January 28, 2010

Disappointment and stability

ANOTHER State of the Nation speech. Another evening of disappointment. No tax breaks for sailboat owners. No bailout for those behind with their marina fees. No $8,000 discount for buyers of new boats. Sheesh.

I turn to my inbox for consolation. A message from Sam Psmythe (silent P as in bath) says:

“I saw your piece about how sailboats capsize more easily on the crest of a wave. Well, duh. Surely everybody knows that. What other banalities masquerading as gems of wisdom are you going to parade for us?”

Well, just between you and me, old Sam is pretty nifty with words, but he’s a bit of a know-all. I think he has forgotten that new generations keep coming along, generations that have to learn all the same old things all over again, because humans don’t seem to be able to pass on the seeds of experience to the fruit of their loins.

So while Sam may think he’s heard it all before, I cater to the newbies, the neophytes who are anxious to learn how not to kill themselves at sea. And interesting things happen at sea, believe me. Your boat loses stability in broken water, for a start. Yes, we’ve been there, done that. Thanks, Sam.

But do you know why boats so often broach, roll broadside on, and capsize when they’re running before the wind in large waves? It’s because when a wave breaks under your stern you have practically no steering power to keep her running straight. The rudder is suspended in foam, not water, and it can’t do its job. If you’ve ever been dumped by a big breaker while body surfing you’ll know the feeling of not being able to float high enough to get your head above water.

And if your boat heels to 45 degrees, you don’t have much steering ability, either. Think about it. The rudder is trying to lift the stern toward the sky as much as it is trying to turn the boat sideways. And, of course, if you do a 90-degree capsize you can’t steer at all. If the rudder isn’t totally out of the water, as it would be on a tubby light-displacement boat, it will be horizontal and unable to turn the stern either way.

Stability at sea is always a fascinating subject for sailors, whether they actually get away from the sight of land or not, and one of the very basic facts about boats is that stability comes as a cube of the length, other things being more or less equal. This means that a 30-footer is 72 percent more stable than a 25-footer, which explains why a 30-footer can stand up to its canvas so much better. It also explains why a 30-footer costs so much more than a 25-footer. But that’s another subject for Sam to get enraged about. Some other day, perhaps.

Today’s Thought
It hath been an opinion that the French are wiser than they seem, and the Spaniards seem wiser than they are; but howsoever it be between nations, certainly it is so between man and man.
—Bacon, Essays

Boaters’ Rules of Thumb #7
Statistics from cruiser in Mexico, the Caribbean, and the South Pacific show that the average yacht spends 10 percent of the time at sea, 5 percent tied to docks, and 85 percent at anchor. This emphasizes the importance of good and easily handled ground tackle, and an efficient dinghy.

Golfer: “You must be the worst caddie in the world.”
Caddie: “Oh come now — that would be far too much of a coincidence.”


Bob K7ZB said...

Keep 'em coming, John!

I have been sailing only a few years now on my own boat and have found your columns very useful. For example, last summer sailing on Lake Michigan I found that I was experiencing things you would write about the following week. So, I do think you might work on the synchronization of your columns as we get closer to the season - it would help to know what's going to happen to me BEFORE it occurs for a change!

Sabre 28

Steve, SV Arcturus said...

Your explanation of why boats tend to broach sort of missed the main reason. The basic reason is that if one looks at a cross-section of a series of waves, the internal circulation of the water is a regular pattern. The water within the top of a wave moves in a circular pattern with the water moving up the back of a wave, flowing down the face of the wave, and back around the lower part of the wave to return up the back of the wave. Below the trough, the water rotates in the opposite direction, the top of that circle moving the water in the trough toward the wave face.
The result is that when a boat is running downwind, when the stern is near the top of the wave, that water is moving down the face of the wave in the same direction the boat is. This means there isn't much flow past the rudder, thus it generates little lift resulting in poor or no steering. This condition is always in place near the top of a wave of any size, so anytime one starts down the face of a wave, there's a period when the rudder is in the upper face of the wave when steering is degraded.
Meanwhile, when the bow enters the trough, the water flowing backward tries to slow the boat and, if the boat is a bit diagonal to the wave direction, tries to shove the bow to the side.
The biggest problem that results is that in waves of a certain size (which depends on the length of the boat), the bow is near in the trough where the water is moving backward while the rudder is in the wave face where it has poor steering ability. If the boat gets a little askew on the wave face: bow shoved up the wave, stern shoved down the wave, no ability to steer to correct the heading...Broach
Aerated, breaking crests don't help, but one can broach easily in non-breaking waves if the size is 'wrong'.
Now ask me why sterns walk when backing down. :-)