NOTE: Walt offered a very good explanation of how water ballast works while I was typing this, but since I typed it all I'll leave it
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Here is a link that explains it way better than I can.....
http://h260.com/water_ballast/water_ballast_index.html
A number of water ballast boat owners have tried to knock he boat down on purpose and a few have succeeded for a couple seconds but as far as I know they have all popped right back up without even taking water into the cockpit. MacGregor does recommend not sailing with the pop-top up but there again I seen a number of people doing that with the boat in an extreme heeling attitude and everything is fine. We always have ours down sailing being the chickens we are
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Sum
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That is a common misconception but not true. What is true is what you said about the lower the weight the more effect it has. One reason the water ballast boats need more weight is because it is higher and less effective.Im sure someone will explain it better but you are pretty much right on with your idea. The reason the water ballast boats are tender because the water ballast is pretty much neutral when it is in the water. It does really provide mass until you start taking it out of the water..

Here is a link that explains it way better than I can.....
http://h260.com/water_ballast/water_ballast_index.html
A number of water ballast boat owners have tried to knock he boat down on purpose and a few have succeeded for a couple seconds but as far as I know they have all popped right back up without even taking water into the cockpit. MacGregor does recommend not sailing with the pop-top up but there again I seen a number of people doing that with the boat in an extreme heeling attitude and everything is fine. We always have ours down sailing being the chickens we are
Sum
[FONT=Arial, sans-serif]============================[/FONT]
Our Endeavour 37
Our MacGregor 26-S Pages
Our Trips to Utah, Idaho, Canada, Florida
Mac-Venture Links