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  #221  
Old 03-30-2012
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Re: Full or fin keel?

The paper I was referring is from the Australian Maritime College. I cannot post it since it not free but I guess I can post this one By Richard Birmingham from the School of Marine Science and Technology, Newcastle University, UK (2004). On this paper he explains why a boat with an intact mast returns easily to its feet:


Another result of recent research has caused considerable discussion due to its unexpected nature:

Should a capsized vessel retain its rig intact it is usually assumed the damping effect of the rig would impede any action tending to right it. In theory a vessel capsized by breaking waves may be righted again by another wave with similar energy. However as the mast in the water would have a damping effect not present if the vessel had been dismasted it has been assumed that this would slow the righting action.

Experimental result from the towing tank in the Australian Maritime College, Tasmania, found that a dismasted vessel required more waves to right it than
the vessel with the intact rig . ... the effect can be demonstrated, and it is argued that the damping effect is in fact helping the righting process.

The explanatory theory is that the breaking wave force the hull of the vessel at the surface sideways (in sway), while the rig, deeper in the water is
left behind. After the wave has passed the vessel starts to return to 180 degrees of heel (i.e. fully inverted) but the damping effect of the mast slows this such that it is still heeled when the next wave arrives. Each wave arrives with the vessel at a reduced angle of heel (160 degrees, 140 degrees etc.) until the angle of positive stability is reached, and the vessel re-rights.

In contrast the dismasted yacht returns to 180 degrees more quickly, so the progressive righting process does not occur.




Off course this is pretty meaningless other than from the theoretical point of view because almost all sailing boats break the mast when rolled to the inverted position.

There is a small lapse on this paper: when it is said :

"In theory a vessel capsized by breaking waves may be righted again by another wave with similar energy. "

This would only be true if the area under the positive part of the RM curve was equal to the area over the negative part of the RM curve, or saying in a simpler way if the energy required to capsize a boat was the same as the energy to roll it back to its feet.

A modern sailboat stability curve has almost always at least 2 times more area under the positive part and many times about 3 times more positive area.

This means that almost any sailboat will only requires a wave with half the energy of the one that capsized it and many will requires only about 1/3 of the that energy to get back to its feet.

Paulo

...

Last edited by PCP; 03-30-2012 at 07:07 PM.
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  #222  
Old 03-30-2012
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Re: Full or fin keel?

That sounds very interesting Paolo, that is what I was asking for all the time, scientific evidence proving Marchaj was not always right.
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  #223  
Old 03-30-2012
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Re: Full or fin keel?

Quote:
Originally Posted by pvajko View Post
See attached.

Now let us see your paper.
Those photos are from the Wolfson unit tank testing on a work directed by Claughton that gave origin to the paper “THE DYNAMIC STABILITY OF SAILING YACHTS IN LARGE BREAKING WAVES” and yes it is from 1984. My bad I had the idea that it was from the early 90’s.

The work of Marchaj about Seawothiness was also from the 80’s but posterior to that tank testing. Probably I was influenced by the age of Marchaj that was most of its work published in the 60’s and 70’s (we was born in 1918).

I am familiar with that paper and on that tank testing they did not test (I had a quick look to confirm) capsizing with a mast and without a mast and much less return from an inverted boat to its normal position with a mast and without a mast.

What is said on that picture is that they tested the capsize of sailboats without masts and Marchaj suggests as a non tested hypothesis that a sailboat would be harder to return to its feet if it had a mast.

Well, Marchaj was wrong about that. Much more recent tank testing (specifically over that subject) showed that it was the opposite.

Regards

Paulo

...

Last edited by PCP; 03-30-2012 at 08:29 PM.
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  #224  
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Re: Full or fin keel?

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Originally Posted by PCP View Post
I am familiar with that paper and on that tank testing they did not test (I had a quick look to confirm) capsizing with a mast and without a mast
Looks like Marchaj did , see this figure on the left.

Peter
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  #225  
Old 03-30-2012
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Re: Full or fin keel?

This is so far one of the best threads on sailnet but it has not pushed me to pick a winner between full and fin keel. Could you all name types of heavy weather boats that shed water come back upright and fend for themselves? Full or fin or does it have more to do with the sailor than the boat? Thanks for some great reading.
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  #226  
Old 03-30-2012
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Re: Full or fin keel?

Lou,

There is another sticky thread somewhere with one persons "bluewater" boat list. Even that list draws critiques if you will, as that person has a particular set of what the boat should have to be bluewater! As such, a boat may be built to the correct scantlings, but be 20 gals short of water and fuel, and NOT make the list due to insufficient stock water and fuel tankage.

Reality is, there are many boats with ALL types of keels that will survive, roll correctly if you will.........

marty
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  #227  
Old 03-30-2012
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Re: Full or fin keel?

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Originally Posted by Lou452 View Post
.... Full or fin or does it have more to do with the sailor than the boat?... .
This discussion is one area where the poor hapless sailor involved cannot really be a factor anymore. Whether the boat will right itself smartly or eventually is subject to many factors including the design, condition of the boat at the time (holed or not?), sea and wind conditions, and perhaps a good dollop of luck (or lack thereof)..

Where the sailor is during the event is yet another factor.. locked below? on deck and 'hanging on for dear life'? underwater at the end of a tether? knocked free and floating away? Conscious? Not part of the full/fin debate, obviously, but part of the overall outcome.

This is a decision that most make on another 'many factors' basis. Speaking personally I like a boat with good weatherly performance, will move in light air, with an interior I can comfortably live aboard for months at a time. I also really appreciate a boat that will back up predictably in close quarters maneouvering, which pretty much eliminates a full keel, or even most long keel/skeg rudder designs ( I LOVE the Passport 40, for example, and it would be way up my list for offshore sailing, but not for BC coastal cruising...)

And it's another one of those areas where one camp will never convince the other.. but if people reading it all learn something from it, or at least take away a balanced view of the issue then I suppose that's all to the good.
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  #228  
Old 03-30-2012
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Re: Full or fin keel?

Quote:
Originally Posted by pvajko View Post
Looks like Marchaj did , see this figure on the left.

Peter
That is a theoretical study of the capsizing possibilities based on the maximum angle of roll with and without a mast regarding roll moment of inertia. What that graphic shows is that according with his calculations the boat without a mast can roll to the point of capsizing while the boat with a mast remains safe. That was not tank tested or tested in any other way

As I have said the results of those tank testing sessions were published by Claughton and they did not experiment with boats with masts and without masts, much less regarding the influence of a mast in an inverted boat regarding the righting itself up, that was my point anyway.

The results of those tank tests were a bit deceiving and results showed that there was not a big difference between the 3 different types of hulls they have tested. A relatively small breaking wave could capsize any of the boats. Also showed that a significant increase in weight of the boat make no significant difference.

I guess that now, almost 40 years after those tests, with the evolution of computer naval software and tanks, there is conditions to testing with much finer detail but tank testing is very expensive and that kind of research would cost a fortune that nobody is willing to pay.

Regards

Paulo

...

Last edited by PCP; 03-30-2012 at 09:49 PM.
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  #229  
Old 03-30-2012
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post

In science I learned that one can usually devise an experiment to get the results desired.

A mast will hang down deep, well below the wave action, in deep still water. So if beam on, the waves could carry the hull along while the mast stays behind, thus in effect starting the roll towards righting.

But generally a mast and rigging stabilize the rolling of the hull, so generally it would greatly slow the rolling back from an inverted position. If the boat was not beam on to the waves, I would think it would have to wait until (and if) wave action slewed it around to beam on. A rogue wave may not have another behind it strong enough to push the hull beyond the mast. If wind capsized the boat, the waves might not be strong enough, and lacking the leverage of the mast, the wind will slide over the hull.
Quote:
Originally Posted by PCP View Post
If it was that way we would still be in the dark ages. It all depends on who makes the testing, amateurs interested in proving something or scientists trying to broaden knowledge on hydrodynamics. In this case it was the last one. The Tank testing was conducted in a main university by researchers.

Regards

Paulo
Unfortunately I was referring to University researchers. I was a science "nut" until I got to university and found out that 90% of people in science are just doing a job. The research is under a professor, but carried out by graduate students - using undergrads for help. They know the results desired by the organization providing the funding. There is no "pure science" any more, don't believe all that you hear. We are in many ways in the dark ages, due to funding and politics.

Do I have a good scientific mind? Compare what I decided by viewing the physical problem of the "inverted boat with mast" in my mind. It matches exactly what your paper reported, except I go further because I've seen wave tanks and know they can make identical pure waves and control the orientation of the boat, unlike in real life.
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  #230  
Old 03-31-2012
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Re: Full or fin keel?

Quote:
Originally Posted by skygazer View Post
If wind capsized the boat, the waves might not be strong enough, and lacking the leverage of the mast, the wind will slide over the hull.
Wind alone will not capsize a boat. It will knock it down but when the boat is at around 90 degrees the keel will produce a large righting force and the boat will not go over any farther. It takes waves to pass that point.
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