Light wind Sail trim

In the first of a two part series, we take a look at top tips for light wind sail trim in a cruising yacht.

mainsail trim_gf


While it’s good to have a broad understanding of wind-twisting mechanisms, there are plenty of world-class sailors out there for whom it is quite enough to know that the wind is likely to be freer at the top of the sails than it is at the bottom, and that this effect will be more pronounced in light winds. Thankfully with a set of simple telltales along the leech of the main and behind the luff of the jib you have everything you need to know to set the correct amount of twist for any conditions you’re are sailing in.

Without getting overly technical it is worth knowing that wind is always stronger the higher up the mast it is measured – because surface friction, even over flat water, slows the wind down. So there are usually a few more knots at the masthead than at the boom.

The technically-minded can go into much more detail when it comes to factoring for this twist. Indeed, even the true wind itself is also twisted as it is slowed at the surface according to multiple variables. This wind shear, which is driven by the meteorologist’s old friend the Coreolis effect, works differently on each tack and is more or less pronounced according to atmospheric conditions.

Light wind mainsail set

While you have your telltales to let you know when you’ve got things right, it’s also useful to have a rule of thumb or ‘base’ setup to work from to create twist and deeper draft:

Kicker – Easing the kicker or vang allows the boom to lift and the leech of the sail to open more. In very light winds the weight of the boom may mean that not enough twist can be introduced, in which case a little tension on the topping lift can help.

Traveller and mainsheet – Moving the traveller to windward while freeing the mainsheet to keep the boom at the same sheeting angle allows the top of the sail to twist off.

Backstay – If you have a fractional rig with adjustable backstay, you can ease it so that the mast is straight and the mainsail fuller.

Outhaul – Easing the outhaul slightly will mean that the sail becomes fuller and generates more drive, especially in the bottom part.

Halyard – Loosening the halyard allows the draft of the sail to move aft and become fuller.

Telltales – Three or four light ribbon or wool telltales leading off the mainsail leech near the batten ends will allow you to fine tune. You want to get all of them streaming straight out as a continuation of the shape of the sail at each level. Work the sheet and traveller first to get the bottom telltales streaming, then, leaving the boom at the same sheeting angle, work these lines to lift or drop the boom to introduce more or less twist higher up the sail until all the upper telltales stream too.

Light wind foresail set

Just as for the mainsail, you need your genoa to be relatively full to create power, and relatively twisted to cater for the apparent wind twist further up the sail:

Backstay – As well as creating more draft in the main, freeing the backstay also serves the dual purpose of de-tensioning the forestay. This allows the luff of the genoa to sag to leeward in the middle, creating more draft and therefore power.

Genoa cars – Moving the car aft allows more twist in the genoa.

Halyard – Just as with the mainsail, an eased halyard allows plenty of draft further back in the sail to maximise drive. You only need enough tension in the halyard to take away any horizontal creases.

Telltales – You are trying to get both the inner and outer telltales streaming nicely and parallel all the way up the genoa, with the inside telltales just starting to lift slightly. As with the mainsail, use the sheet tension and car position to set the bottom of the sail first. Then, if the top outside telltale is lifting, the car should be moved backwards until the telltale flies true.

Both sails

Of course, looking at each of your sails independently is only part of the picture. Your sails, as far as the airflow over them is concerned, work together to form a single aerofoil shape. It is worth tweaking your sail set with the interaction between the two sails in mind.

On many boats, it is completely acceptable for the first part of the mainsail aft of the mast to flutter or even be slightly backwinded. Our concern is with how the wind is behaving over the whole sailplan, which is why, for instance, we don’t have telltales at the luff of the mainsail.

If the genoa telltales and mainsail telltales are all streaming properly, the overall flow across the sails, as a single foil, is good. It’s easiest to see down on the lee side when sailing closehauled, looking up the leeches of the sails. The twist of the mainsail should match the twist of the genoa, so the leeches should appear as parallel as you can get them.

“If there is more twist in the jib than the mainsail, the air between the sails will be forced upwards, which will be indicated by the upper mainsail leech telltales lifting.”
