Join Date: Feb 2000
Location: Annapolis, Md
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Re: sailing past close hulled?
A proper answer to Sherpa's question has a lot of parts that come into play to fully understand the answer. The ASA 101 book does have a good, albeit simplified answer to the question, and Sailormon 6's answer provides a major portion of the explanation.
Another part of the question asked whether pulling the sail to windward of the center would allow the boat to point higher. The answer to that starts with the way a sail generates forces. Like any wing, as long as the airflow remains parallel to the surface of the sail for the entire surface of the sail, a sail generates it's forces over the entire surface of the sail, with the highest proportionate forces occuring near the leading edge of the sail, and the relative lower forces per square area occuring near the trailing edge. As a slight simplification, those forces at any point on the sail will be perpendicular to the surface of the sail cloth.
So for any angle of attack, (the angle of the sail relative to the apparent wind) there will be drive(forward forces), side forces, and drag(forces resisting forward motion). As the angle of attack is reduced, the side forces and drag increase relative to drive.
This occurs for a variety of reasons. To begin with, drive is diminished at flatter angles of attack. This occurs because at flatter angles of attack, if left unadjusted, the sail will luff.
Luffing is the opposite of stalling. Stalling (adjusting the sail to far to windward) occurs when the angle if attack is too steep so that the apparent wind is too perpendicular to the surface of the sail which in turn results in the air flow separating from the surface of the sail, and stops generating lift.
Luffing occurs when the angle of attack is too flat, and the wind is able to get to the leeward side of the sail and collapse the wing shape of the sail. To prevent the sail from luffing, the sail shape needs to be flattened and that in itself reduces drive.
But also as the angle of attack of the sail is rotated closer to the center line of the boat, a larger percentage of the area of the sail is pointed sidewards, and some of the sail is pointed slightly towards the back of the boat, so that the amount of side force and drag is increasing.
This slows the boat, and since the keel's ability to resist side force is proportionate to speed, and side forces are increased, the boat makes more leeway.
So, More specifically to the question about rotating the boom to windward, if the angle of attack is rotated above the center line of the boat, more of the former side forces are aimed astern and effectively become drag, greatly slowing the boat, and a larger percentage of the drive becomes side forces, causing a rapid increase in leeway, until the boat eventually has no forward motion, and only has leeway. That is effectively what happens when a boat is have to, but that is another topic for another day.
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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay
Last edited by Jeff_H; 02-16-2019 at 11:47 AM.