Weather gage
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The phrase "to have the weather gage" (or "gauge") describes the favorable position of a sailing vessel relative to another with respect to the wind. It is any position upwind of the other vessel. An upwind vessel is able to maneuver at will toward any downwind point, since in doing so the relative wind moves aft. A vessel downwind of another, however, in attempting to attack upwind, is constrained to trim sail as the relative wind moves forward. Furthermore, in beating to windward, the vessel heels under the sideward pressure of the wind. This is a great trouble to gunnery, as cannon on the windward side now point to the sky, while the leeward gun ports aim uselessly into the sea, or in heavy weather may be awash. A ship with the weather gage, turning downwind to attack, may alter course at will in order to bring starboard and port guns to appropriate elevations.
The term is from the Age of Sail, and although entirely antiquated has had a literary birth in the popular seafaring novels of Patrick O'Brian. In today's modern sailing craft the significance of the weather gage is much diminished. Whereas a ship of the line in the Napoleonic Wars struggled to make any progress better than 90 degrees to the wind, a modern America's Cup racer can point within 30 degrees of it; yet such combatants still seek the weather gage, as can be seen in the aggressive circling chases that precede the starting gun.