October 3, 2010

Aerodynamic perfection

I NOTE WITH INTEREST that Arvel Gentry’s Ranger 23 is for sale. So what, you say? Well, this is no ordinary Ranger 23. This is the Ranger 23 that America’s premier sailing aerodynamicist has owned for about 38 years.

Gentry is the self-styled Seattle weekend sailor who was “interested, amazed, and amused” by attempts in the sailing literature to explain how sailboat sails actually work.

In order to clear up some misconceptions stemming from “some completely false and misleading ideas published in standard sailing references,” he sat down in 1971 to write a landmark 12-page paper named The Aerodynamics of Sail Interaction.

This was an attempt to explain to dummies like me the magic whereby a sail can move a boat against the wind. I have read it several times and am very little the wiser for it, except that the conclusion seems to be that the air pressure on the leeward side of a cambered airfoil is lower than the pressure on the windward side. But I knew that already.

Gentry scoffs at attempts by people less gifted than professional aerodynamicists to explain this phenomenon, and he cringes at theories that have comforted me all my sailing life, such as the one that says the air to windward of the jib takes a short cut across the camber, while the air going around the leeward side of the jib has farther to go, and so must go faster to catch up. And as we all know from kindergarten physics, if you speed up a gas, the pressure drops. Thank you, Dr. Bernoulli.

This is an overly simplistic explanation, of course, and I can see why it makes Gentry shudder; but for people like me it’s a comforting explanation. It explains about as much as I need to know to fit the facts I can see with my own eyes. It’s on a par with the kind of celestial navigation I taught myself from Mary Blewitt’s little book. She espouses the pre-Copernican theory that the sun goes around the earth, and all I can say is that if you can make yourself believe it, celestial navigation becomes a whole lot easier. People like Copernicus and Gentry just make things more difficult when they come along with their new-fangled theories that don’t change anything for practical sailors.

Gentry had a chance to put some sense into my head when Good Old Boat magazine once asked me to interview him for an article. But he refused to meet me and talk sailboat aerodynamicism buddy to buddy. I don’t hold it against him. If it weren’t for Spellchecker I wouldn’t even be able to spell what he does.

Incidentally, if you;re interested in his boat, the advertisement for his Ranger 23 doesn’t give a price. But I can tell you that he paid $28,440 to have it restored in 2001, when it was 29 years old. Aerodynamic people obviously earn good money.

You can get the full details at http://www.arvelgentry.com/

Today’s Thought
The higher we soar on the wings of science, the worse our feet seem to get entangled in the wires.

— Anon, The New Yorker, 7 Feb 31

Boaters’ Rules of Thumb, #102
Human horsepower. It’s fairly well established that the average man in good condition can produce about one-quarter horsepower for about 40 minutes, and between one-sixth and one-seventh horsepower for several hours at a time. Maximum hp from a highly trained male athlete for a burst of a few seconds is a little less than 2 h.p. Interestingly, you can row a dinghy at a reasonable clip in settled conditions by using just one-sixth horsepower.

“How do you like your new babysitter, Johnny?”
“I hate her, Mom. If I was bigger I would grab her and bite her on the back of her neck like Dad does.”

1 comment:

Aaron Headly said...

Here's a method of conceptualizing what a sail (or a wing) does that I've always found useful:

What you are trying to do with a sail is bend the wind a little, but not 'break' it (stall the sail).

Just like when you bend, say, a rod, the wind 'pushes' back when you bend it; this 'push' is what makes a sailboat go.

If you try and bend the wind too far (over sheet the sail), the wind 'breaks' (just as a rod would, to stay with the analogy), and then it stops 'pushing' back in the helpful way.

Also: as you point up, there is less and less remaining angle to usefully bend the wind; eventually you can't get any more 'push' and you have to fall off to stay sailing.

I am aware that this sounds like an over-simplification, but it actually matches the physics.