Sailing ship stability

Warship design and construction, terminology, navigation, hydrodynamics, stability, armor schemes, damage control, etc.
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marcelo_malara
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Sailing ship stability

Postby marcelo_malara » Mon Mar 27, 2006 4:33 pm

I sail frequently in a (modern) sailing ship, 35 feet long. Many times we met strong winds, the ship heeling may be 30 degrees, and once the mast touched the water. The question are:
-how many degrees can heel a ship like this without capsizing?
-somebody posted in other forum that the ballast of the keel actually lowers the centre of gravity BELOW the centre of buoyancy in this ships, is this true?

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Ulrich Rudofsky
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Postby Ulrich Rudofsky » Mon Mar 27, 2006 7:16 pm

There is quite a bit of difference between a modern 35 ft keel sailboat and a ship when it comes to self-righting. The boats I have sailed (35-50 ft) can handle a knock-down with the spreader in the water quite well, spill the wind and come back up, and generally go down one more time before you can get the genoa or spinnaker under control. A ship's metacenter may not allow that kind of recovery from a severe broach and there is a point of no return. Foeth would know what that is.
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Matthias
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Postby Matthias » Mon Mar 27, 2006 8:44 pm

damn, it's frustrating to know something but not having the instrument to explain it in another language...

I'll wait for Foeth too... :stubborn:
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Ulrich Rudofsky
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Postby Ulrich Rudofsky » Mon Mar 27, 2006 8:55 pm

Matthias: Go to: http://www.kbismarck.com/archives/index.html 2.3.6. Inclining tests
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Postby foeth » Tue Mar 28, 2006 8:52 am

It is possible have have your center of gravity (G) below your center of buoyancy (B). For instance, sailing ships racing in the america's cup displace about 25 tons. 20 tons is in the keel! Pic:

Image

But normally you have G above B. A ship that also always has G below B is a submarine, otherwise it would immediately 'capsize'.

Back to ships, and it helps to envisave a floating block (quite easy). G is above B!. Now, the 'dent' the ship makes in the water is now cube shaped and its center of gravity is called B. I'm stealing pics around the net :)

Image

If you give this block an angle, the underwatershape shanges. Try not to think that you are rotating the ship, try to see it as rotating the waterline. What happens is that a part of the ship is now dry, another part is now wetted. This means that the point B will shift towards the part of the ship that has now entered the water as the center of gravity of the submerged body changes accordingly with its shape.

Image

The above pic illustrates this. Notice that a line drawn straight up intersects the mid line? This point is called the Metacenter. (Note that this point is also not fixed, it depends on your initial angle etc)

How much does B shift? This depends on two things. 1) How wide is the ship and 2) what is it's total displacement. If the displacement is very small and the ship is very wide, the part of the ship entering the water is relatively very large and B will shift a lot to the wetted side. Of course, it won't move if the ratio's are reversed. Concluding: the ratio of the waterline(!) and the displacement determine stability. (actually it's the ratio between waterline moment of intertia and displacement that determine the distanve from B to M)

Why is this important? Think if the block under an angle. If you draw the block under an angle, G is above B, with the forces acting on it, you'll see that G tries to capsise the ship. Well, with stabilty and the change of the underwater hull shape, B will shift. If it shifts more outboard than G, the resulting forces produce a moment that will turn the ship back into its initial position. Otherwise it capsizes. The pic below illustrates this perfectly

Image

Each ship has its own 'point of no return'; any further and G will be outboard of B. Some ships (life boats!) are so stable they'll never capsize. Note that if your ship is fully watertight, that even if it does capsize and floats upside down, it is stable again. Normally a hatch or porthole or something else nasty hits the waterline and ships takes in water.

Note that when the deck hits the water, the waterline width drastically dimishises with disastrous results for stabilty. And this is all static. If you have some rolling speed, ths ships inertia may take it beyond its point of no return. Dynamic stabilty is thus quite important!

A second caverat are waves: they can change you waterline. If you have a wave crest amidships, the waterline can be very slender near the bow and stern (wave troughs near the bow). This low of waterplane area has been the cause of spontaenous capsizes of otherwise quite stable ships when a wave approximately as long as the ship itself with more or the less the same speed as the ship met.

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Postby Karl Heidenreich » Tue Mar 28, 2006 3:28 pm

A couple of doubts: in the second image from the left, the G and the B counteracts against the rotation of the vessel, creating a moment that stabilizes the ship, isn´t it? Then, in the last one, the B goes below the G, and to it´s left as matter of fact, and the rotation is "clockwise", so in favor of the rotation beyond the "point of no return"?
The last time I saw something related with fluids was 18 years ago and now seems to me like Mesopotamian Metaphysics! :oops:

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Postby marcelo_malara » Tue Mar 28, 2006 3:37 pm

Foeth, returning to sailboats, can they capsize? I saw some images of capsized racing boats with their keels pointing to the sky.

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Postby Matthias » Tue Mar 28, 2006 3:38 pm

Ulrich Rudofsky wrote:Matthias: Go to: http://www.kbismarck.com/archives/index.html 2.3.6. Inclining tests


Uh, you're right, I forgot I had it... :oops:
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Postby foeth » Tue Mar 28, 2006 4:07 pm

marcelo_malara wrote:Foeth, returning to sailboats, can they capsize? I saw some images of capsized racing boats with their keels pointing to the sky.


I suppose you just answered your own question!

Karl Heidenreich wrote: in the second image from the left, the G and the B counteracts against the rotation of the vessel, creating a moment that stabilizes the ship, isn´t it?


Yes

Karl Heidenreich wrote:Then, in the last one, the B goes below the G, and to it´s left as matter of fact, and the rotation is "clockwise", so in favor of the rotation beyond the "point of no return"?


I don't quite follow, but if you mean that rotating any furher means the rotation will topple the ship: yes. The outer right picture shows a ship on a very tricky point. A seagull landing on the wrong side of the ship can capsize or stabilize it (Should you ever be able to position a ship in such a delicate position and train a seagull)

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Postby marcelo_malara » Tue Mar 28, 2006 4:39 pm

So Foeth, can you tell which is the angle of vanishing stability of a sailboat? Those who has G below B, are really uncapsizeble?

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Ulrich Rudofsky
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Postby Ulrich Rudofsky » Tue Mar 28, 2006 4:57 pm

I think we are all getting an education from EJ Foeth here. Matthias: If you want to "turtle" a sailboat, all you need is a knockdown with the hatches open Try it. It works! Also if it lies on its side, standing on the keel may be enough to pop it back up; a seagull might not do it. :lol: Of course I talking about small sailboats.
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foeth
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Postby foeth » Tue Mar 28, 2006 6:53 pm

can you tell which is the angle of vanishing stability of a sailboat? Those who has G below B, are really uncapsizeble?


Depends on the design what that angle is of course. As far as sentence #2 is concerned... :think: :think: :think: sounds logical! As long as the boat doesn't take in any water...

I don't have any sailing experience, but I'd probably be careful with Ulrich's advice ;) I have hear people trapped inside sailing boats completely upside down. They opened the hatch and let water in. The boat got unstable and turned turtle: back up! It IS possible...

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Postby Matthias » Tue Mar 28, 2006 8:39 pm

Ulrich Rudofsky wrote: Also if it lies on its side, standing on the keel may be enough to pop it back up; a seagull might not do it. :lol: Of course I talking about small sailboats.


Well, if I'm not mistaken it's the standard tecnique to re-capsize it on her keel, isn't it?
I hope I could try it this summer, I am seriously intentioned to follow a practic course to get a sailing licence.Luckily my father boss is a good sailor and he promised he'll help me. :D

And also, I could come here and learn from those who know the subject better then me.. :wink:
"Wir kämpfen bis zur letzten Granate."



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