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:

http://www.sailfeed.com/2012/10/coast-guard-boardings-and-your-fourth-amendment-rights-part-1/

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

Tailpiece

“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.)

Inertia and seaworthiness

IT COMES AS A SURPRISE to many boaters to learn that inertia is of great importance to seaworthiness. It is one of the principal ways by which a boat resists being capsized by waves.

Inertia sounds like the feeling that afflicts you when you settle down on the sofa to read a good book or watch a movie on television. It sounds like the feeling you get when you know the lawn needs to be trimmed, or the dishes need to be washed. It is, of course, the full-blood mother of procrastination.

But in physical terms, inertia is the property of matter that makes it want to keep moving when it’s already moving. And it makes it want to keep still when it’s still. In other words, inertia resists change.

Therefore, if a wave breaks against the side of a boat that has significant inertia, it will not immediately throw her over on her beam ends. The boat’s inertia will resist any sudden change, and the more inertia the boat has, the more it will resist.

Deep, heavy boats have a lot of inertia. Heavy-displacement boats have as much as five times the resistance to being rolled over that ultra-light boats of the same length have, according to research scientist and naval architect Tony Marchaj.

A heavy mast on a sailboat or a tall tuna tower on a sportfisher provides considerable inertia via leverage, and takes a lot of jerkiness out of rolling. At the same time, it tends to prolong the roll and perhaps exaggerate it. You always have to be careful about adding weight too high up.

Inertia also affects hobbyhorsing. It makes a boat press her bows deeper in the water as a wave arrives, and throw them higher in the air as the wave passes by. This detrimental effect may be mitigated substantially, if not completely cured, by moving heavy weights away from the ends of the boat and placing them more toward the center. Now, you might think that this is counter-intuitive; that the bow will rise quicker and higher if it’s light and free to rise to waves. Well, that’s certainly true, but the fact is that the bow won’t be fighting the waves by being forced through them. It’s riding buoyantly over them. Your ride will be jerkier, but your speed will improve, and you won’t suffer that very frustrating business of standing dead still while the bow rears and plunges in the same darned hole in the water.

Today’s Thought
Change doth unknit the tranquil strength of men.
— Matthew Arnold, A Question

Boaters’ Rules of Thumb, #153
On a long cruise of, say, a week or more, every crewmember needs a place on board where he or she is guaranteed privacy. A bunk is the most suitable spot and a curtained-off pilot berth is a sailor’s dream of heaven. It’s important that crewmembers respect and preserve each other’s private retreats. Even a special drawer or cubby-hole allocated to one person alone can make a big difference.

Tailpiece
“Grief, who’s that ugly girl in the corner over there?”
“That’s no girl, that’s my son.”
“Oh geez, I’m terribly sorry. I didn’t know you were his mother.”
“I’m not. I’m his father.”

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