Sail Trim by Point of Sail Calculator

Sail Trim by Point of Sail

Each point of sail requires different sail trim. The fundamental rule is to ease the sails until they just luff, then trim until the luff stops. That is the optimal angle for that wind direction.

Standard points of sail:

Mainsail trim by point:

• Close-hauled: main sheeted hard, traveler centered or to windward, vang pulled
• Close reach: ease sheet 1-2 ft, traveler down
• Beam reach: boom about 30-45° from centerline
• Broad reach: boom 60-80° from centerline, vang firm
• Running: boom near 90°, watch for accidental gybe

Headsail trim:

• Genoa cars: forward in light air (more depth) / aft in heavy (flatter sail)
• Tell tales: the windward and leeward pair should both stream aft
• Leeward one breaking up → the sail is stalled, ease the sheet
• Windward one lifting → you are pinching or over-eased, trim in or bear away

Sail twist guidelines:

• Light air (under 8 kts): more twist (top of sail eased more) for flow
• Medium air (8-15 kts): standard twist, both tell tales streaming
• Heavy air (15+ kts): less twist (top of sail more flat) for power control

Vang and Cunningham:

• Vang: controls leech tension on running and reaching, pull harder downwind
• Cunningham: flattens the sail in heavy wind by pulling luff down
• Outhaul: flattens the bottom of the main, pull harder in heavy air

Wind speed considerations:

The “tell tale rules”:

1. Both windward and leeward tell tales streaming aft = trimmed correctly
2. Windward tell tale lifting / fluttering = angle of attack too small. Bear away, or trim the sheet in
3. Leeward tell tale fluttering = the sail is stalled, angle of attack too large. Head up, or ease the sheet
4. Top tell tale lifting = too much twist; pull vang or sheet
5. All tell tales fluttering = bad shape; check halyard tension

Rules 2 and 3 are the ones people reverse, so it is worth holding on to the reason rather than the words. The windward telltale sits on the side the wind arrives from, so it lifts when air starts curling round the wrong side of the luff, which happens when you are pinching or the sail is over-eased. The leeward telltale sits in the accelerated flow on the back of the sail, and it only breaks up when that flow separates, which means the sail is over-trimmed and stalled. One handy memory: the fluttering telltale tells you which way to move the tiller. Windward flutters, tiller to windward.

Common mistakes:

• Over-trimming (everyone’s default in light air): boat slows, points lower
• Forgetting traveler: main sheet alone doesn’t give full control
• Wrong genoa car position for wind speed
• Not adjusting for wind shifts: set and forget = lost speed