March 17, 2011

Capsizing on the wavetops

ONE ASPECT OF SEAWORTHINESS that has interested me for years is rarely discussed in the technical press and probably little understood by the experts. It’s the fact that a sailboat is very prone to capsize on the top of a big wave or swell attacking from the side.

I have seen a video of this phenomenon in which a Star-class dinghy simply gets blown flat on her side after rising to the very top of a swell, and it’s my guess that keelboats are just as vulnerable. It’s not the water hitting the hull that causes the capsize. It’s an interesting physical phenomenon.

When a boat of any kind is lifted by a broadside swell, there comes a moment at the top when she is almost weightless. It’s the feeling you get when an elevator starts to descend, and your stomach rises to your chin. Because the boat is weightless, almost all of her stability disappears. Both form stability, from her beam, and gravitational stability, from her heavy fixed keel, are reduced to nil in the moment when the upward movement stops and the downward movement begins.

In that moment, there is nothing to counteract the wind pressure in the sails. She simply gets blown flat on her side in a second.

Most of us have recognized that moment without realizing its import. It’s a strange feeling at first, and highly conducive to seasickness until you get used to it. Even in light winds it often results in an awkward lurch to leeward, and in heavy winds, as I’ve said, there’s every chance of a lightning 90-degree capsize.

I have never seen any studies on this phenomenon and I don’t know what you can do to lessen the chance of capsize, except to change course just before the critical moment, so that the hull is no longer on a beam reach. That’s my intuitive reaction. Either pulling off or heading up for a few seconds would probably help but I can’t explain why; and maybe I’ve got it quite wrong. Maybe it wouldn’t help at all. Weightless is weightless, no matter which way you turn. In any case, I offer this as food for thought. Actually, more like food for sober reflection next time you find yourself bobbing up and down among big swells in a capful of wind.

Today’s Thought

He who has suffered shipwreck fears to sail
Upon the seas, though with a gentle gale.
— Robert Herrick, Shipwreck

Boaters’ Rules of Thumb, #173
After running aground, your first action in a sailboat should be to try to spin the boat about to face deep water. In a powerboat, it usually makes more sense to keep the propellers in deeper water astern, and apply full power astern.

Tailpiece
I hear that the fastest growing organization in the States is Athletics Anonymous. When you get the urge to hike or jog or work out at the gym, they send a man over to drink with you until the feeling passes.

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

9 comments:

Aaron Headly said...

Huh. Now you've got me thinking about this too.

I don't think it has to do with 'weightlessness', what as all the mass moments would remain in place as the apparent weight drops (they would all apparently decrease, but all by the same amount).

My suspicions point toward changes in buoyancy, particularly towards changes in the center of buoyancy as the keel crosses the peak of the wave.

All of the center of buoyancy diagrams you see show the hull floating on flat water. As the boat heels, more hull is submerged on the low side, so the center or buoyancy moves to the low side. This acts as a sort of lever on the hull, pushing it back towards the upright. I'm not sure what happens when the waterline isn't straight across (as when it crosses the peak of a wave on a reach).

An aside, for sailors: 'tenderness' and 'stiffness' are actually terms referring to how hard the hull 'pushes' back towards the upright when it's heeled. With rounder hull cross-sections (and hulls with pronounced deadrise), the center of buoyancy shifts less, so the buoyancy has less leverage, and the boat will be more tippy ('tender').

I'm going to be gnawing on this one for a while. Thanks for that, Mr. Vigor.

bob said...

If the boat is stationary, your analysis is spot on. But if she is moving then the lift generated by the keel must be considered.

frabs said...

John, the Star class is not a dinghy but a keelboat.

Does your analysis take into consideration that there is more air than water in the crest of a breaking wave?

Anonymous said...

First, as far as I know there is no such thing as a Start dinghy. They are keel boats.

Not sure why you would be more likely to capsize at the top of the wave unless the wave drops out from under you at rate faster than gravity. This may result in more of the hull being out of the water than usual.

Gravity is still working on the keel and the keel still is going to resists lifting up.
BoB

John Vigor said...

Yes, I misspoke when I referred to the Star as a dinghy since technically she is a keelboat. In practice, however, she's sailed like a dinghy and behaves like a dinghy -- crew hanging out over the side, suction bailers and all.
And the fact that a keelboat can be blown over suddenly at the top of a wave makes the phenomenon even more interesting.

John V.

John Vigor said...

I've found expert support. Sir William Henry White wrote in his book Manual of Naval Architecture in 1877:

It has been shown that a ship accompanying the motion of the waves, and heaving vertically, is subjected to accelerating forces which affect her apparent weight, just as similar accelerating forces affect the pressure of the water in the wave. Actual observations have proved the apparent weight of a ship to have varied about 20 per cent, above and below the true weight ;* and the importance of this variation must not be overlooked, because the righting moment at any instant involves as a factor the apparent weight, which may be above or below the true weight. Take the case where it is less than the true weight—i.e. the upper half of the orbits, or the upper half of the waves. Then, since the force of the wind is not affected by the wave motion, it must during this time have a greater inclining effect upon the vessel than the same force of wind would have in still water; and in vessels of low freeboard, such as the Captain, where the curve of stability is exceptionally small in area, and flat-topped (see Fig. 47), this virtual loss of righting moment due to the vertical heaving motion may prove another cause of danger. Of course, in the lower half of the orbit the apparent weights and the instantaneous righting moments are greater instead of less than in still water. And it must also be remembered that for a ship on a wave the vertical accelerating forces become zero, the true weight being the apparent weight, at nearly the same time that the wave normal reaches its maximum inclination (as in Fig. 72). Both these are circumstances telling in favour of the ship; but at the same time the subject now briefly mentioned is certainly one deserving attention in discussions of the safety of rigged ships rolling amongst waves.

PS: This book is still print.

John V.

frank said...

- and don't neglect the orbital water motion within the wave!

a sufficiently peaked wave can break and throw (or lose!) its own top horizontally forward, leaving the main body of the wave behind - and that is where the keel (or centreboard of a dinghy) is - back in the wave whilst the top surges to leeward carry ing the hull ....

OZTayls said...

It's nothing to do with weightlessness, as the vertical velocity is nowhere near enough.

What Frank is hinting at is the shear forces within a wave. That is, there is a difference in horizontal velocity at various depths of its amplitude. I suspect that the faster the waves are travelling, the greater this differential will be. The horizontal force on the hull becomes higher than that on the keel, so the boat heels momentarily. Add the force of wind on the sails. Now, the wind velocity on the peak of the wave is at its greatest as well. With the sail/keel forces momentarily in opposite directions, the hull (which is the fulcrum) is more prone to be laid on its side.

Anonymous said...

Hi John,

What you posted on has been described by Marchaj. Look up his 'Seaworthiness'.

Regards,
nick