Jib sheet settings

Moderator: GreenLake

Jib sheet settings

Postby Breakin Wind » Wed Oct 03, 2012 11:39 pm

When sailing close hauled, as a matter of practice, should the jib be tighter (less curve) in high winds or low winds? And I assume the opposite to be true for a looser sheet (more curve)? When reaching do the settings reverse?

Thanks - Scott
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Postby PassingWind » Wed Oct 03, 2012 11:43 pm

I only adjust the jib blocks depending on the wind. Heavy winds move them back(tighten foot),lets the wind spill out the top. lighter winds move them forward(tighten leech) catch more wind

In normal winds you adjust the blocks so the airflow is equal across all the telltales..
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Postby GreenLake » Thu Oct 04, 2012 1:01 am

Scott,

there is really more than one curve in your jib. The basic curve is probably the one you are thinking of, and it's along the chord. Simplified: more curve gives more sail force. That force contributes both to driving your boat and to heeling it. In stronger winds, say > 12kt, you have all the power you need, and by flattening your sail you depower it a bit and that should also reduce heeling forces.

In lower winds you want progressively more curve to get more driving force, even if it hurts your ability to point. I'd say, the range where you paly that is 4-7 kts. Higher if you need the power to cut through chop. At really low winds, too strong a curve is not good, so you flatten the sail again. Drifting to perhaps 2kt? (I'm not sure you'd want to quote me on precise speeds here, we are talking ball park).

Basic sheet tension sets most of that curve.

The next most important curve is the one called twist. You have more twist when the foot of the sail is sheeted in harder than the area near the top. As you sight up your sail, the top is blown further out.

In winds under 6kt, the windspeed changes rapidly with height. You need twist to account for the fact that the apparent wind changes direction (your boatspeed doesn't vary as you go up the mast, but the wind speed does, and when you combine the two you get different angles for the resulting wind).

In moderate winds you want less twist as above about 6kt the true wind suddenly goes equally fast top and bottom. However, in really high winds, >12 you may want to spill some wind, and increased twist will help you spill wind at the top (where it would otherwise add disproportionately to the heeling moment).

Twist is set by whether the blocks are forward or back. Forward makes the sheet pull more downward, reducing twist.

The third curve is the amount of headstay sag - the front (luff) of your sail isn't on a straight line, because the pull from the sail always bends the headstay a bit. Beyond adding a bit more tension to your rig in higher winds, I don't have any basic suggestion. Might be a bit of a black art when you are starting out.

Now, if you fix several pairs of telltales from bottom to top (about a foot behind the luff) you can see whether your twist is correct. In the medium and medium low wind range you want all pairs to stream evenly, correcting twist to make that happen. (That's the long version of what @PassingWind wrote above).

When reaching, you let out the sheet so that the angle of the sail increases. The goal is to get the same angle to the apparent wind as for close hauled. Because your sheet now also pulls in, not just back and down, you won't have the same control over the sail shape as you have for upwind sailing.

However, the change in angle means that more of your sail force ends up driving your boat instead of heeling it, so you get good (or better) forward speed even if your sail shapes aren't as optimized.

Some boats have additional controls - barber inhauls. These have the effect of moving the sheet inboards a few inches. In moderate winds 8-12kt, they can increase the pointing ability by a bit, allowing you to get a better curve for the sail, while it's sheeted closer to the center line (without them, you'd need to pull too hard on the sheet to attempt to get the same angle). The inhauls are just a block that rides on the sheet and a line that pulls that block inward. You don't use them in low winds, high winds or reaching (or running).

The values for speed and locations I've given are rather rough, so if anyone has better numbers, let's hear them.
~ green ~ lake ~ ~
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Postby PassingWind » Thu Oct 04, 2012 7:43 am

Scott here is a simple way of testing you jib lead placement.

"So the test to determine proper jib lead placement is to head the boat up slowly until the jib begins to luff If it luffs at the top first, the lead is too far aft. If it luffs at the bottom first, the lead is too far forward. But if it luffs the full length of the sail all at the same time, the lead is set in the right spot. "
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