Trimming the Jib

The Lead Car — One Slider, Two Jobs

The jib sheet doesn't run straight from the clew to your hand — it turns a corner through a small wheeled car that slides on a track bolted to the deck. That car sets the angle of pull on the clew, and the clew is the single corner where the sail's two loose edges meet: the foot (along the bottom) and the leech (the trailing edge). Pull tension splits between them depending on where the car sits.

• Car forward — the sheet pulls more downward. That loads the leech, pins the trailing edge tight, and stands the top of the sail up straight. The foot goes rounder (more depth low down). Less twist.
• Car aft — the sheet pulls more backward, along the line of the foot. That tensions the foot flat and takes the load off the leech, so the top of the sail is free to fall away to leeward. More twist.

Moving the car also drags the sail's deepest point — its draft — around: forward car deepens the lower sail and moves draft aft; aft car flattens it. But the headline effect, the one you steer by, is twist.

The ½-luff rule — a lead you can set in seconds. Before you even look at the telltales, there's a geometry trick. Sight along the jib sheet and picture that straight line carrying on forward through the clew. Where does it point on the luff? Aim it at about the middle of the luff and the car is almost always in a good spot — that's the angle where the sheet's pull is shared evenly between the foot and the leech. Point high on the luff → the car is too far forward; point low → too far aft. It gets you into the ballpark instantly; the luff test in Lesson 4 dials it in.

Twist, and Why the Wind Aloft Is Different

Twist is the sail opening progressively toward the top: the chord up high points further out to leeward than the chord down low. It looks like a fault. It isn't — it's the sail matching a wind that genuinely changes with height.

Two things stack up as you look up the mast. First, friction with the water slows the true wind near the surface, so it blows harder aloft — the wind gradient. Second, the boat's own forward motion makes a headwind that's the same at every height. Add a bigger true wind to that fixed headwind up high and the apparent wind angle opens up — it comes from further aft at the masthead than at the deck. Your sail has to open by the same amount up high, or the top will be trimmed wrong while the bottom is trimmed right.

So how much twist you want depends entirely on conditions:

• Flat water, steady breeze — less twist. Move the car forward, stand the leech up, and work the whole height of the sail hard to point as high as possible.
• Overpowered / heavy air — more twist. Move the car aft. Letting the top blade open dumps the strongest, highest wind cleanly out the top, cutting heel without dropping the sail.
• Light and choppy — more twist. Chop pitches the rig and keeps stalling the sail; a twisted, forgiving shape has a wider "groove" that keeps air attached as the boat lurches.
• Gusty — more twist as a permanent gust valve, so each puff spills off the top instead of laying you flat.

Reading the Telltales

Short ribbons of yarn stitched near the luff, one pair each low, middle and high. They show whether the air is glued to the sail or has torn away — the single most useful instrument on the boat, and it costs a dollar.

The rule never changes: the ribbon that misbehaves tells you which way to move. Windward lifts → ease or bear away. Leeward stalls → trim or head up. You chase the windward one when sailing upwind, keeping it just on the edge of lifting for maximum height.

One System — The Luff Test

Lead, sheet, steering and telltales aren't four facts to memorise — they're one loop. Here's how a sailor closes it on the water.

The three telltales let you set the lead exactly, without guessing. Get the sail roughly trimmed, then steer up slowly toward the wind and watch which windward ribbon lifts first:

• All three lift together — the twist matches the wind's shear perfectly. The lead is spot on. This is the target.
• Bottom lifts first — the top is held too tight (too little twist). The car is too far forward: move it aft.
• Top lifts first — the top has fallen open too far (too much twist). The car is too far aft: move it forward.

Then the sheet sets how deep and how hard-on the whole sail is, and steering sets the angle it all meets the wind at — change any one and the telltales answer. A gust? The apparent wind swings and the top loads up first — which is exactly why you carry twist as your gust valve. Trim, steer, and the little ribbons: cause, effect, correction, forever.