Air should flow smoothly across both sides of the sails. Ideally, you would only notice a bit of disturbance in the forward part of the mainsail because the air there is in the shadow of the mast. The sails will look as if they were rigid, with no puffiness or flopping. We call this flopping of a sail luffing because it starts at the luff (leading edge of a sail) and works its way aft. Adjustments to sail shape are required to keep the boat in balance and sailing as efficiently as possible.
A more sensitive way to judge the correct trim is through the use of telltales. These are strings of yarn on each side of the luff of the head sail (forward sail) and on the leech of the mainsail.
Here we see the telltales in blue are streaming back nicely. Think of the head sail and mainsail as one airfoil and trim accordingly. Head sail and main work together to increase their efficiency. The head sail increase the flow of air over the main; in effect funneling more air across it than would otherwise be the case.
Left we are looking down on the main from directly above showing how air flows across the sail.
In light to moderate winds we focus on powering up the sails making them fuller with greater curve, or draft. In stronger winds, 15 knots or more apparent, we are making the sails less powerful by flattening them or reducing their size by reefing. Less powerful sails reduce heel and keep weather helm at an appropriate level.
Let's start out on a beam reach and see what that looks like from above the boat. All the points of sail are covered in Points of Sail.
Even though we are on a beam reach (the apparent wind is coming directly over the beam of the boat) trimming the sails for a close reach, broad reach, and run is somewhat similar. We ease the sails out until the telltales flop around (stall) then trim in the sails just until the telltales stream straight back. We aim the boat where we want it to go and adjust the sails accordingly. On a broad reach or run we may choose to use the boom vang to hold the boom down. Otherwise the vang is kept loose.
If our destination lies straight upwind then we will need to go as close to the wind as possible. This point of sail is called close hauled or beating. Here, we set the sails to their close hauled position and adjust the course of the boat to achieve our goal of streaming the telltales straight back. By making small adjustments the sails will fill with air and become full or, by steering more towards the wind, they will start to luff and lose power.
We are trying to sail as close to the source of the wind as possible because we want to go somewhere that lies upwind of our present position. Here, trim in the main so the boom is close to the center line of the boat and trim the head sail in all the way. Then, make small course changes as you go upwind to keep the telltales streaming straight back. If weather helm becomes a problem then move the traveler to leeward. The mainsail is adjusted with both the main sheet and traveler. Using the traveler alone allows us to change the angle of attack of the sail without changing its shape. If there is still too much weather helm it may be time to reduce sail by reefing.
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