Car position for jib sheet aft/forward

[requiem]

Originally Posted by NPCampbell

We only just touched on this in my ASA 103 class and I haven't wrapped my head around this yet. I realize that the twist is trying to compensate for the situation in which the wind aloft is faster (and therefore at a different apparent wind angle) than the slower wind at the foot of the sail. However, is the change in apparent wind angle aloft always in the direction of the twist? Can you ever have a situation where you need to twist the head of the sail in the opposite direction to windward (which I'm assuming is impossible to achieve)?

Assuming the wind speed aloft maintains the same direction, but increases in speed, in all cases the apparent wind shift will be aft (when the true wind increases). Here's a diagram:



Apparent wind is the vector sum of the true wind and the boat wind. The length of each arrow represents the speed. When the boat wind speed and true wind speed are similar, the direction of the apparent wind will seem to split the difference.

Any increase in true wind will push the origin of the apparent wind vector further away from the boat's centerline, and with the point of the arrow unchanged, this rotates the apparent wind direction aft.

For the apparent wind aloft to move the other direction I suppose you'd need some sort of inversion layer where the winds aloft are either moving slower or in a different direction.

I haven't read too many others, so Stu's suggestion may be better, but one useful booklet I ran across was Ivar Dedekam's Illustrated Sail & Rig Tuning; it's very short at only 78 small pages and has close to a 1:1 ratio of diagrams to text, so it provides very clear explanations of all these trim questions.

[skipperearly]

Strange terminology "boat wind" . The only wind you feel on a boat is the apparent wind. Call it boat speed and all is clear.

Originally Posted by requiem

Assuming the wind speed aloft maintains the same direction, but increases in speed, in all cases the apparent wind shift will be aft (when the true wind increases). Here's a diagram:



Apparent wind is the vector sum of the true wind and the boat wind. The length of each arrow represents the speed. When the boat wind speed and true wind speed are similar, the direction of the apparent wind will seem to split the difference.

Any increase in true wind will push the origin of the apparent wind vector further away from the boat's centerline, and with the point of the arrow unchanged, this rotates the apparent wind direction aft.

For the apparent wind aloft to move the other direction I suppose you'd need some sort of inversion layer where the winds aloft are either moving slower or in a different direction.

I haven't read too many others, so Stu's suggestion may be better, but one useful booklet I ran across was Ivar Dedekam's Illustrated Sail & Rig Tuning; it's very short at only 78 small pages and has close to a 1:1 ratio of diagrams to text, so it provides very clear explanations of all these trim questions.