capsize ratio

[Scott k]

CAPSIZE RATIO = BEAM / (DISPLACEMENT / 64) 0.333

Is this formula overly simplified? Wouldn't keel design have a significant influence? IE. full keel vs.fin, amount of ballast, lead vs.iron, placement of ballast and etc.? Furthermore, what effects would heavily provisioning for an extended cruise have on this formula?

Scott

[Kai Nui]

I am guessing you got some info from the link that GORD posted the other day. I checked the numbers on a couple of boats I am familiar with, and found them to be highly suspect. The site does note that the numbers are generic, and cites the sources. Yes the capsize ratio would be affected by the factors you mention. Although the numbers on the site are based on the designed weight and hull, adding 2000 pounds of equipment and provisions will certainly change the numbers.
It is a good starting reference.

[CaptainK]

Scott k,

I totally agree with kai. When you add weight or subtract weight. The capsize ratios change.

Hope you find what you're looking for.

Regards,

Kevin
CaptainK

[Catamount]

This capsize ratio was only intended to be a preliminary screening tool (see "Desirable and Undesirable Characteristics of the Offshore Yachts" edited by John Rousmaniere, published in 1987by Norton). Values less than 2, probably OK for "offshore" sailing, values of 2 or greater -- a more detailed stability analysis would be warranted.

Obviously there is a lot more to determining stability and resistance to capsize than just beam and displacement, such as the distribution of that displacement (vertical center of gravity), etc...

Regards,

Tim

[Jeff H]

I know that I have explained this on this forum before but here it is again, both of these formulas were developed at a time when boats were a lot more similar to each other than they are today. These formulas have limited utility in comparing boats that otherwise are very similar.

Neither formula contains almost any of the real factors that control motion comfort or seaworthiness. Neither formula contains such factors as the vertical center of gravity or bouyancy, neither contains weight or buoyancy distribution, and neither contains any data on dampening all of which really are the major factors that control motion comfort or likelihood of capsize. Weight alone has no bearing on motion comfort and stability.

I typically give this example to explain just how useless and dangerously misleading these formulas can be. If we had two boats that were virtually identical except that one had a 500 pound weight at the top of the mast. (Yes, I know that no one would install a 500 lb weight at the top of the mast but teak decks, heavy decks, wooden or steel spars can easily have that kind of impact.) The boat with the weight up its mast would appear to be less prone to capsize under the capsize screen formula, and would appear to be more comfortable under the Motion Comfort ratio. Nothing would be further than the truth. That is why I see these formulas as being worse than useless.

Jeff

[Scott k]

I was just curios about the formula since it seemed just too simple and does not take into account any factors other than disp. and beam.

.........so the bottom line is that I would be better off to forget these ratios and focus
on those boats that have PROVEN themselfs for the type of sailing I'll be doing.

Thanks guys
Scott

[Kai Nui]

Yes and no. This is a good starting point. You can eliminate boats with over a 2.0 or even in the upper range, using this formula. After that, you have a much narrower field to select from.

[Alan Wheeler]

I think it's important to throw in here, that capsize stability is different to roll/motion stability. Don't consider capsize stability as a measurment to what a boats movement will be.

[BC Mike]

Two boats I am familiar with are my own Tanzer 8.5 and a C&C 29. I am sure the capsize screening test could not tell you which of these boats can carry sail better and which one will capsize first. Assume that both boats wiegh the same and the keels weigh the same. The C&C has the majority of the wieght higher in the keel, while my boat carries the weight low. The C&C has a taller rig with more weight up high. Consequently it is a faster boat in light to moderate breezes, and we sail over the top of them when it blows, and we stand up better. He has the better boat for Wednesday night races, I have the better boat for adverse conditions. We need to do research on the boat in mind, for the conditions we want.
Michael