July 31, 2011

Power of the wind

I OFTEN WONDER at how little power it takes to move a boat through water. For centuries we have moved bulky goods long distances by water and it has proved to be most economical. But, of course, it’s not fast. And neither are most sailboats, especially the ones designed to carry reasonable loads.

It’s interesting to note how little power it takes to drive a sailboat. For instance, a dinghy sail of 75 square feet generates only about 1 1/2 horsepower in Force 4 winds — 11 to 16 knots.

If you can fly 500 square feet of sail, that same breeze will rustle up 10 horses to push you along. That’s certainly underpowered according to the engine sizes we’ve got used to in motorboats, but nevertheless it works for sailboats if the hull shape is slippery enough.

But then, again, your horsepower under sail depends on the speed of the wind as well. Energy equals mass times the square of its speed, so a Force 6 wind of 22 to 27 knots does not generate twice the horsepower of a wind blowing at 11 to 16 knots. If you double the wind speed, the horsepower goes up four times, which is why most small sailboats have to start reefing when the wind gets above 15 knots or so.

And what happens when there no wind? Well, one human being in good condition can produce about 1/4 horsepower for about 40 minutes, and the maximum output from a trained male athlete is a little less than 2 horsepower — but for only a few seconds. Rowing a dinghy at 3 or 4 knots in calm water takes about 1/6 horsepower and you can keep this up for several hours at a time, but it becomes obvious that moving a bigger craft, by rowing or sculling over the stern, is much heavier, slower work, and for short distances only.

Today’s Thought
You shall have joy, or you shall have power, said God; you shall not have both.
— Emerson, Journals.

Boaters’ Rules of Thumb, #229
The old rule was that wood will not rot if it is always kept thoroughly wet or thoroughly dry. But many wooden boats are being sheathed with fiber cloth and epoxy resin these days. The theory is that without sufficient water, and lacking oxygen, rot fungi cannot grow inside epoxy-coated wood. But no epoxy coating is a total barrier to water vapor, and I always presume that no matter what you do, water will find its way in sooner or later, which is why it might be preferable to paint at least one surface with ordinary oil-based paint. That will allow trapped moisture to escape.

“Does you dog have an impressive family tree?”
“No, he just uses any old one.”

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

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