The truth about sail twist

WHEN WIND BLOWS over water there is, of course, friction generated at water level. Some of the energy generated by this friction is turned into wave energy. But what’s more interesting is the fact that the speed of the wind increases with its  height above water level.  Whereas friction slows down the wind at water level, there is nothing to prevent it reaching its full potential speed higher up.

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.  

Today’s Thought

Our knowledge is a little island in a great ocean of nonknowledge.

— Isaac Bashevis Singer. NY Times, 3 Dec 78

Tailpiece

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.)

Apparent wind dilemma

THE APPARENT WIND has been an abiding puzzle for me. In my racing days, especially in one-design dinghies, I was never sure about the best way to go to windward. That is, whether I should pinch or foot.

What always made the decision difficult was the simple fact that the faster you go, the more the apparent wind comes from ahead. And the more the wind draws ahead, the more you have to pull off to compensate, and the faster you go. Conversely, the slower you go, the higher you can point.

It's all very well going as fast as you can, but when the boats around you are going slower but pointing higher, you get a gut feeling about velocity made good, that is, the real progress you are making toward the windward mark. Actually, it's not your gut that does the feeling. It's your brain, but for some reason it manifests itself in your gut.

Your brain says to your gut: "He's mad. Tell him to point up. He'll get to the mark much quicker if he goes slower but points higher." The gut says to the brain, "No, no, the plan is to take the slightly longer route, not point so high but go faster. Cover more ground more quickly."

"Won't work," says the brain. "He always does this, and never wins. He is the epitome of hope ignoring reality. Doesn't bitter experience tell him anything?"

"Don't ask me," says the gut. "I'm just the messenger. I didn't volunteer for this job and I don't get paid for it."

And then, just to confuse the issue, a freeing gust comes along, and naturally I am able to point up. So I do. Can't help myself. But at the same time the boat speeds up, so the apparent wind hauls more ahead, and I have to pull off some more to keep the jib filled. Back to square one.

Meanwhile, the other boats that were pointing up all the time still seem to be pointing higher than me. I ask the crew (my wife) in a perfectly calm voice to make sure the jib is sheeted in as far as it will go, because I don't seem to be able to point properly. And to do it rather quickly if she doesn't mind. She says: "If you scream at me once more I'm going to jump overboard."

So I don't have much choice, really. What it comes down to is that I have to point lower than the others AND go slower than them, too. It's the usual recipe for disaster. VMG gone to hell. Crew grinding her teeth and not speaking to me. The fleet disappearing ahead. And it's all the fault of that damn mysterious apparent wind. Again.

Today's Thought
The way of the Wind is a strange, wild way.
— Ingram Crockett, The Wind.

Tailpiece
"Doctor! Doctor! Help me! I think I'm shrinking!"
"Now calm down, Mr Jones, there's nothing to be done. You'll just have to be a little patient."

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)