On a modern sailing yacht we use our sails in order to sail both ‘upwind’ and ‘downwind’, unlike the masters of older square riggers which were restricted to the reach and the run and the Trade Winds that guided their fate.
Many of us may still prefer the relative comfort of a broad reach. Indeed it is regularly quoted that ‘gentlemen never sail to windward’, but for those more ambitious sailors amongst us, the efficient trimming of a sail upwind can enable us to sail efficiently on all points of sail, and very close to the prevailing wind, thus opening our horizons to more than just the downwind ports!
The shape of a sail and the smooth flow of the air passing over it will have an effect on your yacht's ability to point high on the wind and/or sail fast and efficiently on a variety of headings and in varying conditions. In order to trim our sails we can use short pieces of wool or cotton stuck to the sail known as ‘tell tales’. They are usually placed strategically along each side of the luff (the leading edge of the sail) on headsails and the leach (the back edge of a sail) on mainsails with others placed in the middle of the sail to illustrate airflow over the deepest point (or draft). They are placed equidistantly along both sides so that airflow can be observed as the telltales fly or flutter.
For a sail to be at it’s most efficient we usually want the air flowing across it to be undisturbed on both sides and over its entire surface area. In simple terms, if all your tell tales are flying horizontally, or 30 - 45 degrees above horizontal at the luff, then your sail is pretty well trimmed. Of course, there are exceptions to this and certainly, as you bear away past a beam reach, the ability to use tell tales to trim your sail also falls away. The aerodynamic effects of lift you were looking to enhance are lost and your sail becomes more like a kite, collecting air and being pushed along by it.
In simple terms, if your windward telltales at the luff are flying but your leeward telltales are not, you need to change the angle of attack of your luff to the wind. You do this either by changing the heading of your boat, or by trimming the sail. In this instance, ease the sheet until both windward and leeward telltales are flying equally, ideally with your windward telltale flying at about 30 degrees above the horizontal. If your windward telltales are fluttering upward and your leeward telltales are flying horizontally, you may need to trim on the sheet until both are again flying equally.
Of course, advanced sail trim may require you to go ‘up a gear or down a gear’ and this may require you to change the shape of the sail and the angle of attack accordingly. In a high gear in flat water, a fast sail boat, close on the wind, might have a flat headsail with its windward telltale flying vertically at the luff and its leeward flying horizontally. In contrast, in rough chop, the luffs telltales may be flying horizontally, with a deeper draft creating a deeper belly and more power to punch through the chop.
Telltales on the mainsail are more usually located at the deepest point of draft and at the leach. Trimming a mainsail is all about creating twist and angle of attack. In simple terms, look at your leach. If you are on a beat, the foot of your sail should be on the centre line of the boat and you trim the mainsheet to increase or decrease twist and power. If the top tell tale is just fluttering out of sight every now and again your main is relatively well set. Then control it in the gusts by easing or dumping the main traveller. On cruising yachts you may be forced to counter round-ups in gusts by dumping the mainsheet, although this is best avoided if you have a wide traveller that is easily set and is better for this function.