March 15, 2009

The pull on a sheet

THERE ARE TIMES when I wonder what sort of loads my jib sheets are carrying. Those times usually come when we are screaming along on a close reach in 20 knots and (for a change) I have nothing else to worry about.

I look at the working sheet, and its bowline, and the lead block, and the winch, and the cleat, and my little brain wonders how many pounds of tension are being created in all those spots, and what would happen if something suddenly gave way.

The modern rope we use for sheets and halyards is a wonderful material, strong almost beyond belief, but it does have its limits, of course, and if your brain demands peace of mind while you’re sailing, maybe you should be quite sure all your running rigging is adequately sized.

You can make a pretty good estimate of the strain your jib imposes on its sheet. You simply square the wind speed in knots and multiply the answer by the sail area in square feet. Then you divide the answer by 232. This gives you the approximate pull on the sheet in pounds.

For example, let’s say you have a 200-square-foot jib, and the wind is blowing 20 knots. Square the wind speed, 20 x 20, and you get 400. Multiply that by the sail area, 200, and you get 80,000. Divide that by 232 and the result is 344.8 pounds, say 345 pounds.

That’s the weight of two big men, which explains why you need the help of a winch to trim the jib when it’s blowing hard. And here’s another thing: wind pressure in the sail rises as a square of wind speed, so the greatest pull you’ll experience on that sheet is likely to be double the 20-knot figure, or in the region of 700 pounds. If you want peace of mind, make sure your gear can handle it.

Today’s Thought
The winds and waves are always on the side of the ablest navigators.
—Edward Gibbon, Decline and Fall of the Roman Empire.

Another notice we noticed:
In an optometrist’s office:
“If you don’t see what you’re looking for, you’re in the right place.”

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