Sail trim refers to the various means by which the sails are configuration to harness power from the wind.

Sheeting Angle

As discussed earlier in the section on wind forces, the primary variable used to control the sails is the sheeting angle, which in combination with the boat heading controls the sail angle of attack with respect to the wind. The sheeting angle is controlled by – you guessed it – the sheets.

The mainsail has only one sheet connected to the boom and some point in the cockpit. Since this sheet is connected to the boom behind the mast and stays, it is free to move back and forth and only one sheet is needed to control the angle of the boom.

The jib however has two sheets that wrap around either side of the boat. Since the jib is in the front of the boat, its sheets must be guided around the mast and side stays. If only one sheet were used, it would get caught on the mast when the jib flips from one side of the boat to the other. So for the jib, there are two sheets, both attached to the clew of the jib but fed around opposite sides of the boat.

Only one jib sheet actively controls the jib at a time. On a starboard tack (the wind coming over the starboard side of the boat) the jib sheet on the port side is used to control the sheeting angle and the starboard side jib sheet is loose. When the boat changes tack to a port tack, the formerly active (port) sheet is let loose and the opposite (starboard) is brought in to control the sheeting angle.

Shape

The shape of the sail – its twist and draft – can also be controlled to some extent. The jib shape is controlled primarily by the location of the fairlead, and the mainsail shape is controlled by the traveler. Moving the fairlead aft or the traveler windward will tend to flatten the jib or mainsail, respectively.

If you find that the mainsail has excessive twist or draft, it is most likely due to one or more of the following:

The boom vang is too loose
The topping lift is still engaged
The main halyard is not fully brought in
The clew outhaul is too loose

Items 1 – 3 lead to the mainsail being held too loosely in the vertical direction along the mast, while item 4 leads to looseness in the horizontal direction along the boom. Any such looseness will lead to excessive twist and draft.

Telltales

Over time, you’ll develop a feel for the adjustments that need to be made to the sail. Small adjustments to the sheeting angle or sail shape will cause the boat to seem to “lock in” and accelerate. However, before you develop this instinct – but also when you’re an experienced sailor – telltales are useful for determining the most efficient sail trim.

A telltale is a small piece of yarn, nylon, or other light line attached to the sail – or sometimes the stays – that indicates the wind flow at that point. Telltales may alternately be called yarns, wools, streamers, or tufts.

Most useful for trimming the sails are the two telltales attached to either side of the sail near the luff and one attached to the leech. Typically, telltales on the port side of the sail are red and those on the starboard side are green.

Using Telltales to Trim the Sails

When the sails are set properly, the telltales should be running parallel to the sail face on both sides of the luff and at the leach. When the sheeting angle diverges from its most efficient setting, the telltales begin to flutter on one side of the sail.

In general, to correct a misaligned sail, the sail should be brought towards the fluttering telltale. This correction can be made by either adjusting the sheet or changing the boat heading. For example, when the telltales are fluttering to windward, the sheet can be brought in (to harden up the sail), or the boat can be steered away from the wind (to fall off).

The luff and leach telltales can also be used in isolation. For example, sometimes the luff telltales may look good, but the telltale at the leech may be fluttering. In this case, only the leech of the sail needs to be adjusted. To make this adjustment without affecting the leach, the shape of the sail needs to be changed. A windward fluttering leech telltale would call for more curvature (draft) and vice versa.