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

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

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

[davefromoregon]

How meaningful is this ratio when comparing two boats where the numbers are close together? ie 2.0 and 2.05?

[Paul L]

Originally Posted by davefromoregon

How meaningful is this ratio when comparing two boats where the numbers are close together? ie 2.0 and 2.05?

Meaningless difference.

[denverd0n]

Originally Posted by Scott k

Is this formula overly simplified?

ALL such formulas are overly simplified. Take all of them as a very rough guide and nothing more.

[Adelie]

Originally Posted by 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.

I think Wheeler has brought up a very important distinction here, capsize resistance is a single quality, distinct from stability, seakindliness, tracking ability, motion comfort, hull/deck/rig strength, angle of vanishing stability, downflooding angle and seaworthiness. Seaworthiness is really a somewhat subjective conglomeration of all the other qualities in a boat.

Originally Posted by 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.

Did I miss something? Jeff H keeps referring to plural formulas in the above post but I only see 1 mentioned anywhere previously in the thread. What is the second formula mention he is discussing.

Originally Posted by Jeff H

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

The research that went into the development of the CSF showed that a boat with it's mast removed has a higher peak righting moment, increased angle of vanishing stability and is MORE likely to capsize in breaking wave conditions. The mast compromises stability but improves capsize resistance.

This is the point where it became very obvious to the folks doing the research that capsize resistance is related but very distinct from stability. Roll moment of inertia is the primary means of resisting a capsize due to breaking waves, the stability curve play only a small part. Turns out a mast contributes about 1/2 of the roll moment of inertia of a boat.

The key distinction being capsize resistance to breaking waves is a dynamic event where mass and roll moment of inertia (meaning mass distribution) govern response and stability is a static phenomenon where weight, center of gravity and hull shape govern response. Yes the two are related and have affects on each other, but the relationship is complex conclusions about capsize resistance based on stability are specious.

Yes, the CSF formula is a vast simplification, the primary simplification is that weight and beam are used as a proxy for roll moment of inertia.

There are various ways to defeat the formula like removing the mast which would seem to be a good thing or building a very light boat with a heavy mast which would seem like a bad thing but would actually have improved capsize resistance. By and large most boat makers build heavy hulls with heavy masts and light hulls with the lightest mast that is economical.

Better would be to look at the actual roll moment of inertia of the boat but that is a difficult property to calculate or to measure.

So Jeffs idea of putting 500lb up the mast is just a good idea taken too far, in breaking waves 25-50lb up the mast would likely improve capsize resistance.

[wolfenzee]

Originally Posted by davefromoregon

How meaningful is this ratio when comparing two boats where the numbers are close together? ie 2.0 and 2.05?

It tells you nothing in comparing those two boat....but what it does tell you about both of those boats is that neither would be a good choice for offshore use.

[Polux]

Originally Posted by 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

Here you have a thread with 18 pages of discussion about that:

http://www.cruisersforum.com/forums/...ios-78528.html

Here you have a meaningful post from that thread:

Originally Posted by mitiempo

I don't believe many naval architects are using the capsize screening formula as it is only slightly useful when comparing similar types of boats - Alberg 30 (1.68)with Cape Dory 30 (1.67) for example. It doesn't take into account where the weight of the ballast is located, nor any other weights for that matter. Nor does it take into account ballast ratio. A heavily built boat with a low ballast/disp ratio could well have a better (lower) number than a strongly built but not overweight boat with a higher ballast/disp ratio.
For many of the lighter boats with bulb keels we see today it is entirely useless.

For example the Pogo 10.50 has a CSFormula of 2.57 but is Category A and has self righting ability well beyond many boats with a CSF or less than 2. Any formula that only looks at beam and displacement is very limited in usefulness.

As far as 1000lbs high up a mast yes an exaggeration. But if you take a boat with a CSF that is considered good, say 1.8, and load it up with gear on deck or anywhere above the waterline for that matter its CSF would, if weighed again be better than before it was wrongly loaded.

An actual righting curve showing the angle of vanishing stability is a lot more meaningful. ..

The capsize ratio does not take into account the boat CG and that is a main factor in what regards resisting capsizing.

Since the mid 90's all boats have stability curves that are needed to certify the boats in the EC. If you want to access a boat stability use those curves.