January 30, 2011

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.

“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.)

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