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post #7 of Old 10-17-2012 Thread Starter
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Re: Still confused about water ballast

Originally Posted by davidpm View Post
Lets do a little mental experiment.
Picture a standard hull with standard fin keel.

You put some weight in the bilge of the boat.
Can be be feathers, lead, people anything.
What was their before was air you are replacing it with weight that weight will change the righting moment of the boat.

Same boat but this time you take a garbage bag fill it with water dive under your boat and tape it to the keel.
Yes if you try move the boat forward the bag will rip off but what about the righting moment.
No change.
If you attempted to tip the boat the effort needed to tip the boat with the bag of water attached to the keel would not change from the effort needed to tip the boat with no bag.

If you think about it anything that you attach to the bottom of the fin keel that is lighter than water will have the effect to float the boat higher and make it tip easier. Anything you attach that is heavier will have the effect to sink the boat a little and make tipping it harder.

If you had a boat design that had a very deep full bilge that was normally full of air and you flooded it that would change the handling of the boat. You are changing the displacement of the boat.

If you had a torpedo shaped tube at the bottom of your keel that could be pumped out or filled with water that would make a big change to the handling of the boat but not the way you would want.
When empty the lever action would make the boat very tender. You could imagine making it big enough that the boat would tip over automatically.

Hey you just invented something new. Design a reverse canting keel.
Your bottom mounted air bladder would be canted to leeword to stabilize the boat. I think you would need really hi-speed pumps plus computer controlled stabilization. It would be a bear to balance.
Ahhh, this is a great thought experiment. So when you replace what was air filled space with water (or feathers), you are adding ballast. This is starting to make sense.

I know it's not ideal, but everything is a compromise I'm just curious about the concept in general. Besides, if I'm correct the mac 26D has a phrf rating of 210, which is well below my lead ballasted hunter 25's phrf rating of 234. So it's no racer, but it's not bad for a cruiser either, water ballast or not...
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