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Sail Trim 101

2008 July 27

A number of controls are at hand for getting sails flying right

A review of sail trim is among the most frequently requested topics by sailors in our sailing classes and offshore passages. They understand that speed is always good, at least on the water, and properly trimmed sails contribute to maximum boat speed. We often spend the first morning at sea in a thorough discussion of sail trim, helping us go faster during the whole passage.


When discussing overall sail trim, it makes sense to start with the headsail for two important reasons:

  • The jib directs airflow over the mainsail.
  • The jib itself creates lift and provides a portion of the boat's forward momentum.

Look at the jib's leading edge, or luff, as you begin to determine its trim. A sail's luff should face directly into the wind so that air streams evenly on both sides. We can call this the sail's "presentation to the wind." Our trimming efforts aim at making the entire sail, from foot to head, present to the wind evenly and at the correct angle.

The sail should be hoisted so that there are no horizontal wrinkles emanating from, or vertical wrinkles along, the luff. Horizontal wrinkles usually indicate a lack of halyard tension, and often coexist with slack forestay tension. Increasing halyard tension flattens the sail (decreasing the draft, or fullness of the sail's curvature) and also moves the deepest draft forward. The halyard must be tensioned more with increasing wind speeds, which tend to move draft aft (Figure 1). A jib usually flies with the deepest draft about 30- to 40-percent aft from the luff.

Vertical wrinkles along the luff, caused by excess halyard tension, are eliminated by slightly easing the halyard. In light winds, an excessively tensioned sail's draft would be too far forward, and the sail would be inefficient.

After the halyard is properly tensioned, concentrate on the telltales. Our most valuable sail trimming aids, there should be three sets of two telltales, one on each side of the sail, evenly set along the sail near the luff. Sailing upwind, our goal is to have all telltales streaming back uniformly on both sides of the sail, and from bottom to top. Adjustments in trim are needed whenever telltales flutter or stream forward, indicating airflow turbulence caused by an incorrect presentation to the wind.

Begin by comparing telltales from bottom to top. Problems can be corrected by adjustments of the jib fairlead position. A forward lead placement causes a more direct, downward pull on the leech, straightening the leech and bringing the upper sail inward. Figure 2a illustrates a sail with its lead too far aft, and the upper sail is presented to the wind at too broad an angle. The turbulence causes the upper telltale to dance forward while the middle and lower stream aft. After moving the lead forward (Figure 2b), the upper sail is brought inward, more in alignment with wind flow, and the telltale matches those lower on the sail.

To determine the proper position of the jib leads, slowly turn the boat into the wind while watching the telltales. As the sail begins to luff, all the telltales should begin to flutter at the same time. If the top telltales lift first, there is too much tension on the foot of the sail and the lead needs to be moved forward. If the bottom telltales lift first, move the lead aft.

This will work well in winds up to around 15 knots, but as wind strength increases, the lead blocks will need to be moved aft. This is a tactic used to decrease pressure in an overpowered sail, opening the sail's leech up to spill excess air out, but normally we want to maximize power by making the top and bottom uniform in the presentation to the wind.

Course changes call for more fairlead adjustments. When wind moves aft and the sheet is eased, the lead must go forward to compensate and close the upper leech. Wind more ahead calls for increased sheet tension and leads moving aft.

Once we've achieved even airflow vertically over the sail with accurate lead placement, monitor the sheet tension by comparing inside to outside telltales. A fluttering outer telltale calls for an ease of sheet tension (Figure 3), while the sheet is trimmed in to make the inner stream aft. An easy way to remember it is the sail moves in the direction of the fluttering telltale. Backstay tension is adjusted for more or less headstay sag. Greater sag causes the jib to be fuller, with a deeper draft. Tighten the backstay to decrease headstay sag, flatten the sail and pull the draft forward.

The slot is the area between the jib and mainsail through which air from the jib is directed aft across the main. This area closes as the main is eased out for downwind sailing, choking that airflow. Outboard sheeting, using either a barberhauler or reaching strut, moves the jib's clew outboard from the jib car track and opens the slot to maximize airflow. This is best used in high winds and big seas, when you want a wide slot to let air escape. When the jib is trimmed right, its lift is maximized and airflow over the mainsail becomes most efficient.


Hoist the main as you would the jib, eliminating horizontal and vertical wrinkles with adjustments in halyard tension. Once again, think of its presentation to the wind from top to bottom first, and then laterally (windward/leeward).

The mainsheet traveler controls the bottom half of the sail directly, and the top to a lesser degree. Upwind, move the traveler so that the lower and middle telltales stream back. This is an accurate setting for the lower sail, but the upper usually needs tweaking with the sheet.

Sheet tension most directly controls the sail's upper leading edge, with a lesser effect on the lower sections. Tensioning pulls the upper portion in to alleviate fluttering of the inward telltale, while easing the sheet allows the upper mainsail to "billow" more, exposing the outer side to more wind to control fluttering of that telltale.

The wind higher aloft is usually stronger and more aft than wind at the surface; this is called wind shear. Therefore, it's often necessary to trim the mainsail differently up high than at deck level, and outer telltale fluttering calls for easing of the sheet until the outer portions receive enough airflow. This is called mainsail "twist," and trimmers are said to "twist off" the upper main.

When the main is trimmed properly in light to moderate breezes, the boom is often parallel to the upper batten, but with more wind or greater wind shear aloft, the top batten must be eased more. Note the similarity between jibs and mainsails in this adjustment; moving the jib lead aft is like easing the mainsheet tension. Both adjustments result in opening of the upper sail.

The depth and position of draft in a sail are important trim considerations because they determine pointing ability, sail power, and degree of weather or lee helm. Draft depth must be decreased and moved forward as wind strength builds or we change course to point higher. Sometimes depth also must be restricted in very light winds because an overly deep draft allows light air to lose contact with the sail in those conditions. Draft depth is increased and moved aft in moderate to light breezes and when falling off the wind.

Both the mainsail's draft and presentation to the wind are controlled to a greater or lesser degree by a number of possible adjustments. Figure 4, above, describes how these adjustments affect the mainsail.

After the sails have been roughly trimmed, attention is paid to the sails in proportion to how efficiently you want the boat to sail. "Good enough trim" is all that many sailors want as long as the boat moves. Trimmers who desire "racing trim" pay constant attention to the telltales, draft position, luff tension, wind strength and shifts, boat speed, etc., and communicate with the helmsman during a race to maximize boat speed, making small adjustments on a continual basis.

On extended passages, occasional checks of all telltales, noting boat speed and heel angle, are adequate to maintain acceptable trim. Jib telltales indicate the need for adjustments of fairlead position and sheet tension. Check the main with an occasional ease of the mainsheet until the sail luffs a bit, and then tension the sheet just enough to control the flutter. Once the sails are trimmed, we trimmers, or "tweakers," tinker with the many adjustments at hand to keep the sails drawing well. It's a never-ending proposition, because the wind and seas are fluid, presenting different conditions on a constant basis.