Ballast ratios

[john_travers]

Current boat 47% ballast ratio but only draws 1 metre.
Possible purchase 28% ballast ratio boat that draws 1.5 metres.
Not sure if I understand this correctly but could the keel be thought of as a lever?
In which case a deeper but lighter keel will have exactly the same righting moment as a heavier but shallower one.
i.e. 1 lb at the end of a 10 foot pole exerts 10 ft lb force. The same as 10 lbs on a 1 foot pole. Or am I completely wrong? Will we both be sat on the windward rail with the sails reefed as soon as the wind picks up?
Anyone else moved from a Hi ballast ratio boat to Lo ballast ratio?
Comments much appreciated.

 

[Guest]

This is a highly complex area. C.A Marchaj's superb book "Sailing Theory & Practice", pub. by Dodd, Mead & Co, New York, 1964, Section 3, pp334 on, is a good place to start. The website abebooks.com will have second-hand copies. Be prepared for surprises. For example, if you don't want to sit out so hard, lift up your centreboard. In lay terms, it's the keel you trip over. The practical solution is to take the boat for trial sails in a good breeze before you buy it. Friends of mine did just this recently, and it only took a few minutes to persuade them that the steel hulled vessel ( with steel decks) they were considering buying was too tender. Good luck!

 

[Plum]

Ballast ratio is just one measurement. It tells you nothing about stability, righting moment, ability to carry sail area, etc. You are right to consider keel mass and the depth of that mass as this is a measue of righting moment. Form Stability is another important factor and is the stability derived from the shape of the hull. This is how many light hulls with little ballast get their stability from, but once the hull heels beyond a certain point Form Stability does nothing for you, only a mass at the end of a deep keel will stop the boat falling on it's side. In reality most boats use a combination of form stability and righting moment from a balast keel to stop them from falling over when the wind blows on the sails. Use the figues as a guide only and make your judgemnets based on sailing the boat.