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Old 07-23-2006
RGClark RGClark is offline
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Quote:
Originally Posted by SailorMitch
Bob,

Have you even tacked a sailboat?
No, I'm going by what I've read about tacking into the wind:

The physics of sailing.
"How can boats sail faster than the wind? Lots of boats can---especially the eighteen footer skiffs on Sydney Harbour. Ask a sailor how, and he'll say "These boats are so fast that they make their own wind", which is actually true. Ask a physicist, and she'll say that it's just a question of vectors and relative velocities."
...
"The faster that the boat goes, the greater the relative wind, the more force there is on the sails, so the greater the force dragging the boat forwards. So the boat accelerates until the drag from the water balances the forward component of the force from the sails."
...
"Why are eighteen footers always sailing upwind?
In a fast boat, there's no point going straight downwind: you can never go faster than the wind. So you travel at an angle. But if your boat is fast enough, then the relative wind always seems to be coming mainly from ahead of you, as these arrows show. So the eighteen footers never set ordinary spinnakers: they have asymmetrical sails that they can set even when they are travelling at small angles to the apparent wind."
http://www.phys.unsw.edu.au/~jw/sailing.html

I gather from this that being able to sail at a speed faster than the wind speed is not the norm for the average sailboat, but only for boats especially designed for speed.
The method of tacking into the wind also works with ice sailing where the runners pushing sideways against the ice is what causes a force on the boat with a forward component that allows the ice boat to move at an angle *into* the wind. With ice boats the speeds can exceed more than 70 mph when tacking into the wind, much higher than the wind speed:

Ice Yachting.
"A course of 20 miles with many turns has been sailed on the Hudson in less than 48 minutes, the record for a measured mile with flying start being at the rate of about 72 miles an hour. In a high wind, however, ice yachts often move at the rate of 85 and even 90 miles an hour.
"Several of the laws of ice navigation seem marvelous to the uninitiated. Commodore Irving Grinnell, who has made a scientific study of the sport, says: The two marked peculiarities of ice yachting which cause it to differ materially from yachting on the sea are the ability to sail faster than the wind and that sheets are flat aft under all circumstances. Mr. H. A. Buck, in the Badminton Library, Skating, Curling, Tobogganing, thus explains these paradoxes. An iceboat sails faster than the wind because she invariably sails at some angle to it. The momentum is increased by every puff of wind striking the sails obliquely, until it is finally equaled by the increase of friction engendered. Thus the continued bursts of wind against the sails cause a greater accumulation of speed in the ice yacht than is possessed by the wind itself. When the boat sails directly before the wind she is, like a balloon, at its mercy, and thus does not sail faster than the wind. The ice yacht always sails with its sheets flat aft, because the greater speed of the boat changes the angle at which the wind strikes the sail from that at which it would strike if the yacht were stationary to such a degree that, in whatever direction the yacht is sailing, the result is always the same as if the yacht were close-hauled to the wind. It follows that the yacht is actually overhauling the wind, and her canvas shivers as if in the wind's eye. When eased off her momentum becomes less and less until it drops to the velocity of the wind, when she can readily be stopped by being spun round and brought head to the wind."
http://en.wikipedia.org/wiki/Ice_sailing

Apparently, these experts on ice sailing were not aware some sailboats can sail faster than the wind since that is not the usually state of affairs with the average sailboat. The greater drag of the keel in water explains why ice yachts are able to achieve higher speeds than sailboats.


Bob
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