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Old 26-01-2010, 18:52   #1
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Challenge: Capsize to 180 Most Likely to Continue to full 360 Rollover?

If a boat capsizes I assume there are 3 possible outcomes - it stays upside down; it continues around for a full 360; or it comes back up on the same side it went over. For this question, let's assume a return to the upright condition. If the capsize is the result of a breaking wave, does the momentum and the circular motion of waves favour a 360? Or does the pressure/suction phases of the passing wave knock it over then reverse the direction of rotation to pull it back upright? In other words is a full knock-over more likely to be a 180 or a 360?

If anyone has first-hand knowledge that would be fantastic, but second-hand accounts or learned opinions are most welcome.
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Old 26-01-2010, 19:36   #2
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Once you are at 180 it doesn't matter which way you come back up, but I would vote for momentum pushing you all the way round.
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Old 26-01-2010, 19:55   #3
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Read the Smeeton's book Once is Enough.

Depends on boat design. Narrow boats are much more likely to go 360. Volvo ocean race boats were unlikely to come up from 180 until they added that big balloon on the stern deck. Some few boats are unstable upside down. Others can capsize just past 90 at the other extreme. (Talking monos here.) Don't know if I can find it again, but I believe that I saw a formula relating probability of the number of minutes remaining inverted to certain design parameters of the boat.

The capsize screening formula is an attempt at telling you if a boat is a poor candidate for offshore sailing due to capsize risk. As it only uses beam and weight some consider it a very rough/poor guide.

Capsize screening formula - Wikipedia, the free encyclopedia

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Old 26-01-2010, 20:27   #4
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Found it.

In Desirable and Undesirable Characteristics of Offshore Yachts, Figure 3.2 is a graph that is the result of computing at the sea state where the biggest waves are just enough to roll a boat over that has "x" range of stability and how long it will take for another lesser wave big enough to right it to come along using oceanographic wave spectra data. They only used two data points from the 79 Fastnet to test their computation, but they were pleased with the fit with real data. A boat with a positive range of stability of 150 degrees will spend almost no time upside down, 120 degrees, 2 minutes or less, 90 degrees, 10 minutes or less. I'll bet 2 minutes is eternity when you're in the cabin of an inverted boat.

I've never seen this plot anywhere else and as stated only has 2 real world data points testing it, so take it with a grain of salt.

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Old 28-01-2010, 10:41   #5
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Thanks John. Interesting stuff, but doesn't really answer the question. Assuming a boat didn't start with a list, if a wave turns it perfectly upside down it would be momentarily stable with no righting moment, but with the metacentre below the centre of gravity, will be easily toppled -the question is "in what direction will it likely topple?"
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Old 28-01-2010, 11:47   #6
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If there is enough weather to capsize your boat then I believe the waves may have an influence on which way it will come back around
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Old 28-01-2010, 11:55   #7
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I can see where the drag of the lower or back portion of the wave could roll you back the way you came. As Oh Joy surfed on waves, the forces which forced her towards rounding up, would then change to force her offwind as the wave passed.
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Old 28-01-2010, 13:07   #8
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It depends where on the wave face the boat is when it reaches the 180 point. If the boat is still on the face of the breaking wave, it will most likely do a 360 because the breaking wave is pushing the hull down its face while the sails are in stationary water beneath. However, if the boat is on the backside of the wave, it is harder to say. Angular momentum plays a part but also the hull accelerating towards the trough will play a part.

I think that this is one of those questions that involves too many variable to get a reliable answer. There are examples out there of boats coming up both ways. Also, since angular momentum depends on how quickly the boat is rolling, that is another factor.
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Old 28-01-2010, 13:17   #9
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Quote:
Originally Posted by klem View Post
It depends where on the wave face the boat is when it reaches the 180 point. If the boat is still on the face of the breaking wave, it will most likely do a 360 because the breaking wave is pushing the hull down its face while the sails are in stationary water beneath. However, if the boat is on the backside of the wave, it is harder to say. Angular momentum plays a part but also the hull accelerating towards the trough will play a part.

I think that this is one of those questions that involves too many variable to get a reliable answer. There are examples out there of boats coming up both ways. Also, since angular momentum depends on how quickly the boat is rolling, that is another factor.
Ditto what Klem said .....way to many variables.
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Old 28-01-2010, 13:20   #10
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I would think that the majority of knock downs that would be considered serious or threatening would still be less than 180*, so they would come up on the same side. If the boat does go to 180*, then the big issue is will it right itself before down flooding does it in. I can't see where it makes much difference if it rights itself in a continuing direction or changes direction and returns.

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Old 29-01-2010, 08:05   #11
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Any ideas on the resistance the sail and mast may provide? I would think the moment the sails strike the surface that there would be a slowing of the rotation.

Just a guess. I have broached but never knocked down in the ocean.
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Old 29-01-2010, 09:36   #12
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Having had Oh Joy sticks far enough in the water that I ended up sailing home with six feet of kelp in the spreaders, I can tell ya that the sails being full of water slow the recovery a bit.
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Old 29-01-2010, 09:59   #13
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Originally Posted by cal40john View Post
Found it.

In Desirable and Undesirable Characteristics of Offshore Yachts,
John
John's hit on the best explanation I've seen. To borrow from that book, picture a sphere with a keel. It is easy to spin but no matter how you spin the sphere, it will always settle with the keel straight down. It has 180 degrees of positive stability and 0 degrees of negative stability. Now picture a rectangle with a keel. It is hard to start rolling, but if you turn it more than 45 degrees, it gets easier to roll. More than 90 degrees, it will flip over and stay there. It has 90 degrees of positive stability and 90 degrees of negative stability. Most sail boats fall somewhere between the two extremes, although many power boats don't have 90 degrees of positive stability. Once rolled, the amount of time you spend upside down depends on how often a wave comes along that tips you past your limit of negative stability. Assuming you have more positive stability than negative, if the seas are strong enough to roll you once, it will be easier to roll you back.

One of the other conclusions of the book is that wider beam or higher deck boats are easier to roll, as the lever arm of the beam and the height of the deck give a breaking wave more leverage. They act like the rectangle, resisting roll more than narrow beam boats at first. But tip them far enough and their positive stability drops rapidly just like the rectangle.

It is pretty easy to understand static stability from the image of a sphere and a rectangle. But in rollovers, what matters most is dynamic stability, or how the boat resists the impulse of the wave. Narrower beams and lower decks give the wave impulse less surface area to work on. Ballasted keels and masts act similarly--both are lever arms resisting the rotation of the wave impulse. The mast is just as effective as a ballasted keel because it's lever arm is so much longer. Remember what we're talking about here is a period of about a second when a breaking wave has to start the boat rotating.

To do 360 degree roll, the breaking wave would have to impart enough rotation on the boat to overcome both the positive and negative stability.

I can't do the author justice in a short summary. I highly recommend the book.

Brett
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Old 29-01-2010, 13:01   #14
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Originally Posted by LtBrett View Post
To do 360 degree roll, the breaking wave would have to impart enough rotation on the boat to overcome both the positive and negative stability.
Not in this context - assuming the breaking wave puts the boat over 180, it needs no more energy to continue pushing it around than it does to bring it back up on the same side. Also most ballasted mono-hulls should have very little if any negative stability unless they flood. The mast and sails if they remained standing would provide a lot of lateral resistance, but wouldn't change the distribution of weight. Of course if the keel falls off, that's another story... Anyway my question assumes the boat will return to upright. Thanks for the input though; it's all good stuff.
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Old 29-01-2010, 14:14   #15
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Not in this context - assuming the breaking wave puts the boat over 180, it needs no more energy to continue pushing it around than it does to bring it back up on the same side.
Once the boat is rolling, you have to factor in angular momentum. If the boat rolls to 180 and by some miracle stops exactly there, you are correct. Otherwise, the momentum of the boat makes it easier to keep rolling in the same direction.
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Also most ballasted mono-hulls should have very little if any negative stability unless they flood.
I don't think I've done as good a job as the book at explaining negative stability. It's not practical to make a sailboat with no negative stability, if it's even possible.
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The mast and sails if they remained standing would provide a lot of lateral resistance, but wouldn't change the distribution of weight.
They would if the mast fills with water. Plus all the unsecured crap in your cabin, your water/fuel tanks, even your "new" underwater hull profile have shifted your center of gravity substantially and changed its relationship with your center of buoyancy (google metacentric height).

Bottom line, if you are turtled, a wave, whether the initial wave or a subsequent one, has to overcome your negative stability to right you.

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