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Mainsail controls: what every line does
The mainsail has more adjustments than any other sail on the boat. Learn what each one changes and you can reshape the sail for light air, a building breeze, or a gusty afternoon on the lake.
Select a control on the diagram, or use the buttons below it.
Shape controls
These set how deep the sail is and where the deepest point (the draft) sits. Set them for the conditions, then leave them until the wind changes.
- Halyard
- Cunningham
- Outhaul
- Flattening reef
- Backstay (on boats where it adjusts)
Trim controls
These set the sail's angle to the wind and how much the top twists open. You adjust them constantly as the course or wind shifts.
- Mainsheet
- Traveler
- Boom vang
The controls, one by one
Not every boat has every control. Knowing what the missing ones would do still helps you get more from the ones you have.
Setting up for the wind
Starting points for sailing upwind. Adjust from here based on what the sail and telltales show you.
| Control | Light air | Moderate | Strong or gusty |
|---|---|---|---|
| Halyard / Cunningham | Just enough to remove wrinkles from the luff; Cunningham off | Draft about 40–50% back from the luff | Tight, draft pulled forward |
| Outhaul | Eased for a fuller foot | Moderate | Tight for a flat foot |
| Boom vang | Slack to snug | Snug | On firmly to control twist |
| Mainsheet | Eased so the top telltale flows | Trimmed to the luffing point, then in a touch | Eased in gusts, trimmed back in the lulls |
| Traveler | Slightly to windward of center | Centered | Down to leeward |
| Backstay | Eased | Moderate | On hard |
| Flattening reef | Off | Optional | On; consider a full reef before you are overpowered |
Lake St. Clair is shallow, so waves build quickly and short. Gusts can arrive with little warning, so keep the mainsheet and traveler ready to use.
Practice drill for your next sail
Change one control at a time and watch the sail. Cause and effect is how this sticks.
- Find the luffing point. On a close-hauled course, ease the mainsheet until the front of the sail starts to bubble, then trim in until it stops.
- Watch the leech telltale. Trim until the top telltale streams aft most of the time. Ease the mainsheet slightly if it stalls and hangs behind the sail.
- Add vang, ease sheet. Tighten the boom vang, then watch how the top of the sail twists less.
- Move the traveler. Drop it to leeward and see how the boom's angle and the boat's heel change without touching the sheet.
- Change the foot. Pull the outhaul tight, then ease it. Look at the depth along the boom in each case.
Learn it on the water. Trim is easiest to learn with an instructor beside you. Our ASA keelboat courses cover these controls hands-on.
See ASA courses
