Outhaul
The simplest and most underused mainsail control — shaping the bottom third of the sail
Outhaul Hardware and Systems
The outhaul controls the position of the mainsail's clew (the aft lower corner) along the boom. Tensioning it pulls the clew aft, stretching the foot flat. Easing it lets the clew move forward, allowing the foot to bag out and create depth. The hardware varies from dead-simple to multi-purchase cascading systems, but the function is always the same: control the shape of the lower third of the sail.
An internal outhaul runs inside the boom, exiting through a sheave or slot at the aft end to attach to the clew. The line is protected from UV and chafe, the boom profile stays clean, and there is nothing external to snag sheets or crew. The trade-off is that you cannot inspect the line without disassembling the boom end fitting, and a jam inside the boom is difficult to troubleshoot on the water. Internal outhauls are standard on most modern boats from 25 feet up.
An external outhaul runs outside the boom along the foot of the sail, typically through a block at the boom end and back to a cleat near the gooseneck. It is visible, easy to inspect and replace, and simple to rig. The downside is exposure to UV degradation, chafe against the sail foot and boom, and a slightly messier appearance. External outhauls are common on dinghies, older boats, and boats where simplicity and repairability matter more than aesthetics.
Purchase ratios range from a simple 2:1 on small dinghies to 4:1 or even 8:1 with a fine-tune cascade on racing boats. More purchase means easier adjustment under load but more line to manage and more friction in the system. A flattening reef is a separate, high-power purchase that provides extra outhaul tension beyond what the standard system can deliver — used in heavy air when you need the foot absolutely drum-tight. Many cruising boats have a fixed or barely adjustable outhaul, and this is one of the most common performance losses afloat. If your outhaul cannot be adjusted from the cockpit, you are leaving 5 to 10 percent of your boat's potential on the table every time you sail.
If your cruising boat has a fixed outhaul with no adjustment, adding a simple 2:1 purchase with a cam cleat is one of the highest-value, lowest-cost upgrades you can make. An afternoon of work and twenty dollars in hardware can unlock noticeable speed gains in light and moderate air.
What is the main advantage of an internal outhaul over an external one?
What is a flattening reef in the context of outhaul systems?
What the Outhaul Controls
The outhaul adjusts depth (draft) in the lower third of the mainsail — roughly from the boom up to about 30 percent of the sail's height. This is a large area of the sail, and changes here have a real effect on power, heeling force, and overall balance. Yet many sailors treat the outhaul as a set-it-and-forget-it control, if they touch it at all.
Easing the outhaul allows the foot to bag out, creating a deeper, more powerful section in the lower sail. The extra depth generates more lift (power) but also more drag and more heeling force. This is what you want in light air when the priority is generating enough force to move the boat. A visible curve in the foot — a shelf of fabric below the boom on a loose-footed sail, or a gentle belly in the foot channel on a boltrope sail — tells you the outhaul is doing its job.
Tightening the outhaul stretches the foot aft, pulling fabric flat and removing depth from the lower third. The sail becomes flatter, producing less lift but also less drag and less heeling force. This is the heavy-air setting: the wind is providing more than enough power, and your job is to reduce it. A drum-tight foot with no visible belly is the target in strong breeze upwind.
It is important to understand the outhaul's boundaries. It does not significantly affect the upper sail — the top two-thirds of the mainsail are shaped by the backstay, cunningham, mainsheet, and mast bend. Think of the sail in thirds: the outhaul owns the bottom, the cunningham and halyard tension own the middle, and the backstay and mainsheet own the top. Each control works its zone, and trying to use the outhaul to fix an upper-sail problem is a dead end.
What part of the mainsail does the outhaul primarily affect?
Easing the outhaul in light air creates what effect?
Loose-Footed vs. Boltrope Mainsails
How the outhaul behaves depends significantly on whether your mainsail is loose-footed or attached to the boom with a boltrope (also called a footed or slotted-foot mainsail). The two designs respond differently to outhaul changes, and knowing which you have shapes how you read and trim the sail.
A loose-footed mainsail attaches to the boom only at two points: the tack (forward lower corner) and the clew (aft lower corner). The foot of the sail hangs free between them. When you ease the outhaul on a loose-footed sail, the result is dramatic and visible — a shelf of fabric billows below the boom, creating a deep, powerful pocket. You can see exactly how much depth you have created, which makes tuning intuitive. The sail responds quickly and sensitively to outhaul changes.
A boltrope mainsail has a rope or slug system sewn into the foot that runs in a groove or track along the top of the boom. The entire foot is constrained by the boom channel. When you ease the outhaul, the depth increases within the track channel — the fabric bags upward rather than hanging below the boom. The change is more constrained, less visible from the cockpit, and less dramatic in feel. You get depth, but you have to look more carefully to see it.
Most modern racing and cruising sails are loose-footed because they respond more sensitively to outhaul adjustments, are easier to read visually, and are simpler to rig (no foot slugs or track maintenance). The boltrope foot is still found on some cruising sails, particularly on boats with older boom hardware designed for a slotted foot. If your sail is boltrope-footed, outhaul adjustments still matter — you just need to pay closer attention to the subtle shape changes rather than relying on the obvious visual cue of a loose foot.
How does a loose-footed mainsail differ from a boltrope mainsail?
Why are most modern performance sails loose-footed?
Outhaul Settings by Condition
In light air (0 to 8 knots), ease the outhaul 2 to 4 inches to add depth and power in the lower sail. The foot should have a visible curve — on a loose-footed sail, you will see a shelf of fabric a few inches below the boom. In very light air (under 4 knots), some sailors ease even more aggressively for maximum power, accepting the additional drag because the priority is generating enough force to keep the boat moving at all. The depth provides the extra lift that a flat sail simply cannot produce in marginal conditions.
In moderate air (8 to 15 knots), progressively tighten the outhaul as the wind builds. At 8 to 10 knots, you might have an inch or two of ease remaining. By 12 to 15 knots upwind, the outhaul should be near maximum tension — the foot is flat, the lower third is shallow, and the heeling force is reduced. This progressive flattening mirrors what you do with the backstay and cunningham higher on the sail: as wind builds, reduce depth everywhere to manage power.
In heavy air (15 knots and above), the outhaul should be at maximum tension. The foot is drum-tight with no visible belly. On a reach in heavy air, you might ease the outhaul slightly more than you would upwind at the same wind speed — reaching uses more power productively because the drive is more forward and less sideways. On a dead run, ease the outhaul to increase projected area. Downwind, heeling force is not an issue, and a deeper lower sail presents more cloth to the wind.
Unlike the traveler, the outhaul is a set-it-for-the-conditions control. You do not play it dynamically in puffs and lulls the way you play the traveler or mainsheet. When the wind range shifts — say from 8 to 12 knots, or from 15 to 20 — adjust the outhaul to match the new baseline and leave it until conditions change again. On a long upwind leg in building breeze, you might tighten the outhaul once or twice over an hour. That is normal and correct. The outhaul rewards attention to changing conditions, not constant fiddling.
The outhaul is the forgotten control on many cruising boats. Sailors who carefully trim the mainsheet and adjust the traveler never touch the outhaul after leaving the dock — and they are leaving 5 to 10 percent of their boat's performance on the table. Make it a habit: every time you adjust the mainsheet for changing conditions, glance at the outhaul and ask whether it still matches the breeze.
You leave the dock in 6 knots of breeze. Ease the outhaul 3 inches — the foot bags out and the boat accelerates in the light stuff. An hour later, the sea breeze fills in and the wind builds to 14 knots. Tighten the outhaul to near-maximum: the foot flattens, the lower sail sheds heeling force, and the boat stands up and points. Late in the afternoon, the breeze drops back to 8 knots. Ease the outhaul an inch or two to recover some depth. Three adjustments in an entire day sail — that is all the outhaul asks for, and each one makes a measurable difference.
In 5 knots of wind sailing upwind, what should the outhaul setting be?
How often should you adjust the outhaul compared to the traveler?
Summary
The outhaul controls depth in the lower third of the mainsail by adjusting clew position along the boom — ease it for more power in light air, tighten it to flatten and depower in heavy air.
Internal outhauls run inside the boom for cleaner profiles; external outhauls run outside for easier inspection. Both do the same job.
Loose-footed sails respond more dramatically and visibly to outhaul changes than boltrope sails, making them easier to tune and read from the cockpit.
Light air: ease 2 to 4 inches. Moderate air: progressively tighten. Heavy air: maximum tension. Reaching: slightly more ease than upwind for the same breeze. Running: ease for maximum projected area.
The outhaul is a set-it-for-the-conditions control, not a dynamic one — adjust when the wind range changes, not for individual puffs.
Key Terms
- Outhaul
- The control line that adjusts the mainsail clew position along the boom, tensioning or easing the foot to change depth in the lower third of the sail
- Foot depth
- The amount of curvature (draft) in the bottom portion of the mainsail, controlled primarily by the outhaul — more depth means more power and more drag
- Loose-footed
- A mainsail design where the foot attaches to the boom only at the tack and clew, hanging free between them for sensitive, visible response to outhaul changes
- Boltrope
- A rope or slug system sewn into the foot of the mainsail that runs in a track along the boom, constraining the foot and producing more subtle outhaul response
- Flattening reef
- A separate, high-power purchase system that provides extra outhaul tension beyond the standard outhaul, used in heavy air to get the foot drum-tight
- Lower third
- The bottom portion of the mainsail from the boom to approximately 30 percent of the sail's height — the zone controlled by the outhaul