The author is a retired naval architect. He once testified as an expert witness in a case involving a ship that capsized and sank. This is the fourth installment of an 11-part series explaining stability to a lay audience.
Well, it's more than upsetting when large vessels capsize, it is usually tragic. I was very involved with stability issues at the time of the two major sailing ship capsizes in the 80's. Reporters sought me out and almost always asked the same question, "Was the ship unstable?"
It's a hard question because, if you give the proper answer—"Of course not"—they will run back and file a story with a headline, "Naval architect says ship was safe." That may not be the case at all. If you try to explain, the reporter's eyes usually glaze over. 99.999 percent of the boats and ships in the world are stable.
The ones that aren't fall right over at the dock as soon as the lines are untied. The lines probably don't get untied because someone would notice that they are either leaning against the dock or pulling hard on their lines trying to lay down in the water. The real question is, "How stable?"
Stability is the ability to resist an upsetting or heeling force. There are two basic categories of these forces, usually called, heeling moments (nothing to do with "senior moments" but I'll explain that later).
The first is when something shifts the center of gravity, you know, when that person you weren't quite sure you should invite sailing timidly puts their foot down on the seat of the dinghy right next to the side and suddenly lets go. That's upsetting.
The other is what happens when the wind blows and makes your boat heel. We'll talk first about the later and assume until I tell you otherwise that center of gravity always remains fixed. The cases involving shifting CG will be easier to understand after we have gone through wind heeling.
Here is the classic wind heel diagram:
The wind pushes on the sails which puts sideways pressure on the hull. There is always some leeway so the drag of the hull and keel create an opposite resistance. The result is two offset forces in an opposite direction which creates rotation, in this case called "heel". Powerboats react and heel exactly the same way due to pressure on hull and cabin. The forces are simply less in similar wind strength.
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Part 5: Waterlines. “What resists heeling forces? The real action happens at the waterline.”
Hi Ruv,
Yes. Utterly. We went to the museum thinking we would be there an hour or so and ended up being there the whole day.
My best, Dick
Hi Peter, Oh, yes, it sank. See: https://www.vasamuseet.se/en.