When I was learning to sail back in England it was as crew aboard a dinghy. Each weekend we would compete in club races and my job as crew was to adjust many of the lines as we changed our point of sail. From the start we usually had a beat to windward and whenever the wind was strong we had to tighten in almost everything. Then at the windward mark we would reach or run and the lines had to be eased. Then there was the jibe mark (always a potentially difficult spot) before we were back to beating again.
Learning to sail on small boats has a great advantage in that you don't get side tracked with all the systems there are on a larger boat. The boat just has sails and you learn how to trim them properly for each point of sail without having to worry about anything else. But many boat owners have never had the opportunity to sail small boats and when they get their first sailboat it has on it lines that they are not sure what to do with and so are often ignored. What are these lines for?
A sail works like an aeroplane wing (only it's vertical not horizontal). But because a boats sail is made of cloth, the shape of it can be adjusted to suit the conditions - something that cannot be done easily on a plane. Sometimes it will work best when its as flat as it can be; at other times having a deep curve in it will be more efficient. The lines attached to the sail are there to adjust its shape.
Because the mainsail has more lines than the genoa, I shall just deal with this sail now though a headsail should also be set correctly for the conditions and the point of sail. Figure 1 shows a mainsail with the most common lines attached to it. You will definitely have some of these though you may not have all of them.
WHAT THE LINES DO
The halyard is not only used to hoist the sail but, when it is set, it can be used to adjust the tension in the luff of the sail. When it is taut the luff will be stretched tight and the sail will flatten out; when it is slack, the sail will be fuller.
Not all sailboats use a cunningham on the mainsail though most racing sailboats will have one. On a racing boat you are not allowed to stretch the luff of the sail beyond a certain point (often shown by a black band on the mast). A cunningham works in a very similar way to the main halyard by tensioning the luff of the sail without stretching the top or bottom of the sail beyond the bands. Unless you are racing, either the main halyard or the cunningham can be used to control the tension in the luff.
The outhaul is used to stretch the foot of the sail. When it's tight the sail will be flatter than when its loose.
The leech line is a thin line running down a pocket in the leech of the sail; attached at the head and adjustable near the clew. It is used to stop the fluttering of the leech which will otherwise occur when the wind is forward of the beam. It should be tightened only enough to do this. Too tight and it will produce a hook in the leech of the sail which will act as a brake plus, too much tension on the leech line will often cause something to rip!
In England we call the vang a kicking-strap which partially explains its use. When you are sailing with the wind aft of the beam (broad reaching or running) the pressure of the wind would cause the boom to kick up were it not for the vang holding it down. Set the vang so that the boom is held as nearly at right-angles to the mast as possible. On racing sailboats the vang is also used to adjust the shape of the sail when sailing close to the wind. Racing boats will often have a thin tapered mast and a fractional rig (the forestay doesn't go to the top of the mast). When sailing close to the wind, tensioning the vang puts a bend in the boom and flattens the sail.
While most cruising boats will have a fixed backstay, nearly all racing sailboats have an adjustable one. Tension on the backstay puts bend in the mast and flattens the sail.
Though the mainsheet traveller is not a line connected directly to the sail, the traveller is used to adjust the shape of the sail. When the traveller is set more underneath the boom it makes the mainsheet pull down more on the boom and hence the sail.
Boom topping lift
The topping lift is not a line that is used to adjust the shape of the sail. Its purpose is to stop the boom falling to the deck when the mainsail is not set or when you are reefing it. Though not used to adjust the sail, if it is not released sufficiently when sailing it will adversely affect the shape of the sail.
SETTING THE MAINSAIL CORRECTLY
Each of the sail control lines needs to be adjusted depending on the point of sail (whether close-hauled, reaching or running) and depending on the strength of the wind though here I only have space to cover one situation. Imagine you are sailing close-hauled with mainsail and genoa already set and the wind a moderate 15 knots. How should you adjust the mainsail?
First check to make sure that you have eased out the boom topping lift and that the vang and all reefing lines are loose. Now you can check the halyard tension. Release the mainsheet some and look at the luff of the mainsail. It should be tight without being taut. Too tight and you will see vertical creases along the luff; too loose and the sail will appear to sag between each of the sail slides. Next you can adjust the outhaul which also should have a moderate tension on it. If the outhaul is too tight then you will see horizontal creases along the foot. With halyard and outhaul adjusted you can sheet in on the mainsail keeping the traveller near the middle.
When beating to windward the both sails should be tight in and the mainsail boom close to the centerline of the vessel. If necessary you can move the mainsheet traveller up to the windward side a little to help get the boom in but make sure that the boom does not go past the centerline or the sail will then be acting partially as a brake.
If you have a boom vang then it should be set fairly tight. On most cruising boats it serves little purpose when beating but if you are racing, the boom vang can be set tight to flatten the sail enabling the vessel to carry more sail in stronger winds. Similar comments refer to a backstay adjuster which will only be present on boats set up for racing. When sailing close to the wind the backstay will be set tight producing mast rake aft (which aids windward performance) and mast bend which flattens the sail.
Knowing how to adjust all the lines on your sails will help you sail much more efficiently but because each line needs to adjusted differently for differing wind strengths and points of sail, it is not that easy to learn just how to do this. Generally all lines are set tighter in stronger winds than in lighter winds but even this is not true all the time. When you are next out sailing practice adjusting one line at a time and see what difference it makes. If you have a digital knot log you can use the boat speed as an indication of when the line is set at its optimum.