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post #17 of Old 03-05-2007
Join Date: Nov 2006
Location: Beacon, New York
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Making a powerboat self righting is easy and you donít need a keel or ballast. Think about how stability works. There are two types of stability, first is form stability and second is stability from weight. A log will roll or just sit wherever you place it until you add a weighted keel. Then it has stability from ballast. Tip the log and it will return to upright because of the weight. Or take a flat bottom boat and you have stability from shape. As the boat rolls the center of buoyancy moves outboard of the center of gravity so gravity makes the boat returns to the upright condition without adding any ballast.

If you want a self-righting powerboat without ballast you need to add volume up high so that when the boat is on her side on upside-down the volume is ďon the other sideĒ of the center of gravity and the volume will roll the boat to a point where form stability will take over. In other words make the cabin very high and narrow so it provides volume far away from the center of gravity. And if you make that volume large enough to float the boat while upside-down with the hull out of the water the boat canít stay upside-down very long because she is more unstable there then right side up.

Tank testing is easy and we do it all the time. But we donít use a tank. We tow models early in the day on the Hudson River and get all the information we need. There are well known relationships between models and full size boats and if you are interested I can point you in the right direction.

I will look in my copy of Professional Boatbuilder when I get back to the office and will let you know which issue the article was in. SNAME has publications on this subject and RINA has some great stuff about British lifeboat designs. You will need to read up on this before you go too far. Most text books on Naval Architecture donít have much information thatís useful for something like this because not many boats are designed for this. The papers published by SANME and RINA are a better resource for this.
All the best,
Robert Gainer

SNAME and RINA are Society of Naval Architects and Marine Engineers in America and The Royal Institute of Naval Architects in Britain
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