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Old 16-10-2011, 12:05   #1
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Tipping Point - Ballast Engineering Question

Engineers and Designers-
I'm going to try and draw a diagram attempting to describe the changes in center of gravity, or pivot point of a sailboat. The question of ballast will follow:
A
A
A
A
A
A
BBBBBBBBB
................. WATERLINE
C
C
C
C

A=Mast, B=Boat, C=Keel

The ability of a sailboat to keep upright, at all times, even after a knockdown, is dependent on the boats ballast. Correct? If a boat looses its keel, then over it goes. The punching bozo concept no longer applies. Is it the center of gravity at play, or the center of some other factor. Disregarding shape, what determines the keel weight? How is the waterline determined? Can you exceed the formula for saftey? Can you have too much ballast? Or, conversely, not enought ballast? Above/below the waterline? Must the ballast be at somepoint below the center of ??

If you've been to the Dominican, you'll understand this. What happens when I add a container's load of Bananas covering the deck of a keeled sailboat with no water or fuel in the tanks below? (Which begs the question should you keep the tanks full at all times when underway?) Do the heavy bananas shift the center of whatever its called? The pivot point, because they're on deck? I know it does, but I would like to know some facts about this, from a provisioning standpoint. What might be the do's and don'ts when loading a ship. As a rule of thumb, do you store pillows above the waterline and canned goods and tools below? Does a freighter not tip because of its surface area above water? Or because of its weight below the waterline? or a combination.
I guess you could use a metronome as an example. The sliding weight determines the tick-tock speed of the device. On a sailboat, is there a similar sliding point?

This probably sounds stupid but I'm serious, btw.

Thanks,
Sailcat
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Old 16-10-2011, 12:07   #2
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Re: Tipping point-Ballast engineering question

My diagram got screwed up! A and C are to be centered over B. Sorry, I was afraid of that.
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Old 16-10-2011, 12:45   #3
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Re: Tipping point-Ballast engineering question

The static (not moving)stability of a boat is a function of the boyency and the balast.The beam is also a factor(note the beamy nature of caribbean boats).The difference between the center of boyency and the center of gravity is called the Metacentric hight. Increasing this difference will stiffen the vessel and reducing the difference will alow the vessel to role more easily. There are many books on vessel design that go into more detail about designed stability. The questions you are asking require a lot of explaining,and I'm not that good of a typist.Get a book on yacht design. Watch videos on you tube showing the loading boats with large loads(haitian sailboats,loading a truck onto a boat).There's plenty of infornation available.
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Old 16-10-2011, 12:51   #4
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Re: Tipping point-Ballast engineering question

Thanks, I just ordered a couple of books from Amazon. While I don't want to become an engineering student, I would like to know some basic principles.
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Old 16-10-2011, 12:57   #5
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Re: Tipping point-Ballast engineering question

See "Understanding Ship and Boat Stability"
By: Brian Trenhaile, P. E., Naval Architect & Marine Engineer,
Stability and Trim for Ships, Boats, Yachts and Barges ? Part I
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Old 16-10-2011, 13:00   #6
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Re: Tipping point-Ballast engineering question

Cool!!!!!!! Thanks, Gord. Very nice info and sight. This will be tonight's fare.
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Old 17-10-2011, 15:00   #7
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Re: Tipping Point - Ballast Engineering Question

Okay, Gord. That pretty much was way beyond my comprehension. I'm more of a right brained person. Can you or someone else put it in 5th grade language? Laymen's terms. I ordered a few books that come highly recommended. Until they come I would like to have a better understanding of Pivot or Center of Momentum or weight distribution.
Thanks
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Old 17-10-2011, 19:01   #8
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Re: Tipping Point - Ballast Engineering Question

You could also try C.A. Marchaj's books on the topic: Aero-Hydrodynamics of Sailing and Seaworthiness, the Forgotten Factor.
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Old 17-10-2011, 19:17   #9
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Re: Tipping Point - Ballast Engineering Question

The center of gravity pushes down, the center of buoyancy pushes up... when they meet, she rolls over.... simple enough?

Wikipedia explains it well
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Old 17-10-2011, 19:27   #10
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Re: Tipping Point - Ballast Engineering Question

Quote:
Originally Posted by Sailcat View Post
Do the heavy bananas shift the center of whatever its called? The pivot point, because they're on deck? I know it does, but I would like to know some facts about this, from a provisioning standpoint. What might be the do's and don'ts when loading a ship. As a rule of thumb, do you store pillows above the waterline and canned goods and tools below?
There are several centers to worry about, a center of gravity, a center of lateral resistance, and a center of effort.

The center of gravity should be as low as possible. This is why some folks store anchor chain right atop the keel bolts. When I used to race one-design boats, we'd take the outboard off the transom before a race and stow it on the keel. Also, you want to keep weight out of the ends (bow and stern) to prevent hobby horsing.

The center of lateral resistance, which is that part of the keel that converts leeway into forward motion, will not be significantly effected by how you load the boat as long as you keep weight out of the ends. On a balanced boat, however, it should be in line with the center of effort, which is up in the sails. When your center of effort gets out of whack with your center of gravity, that's when you get weather helm. Trimming/reefing the sails help return to a balanced state, but lowering the center of gravity does even more to accomplish this. That's why you always see crews hiking out on a race boat.
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Old 17-10-2011, 19:40   #11
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Re: Tipping Point - Ballast Engineering Question

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Originally Posted by Sailcat View Post
Okay, Gord. That pretty much was way beyond my comprehension. I'm more of a right brained person. Can you or someone else put it in 5th grade language? Laymen's terms. I ordered a few books that come highly recommended. Until they come I would like to have a better understanding of Pivot or Center of Momentum or weight distribution.
Thanks
Imagine it in these terms, under a normal situation, a boats center of gravity remains in the same place. As a boat heels the center of buoyancy moves outwards from the boats center because now as the water "sees" it, the hull now has a different shape. The center of buoyancy now applies an upwards force that wants to right the boat. As the boat heels, there is a point at which this force is maximized. If the boat keeps heeling, the force is reduced until it goes to zero. When the force reaches zero righting effect, the boat is no longer capable of righting itself.

Yes, there are a number of other potential factors scenarios such as free surface effect, ballast or cargo shifts, flooding and ocean swell. I am just trying to keep it simple.

Does this make sense Sailcat?
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