September 23, 2012

Blowing scientific bubbles

 I GET WORRIED when someone comes along and turns upside down what few scraps of scientific knowledge I possess. (Or thought I possessed.) It makes me wonder how wrong I’ve been about a whole lot of other things I’ve believed all my life

The latest issue of BoatU.S. magazine insists that a boat can float on air.  Well, I can only say that no boat I’ve ever owned was able to float on air, although I can see several advantages straight away.  You wouldn’t need anti-fouling paint for a start, and you wouldn’t have to keep it in water, which would save you a bundle. And you wouldn’t need a trailer to tow it anywhere.

But apparently some boats really do float on air. According to the magazine article, Michael Peters Yacht Design has produced a powerboat hull with what they call a Stepped-Vee Ventilated Tunnel.

This tunnel traps air under the boat and channels it to the stern. This, they claim, reduces friction, and therefore drag.  These hulls can reach 60 mph with two outboard motors, and average better than 1.5 miles per gallon while “cruising” at 45 mph.  That compares with a top speed of 55 mph for a Boston Whaler 37 (which doesn’t have a fancy cushion of underhull air) and fuel consumption of about twice as much. Beneteau also recently introduced a powerboat technology they call AirStep, which draws air in from the hull sides and blows it out under the center of the hull.

The BoatU.S. article says: “Addition of air under the hull at low speeds adds lift to the stern, reducing the displacement of the boat, which flattens the bow wave. The result is a boat that rises on plane sooner . . .”

Now I have always understood that aerated water is less dense than normal water, and therefore less able to support a floating body. If you’ve ever body-surfed, you’ll surely agree with me.  What’s more, adding air under a boat cannot possibly reduce its displacement, which is its weight.

I know we live in exciting times, when colliding particles are producing Higgs’ Bosons all over the place and McDonalds is producing vegetable burgers, but it’s really, really hard to believe that bubbles in the water will lift a boat and hold it up higher so that its bow wave is flattened.

So far this principle is not being touted for sailboats, but I don’t doubt the day will come, probably among America’s Cup racers or the Open 60 class.  Between now and then I am going to have to do some hard thinking, and possibly some rearranging of my scientific prejudices. 

Today’s Thought
True science teaches, above all, to doubt, and to be ignorant.
— Miguel de Unamuno, The Tragic Sense of Life.

“What do you want to be when you finish college?”
“You know, I’ve half a mind to go into politics.”
“Great, then you’ll be better equipped than most of them.”

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


Bill said...

Is this the same as the Hickman Sea Sled? Or derived from it? Also, I think Robb White did something along those lines, too, although I can't remember the name of it.

Have faith. there's science in there somewhere.

John Vigor said...

Bill, As far as I recall, the Hickman Sea Sled was a catamaran, an early cathedral hull, grandaddy of the Boston Whaler.

Dear old Robb White might have experimented with blowing bubbles under his hull, but I never heard of it. He specialized in "sport" boats that were a combination of rowing boats and canoes, boats that would plane at 11 knots with a 3 hp outboard motor. It's a hoot to Google "Robb White boatbuilder" and look up his website. Wonderfully entertaining writer.

John V.