That’s why, when a boat is sailing on a beat, the direction of the apparent wind changes by between 5 and 8 degrees from the bottom of the mast to the top. The actual amount of change will naturally vary according to the height of the mast.
Therefore, the rule of thumb is that the mainsail leech at the head of the sail should lie farther off the wind than the leech near the clew.
The reason for this is that if the true wind speed is higher up aloft, the apparent wind direction up there will be less affected by the boat’s forward speed. It will be nearer the true wind direction. Therefore, the top of the sail does not need to be sheeted as close to the wind as does the bottom.
I notice that your eyes have glazed over. No matter, if you find this fascinating fact very boring, you don’t have to worry. You don’t have to take any action. Your sailmaker knows all about it, and has built the right amount of twist and camber into the sail for your boat.
There are times when you might be able to increase the efficiency of your mainsail by bowsing down the boom and hauling the leech very tight to remove the twist, but mostly only racers bother about that kind of thing, and they already know the difference between true wind direction and apparent wind direction and VMG and lee-bowing the tide and the meaning of telltales and all that other stuff, so you don’t have to bother your poor little brain with it. Besides, it’s comforting to remember that even if they are highly intelligent and do know it all, only one of them can win the race.
Our knowledge is a little island in a great ocean of nonknowledge.
— Isaac Bashevis Singer. NY Times, 3 Dec 78
When the admiral retired, he hired his personal orderly of over twenty-five years to come with him. The admiral explained that the orderly’s duties would be exactly the same as they were in the navy. On the first morning of the admiral's retirement the orderly entered the admiral's bedroom and woke him. Then he slapped the admiral's sleeping wife on the backside. "Okay, honey, “ he said, “it's time you got back onshore!"
(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)