I was taught that the mainsheet is the principal mainsail control used to steer a sailing vessel. One certainly learns that the position of the boom as determined by the main sheet and thus, the angle of the main sail to the apparent wind, is a principal determinant of the point of sail ranging from close-hauled to a dead run. While this general understanding of the mainsheet permits one to operate a mainsail on a sailboat its effects and use is actually far more interesting and somewhat more complex.
It’s important to keep in mind that the mainsheet must be used in conjunction with other mainsail controls including the traveler and vang and also the halyard, Cunningham, outhaul, and running backstay.
In all points of sail except the broad and deep broad reaches and a dead run the mainsheet tension affects the twist of the mainsail. The twist of the main is essential to an efficient sail effect and is far too complex to explain here. Too little twist in the upper parts of the sail will cause the upper part of the sail to stall decreasing efficiency and resultant power. The sail will also produce too much weather helm. Too much twist and the sail will be depowered.
Tensioning the mainsheet pulls the boom toward center and decreases twist of the sail. Releasing the mainsheet permits the boom to rise and swing away from center leading to twist of the sail.
The mainsheet should be adjusted so that the twist is such that the mainsail is efficient and not stalled. The best way to determine if this is the case is to take notice of the telltales on the leech of the sail. If the twist of the sail is correct all of the telltales will stream backwards from the leech.
Too little twist will stall the upper reaches of the sail leading to the top telltale to flutter pointing forward more than 50% of the time. Releasing the mainsheet a little will correct the twist. In contrast, if the mainsheet tension is too relaxed then the luff at the main near the mast will enlarge. In this situation one must tension the sheet.
Once the proper mainsail twist is set with the mainsheet, the position of the boom, and thus the main, should be adjusted with the traveler instead of letting out the mainsheet. This allows the sail to keep its efficient twist in the close reaches and off to nearly a beam reach. This approach prevents the change in twist and loss of efficiency one sees when simply letting the mainsheet run out to sail farther off the wind.
“Blowing the main” achieved by letting out the main abruptly, and especially when sailing upwind, is one way to add too much twist and depower a sail. I’ve seen this tactic employed when a rapid course change was needed to avoid collision and to permit a quick turn off wind.
The importance of the traveler for helm control and the vang for preventing excessive mainsail twist will be reviewed later. I do want to emphasize that these sail controls are used in unison to maximize efficiency of the main.