft to ease off) for the headsail. There are lot's of good discussions on this subject in the archives, along with some links to good teaching websites, David Lady Lillie David
Sail trim and such
George: In addition to the afore mentioned evident changes by David and John, there is also an artificially induced low pressure area several yards forward and somewhat to starboard of “the slot” around which the wind starts counterclockwise rotation in the northern hemisphere. All these factors together will cause sailtrim to run askew if not monitored and constantly updated. Full monitoring requires telltales at various strategic locations up and down the sails, atop the mast and hanging from the shrouds. This usually leads to information overdose which can spoil an otherwise beautiful sail outing for everyone including the dedicated string pulling, block sliding, cunningham hauling, sail tweaking skipper. Dedicated racing skippers (I am not one of those) will have complete charts showing expected tacking results and trim responses for each boat they sail. Isn’t it interesting that the more we learn the more there is to learn. Jim: Why Knot!
George: In addition to the afore mentioned evident changes by David and John, there is also an artificially induced low pressure area several yards forward and somewhat to starboard of “the slot” around which the wind starts counterclockwise rotation in the northern hemisphere. All these factors together will cause sailtrim to run askew if not monitored and constantly updated. Full monitoring requires telltales at various strategic locations up and down the sails, atop the mast and hanging from the shrouds. This usually leads to information overdose which can spoil an otherwise beautiful sail outing for everyone including the dedicated string pulling, block sliding, cunningham hauling, sail tweaking skipper. Dedicated racing skippers (I am not one of those) will have complete charts showing expected tacking results and trim responses for each boat they sail. Isn’t it interesting that the more we learn the more there is to learn. Jim: Why Knot!
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