October 20, 2013

The role of inertia

OF THE PEOPLE who find themselves at sea in a small ballasted monohull, few realize that their continued existence depends largely on the outcome of a constant fight between wave impact and inertia.

It is generally understood that a deep-keeled monohull sailboat of almost any size can be capsized by a breaker plunging down the face of a large ocean swell.  But what is not so well understood is the fact that it’s inertia that resists the initial effect of the wave impact. It’s inertia that prevents sudden capsize.

The deeper, heavier, and longer a boat is, the more inertia she possesses. In fact, heavy-displacement keelboats may have as much as five times more resistance to being rolled over than ultra-light boats of the same length, according to renowned research scientist and naval architect Tony Marchaj.

Now, if you have trouble understanding the physical property called inertia, it might help to know that it has two opposite effects.  Matter that is at rest wants to stay at rest. It will resist any attempt to move it suddenly.  And the more matter there is, the more it resists. That’s why it’s difficult to make a boat with a heavy mast roll suddenly: the mast resists quick movement.

But when matter is already moving, it wants to keep moving at the same speed in the same direction. It doesn’t want to be disturbed, and it will resist any sudden changes.

Now, you should not assume that a mast with great inertia will prevent rolling altogether. A steady force will always start the mast moving. What inertia prevents is sudden movement, so that a wave breaking against the side of a heavy-displacement boat with a heavy mast will not be able to throw her over on her beam ends, as it might a light-displacement boat.

There are limits, of course, to the amount of inertia a heavy mast can produce, and as is usual with everything to do with yachts, there are penalties to be paid. Inertia will certainly slow down the frenzied, jerky rolling of a boat running in the trade winds and let her tick slowly from side to side like the pendulum on a grandfather clock.  But, if she falls into a rhythm that coincides with the intervals of the swells, the distance of her rolls to each side will be amplified, and you will end up with the sickening feeling that she’s never going to recover from a particularly heavy roll and just keep going over forever.

So, the job of the yacht designer is to find that happy compromise between beam, draft, length, displacement, and distributed mass which results in a reasonable amount of inertia, but not so much as to cause sickening rolling, excessive hobby-horsing, or a downgrading of performance. Few people who go to sea in ballasted monohulls appreciate how difficult that job really is.

More information
I’VE HAD SOME SQUAWKS from readers who couldn’t quite believe what I said in my last column about the Coast Guard constantly and deliberately contravening the law of the land and the Constitution of the United States by boarding private yachts for random inspections.

Well, doubters might like to click on this link for more information:

Today’s Thought
Architecture is preeminently that art of significant forms in space — that is, forms significant of their functions.
— Claude Bragdon, Wake Up and Dream

“Have a good time at the party, darling, and be a good girl.”
“Jeez, Mom, make up your mind.”

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

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