Capsize ratio

[lancercr]

Hi, I have a Lancer 28 with a capsize ratio of 1.76. This figure is very pleasant for me and my family, just in case. But I did some research in bigger boats around 40' and I discover that in many recent-new models the ratio goes bigger than 2.0 (with the risk of capsize). Mi question is: bulb keel and very low ballast make this ratio useless?

[Pblais]

Mi question is: bulb keel and very low ballast make this ratio useless?

The ratios are not always everything. They are a only screening tool. They may not always be applicable. In this case it compares beam relative to displacement. Nothing more. You seem to feel the design of the keel may make a difference. I'm sure we could come up with a lot of things that would make a difference.

If you were to take a list of boats and compute all the ratios you might make generalities between them but the numbers are not the whole picture.

This number is a traditional number for assessing offshore boats. The guideline was that it is an issue when the value is more than 2 and going offshore. If it was 1.9 vs 2.1 what would you say? It's just one number comparing just two factors. There really isn't an "add on" formula to compensate for everything you could consider. You compute the number and just take it as just one factor.

Personally, if comparing boats and the numbers are even the slightest bit close you forget about.

[delmarrey]

Conditions will determine what will actually capsize a boat. Where one may capsize in surf, may not capsize in a blowdown. It's hull design vs. ballast. Some keels (winged) are treacherous on a tall wave where others will slide down the side of a swell with ease allowing the boat to stay more up right.

Ballast, lower in the water does create a lower COG but then again if it has a full keel the drag is higher. The race boats use the long keel with the bulb but are usually lighter in weight for their size. It's more for wind resistant rather then off shore stability. They're not good on steep close waves.

Cat's for instance seem to be able to take steep angles UNLESS they have dagger boards down or deep skegs..................._/)

[smm]

In the US, I don't think that there's any standard definition to any of the inputs to this formula or the stability curve. Euro boats certified for offshore work at least have to pass a heel angle test which verifies the stability curve at one point, but in the US there's no guarantee that the numbers actually mean anything at all. For example, a light boat with little tankage and aggressive assumptions about displacement might look great on the capsize screening formula, but when you factor in 100 gallons of fuel/water lashed to the rails it could be much less safe than a similar boat with sufficient tankage at bilge level.

I don't think that looking at boats with a "capsize ration less x" or a "vanishing stability angle greater than y" is going to give you meaningful results. These numbers might be useful to compare two very similar boats; the deep keel and the shoal keel model of the same boat, for example.

-Scott

[Pblais]

Scott,

Of course there are no standard definitions. Who would write them. Mostly because they can't tell all there is to know. You can't do it by computations. Mostly because it's not the boat that matters.

[GordMay]

See the thread “Sailboat Design Ratios":
http://www.cruisersforum.com/forums/...ead.php?t=1197

There are links to several excellent sources.

There’s much more to “stability” than a good CSF* number.

*From Ted Brewer: CAPSIZE SCREENING FORMULA (CSF): Some years ago the technical committee of the Cruising Club of America came up with a simple formula to determine if a boat had blue water capability. The CSF compares beam with displacement since excess beam contributes to capsize and heavy displacement reduces capsize vulnerability. The formula is the maximum beam divided by the cube root of the displacement in cubic feet; B/Displ.333. The displacement in cubic feet can be found by dividing the displacement in pounds by 64, of course.

The boat is acceptable if the result of the calculation is 2.0 or less but, of course, the lower the better. For example, a 12 meter yacht of 60,000 lbs displacement and 12 foot beam will have a CSF Number of 1.23, so would be considered very safe from capsize. A contemporary light displacement yacht, such as a Beneteau 311 (7716 lbs, 10'7" beam) has a CSF number of 2.14. Based on the formula, while a fine coastal cruiser, such a yacht may not be the best choice for ocean passages.

[lancercr]

Ok, thank you for the insight of your analisys. First of all, excuse some bad use of english, but I´m not american.
In fact I always consider that in any field you can't have a single factor which explain multicausal problems. The basis of my question is, of course, safety (the relative safety you can expect in the sea), and it depends also of chance, but basically is the result of many factors, including the experience and knowledge of sailors, the prudence of your decisions and the analysis of sea and wheather conditions.
But the boat you ride is an important factor and there are a discussion, as I saw in internet, about the value of these ratios and or course is only a point of reference, not the final word. In my view, for my lack of experience and knowledge, they seems a strong reference about the boat capacity to handle strong seas. But this idea seems weak when I saw also that Benetaus, Bavarias, Hunters, Catalinas, etc exceed the sacred 2.0
I have a little boat which make me very very happy, but I sail in a relatively safe environment (the Gulf of Nicoya and the Pacific Coast in Costa Rica), so my question may be a rhetorical one, but when I must face winds and waves bigger than ussual with my child and wife (my responsability), I find peace of mind considering that my little boat can heel and move without capsize at least a breaker bigger than 15 feet hit us in one side. And if this happen (a very little possibility here and not if I have something to do about) my boat can handle that and right itself.
So thanks again.

[Weyalan]

After the disastrous Fastnet race of 1979, the subject was revisited in some depth by the RORC and the stability/righting tests for racing yachts was tightened up considerably. Many IOR designed boats ended up having to put additional lead in their keels to meet the new righting requirements. Nevertheless, most righting calculations are based on bare boat / empty tanks, etc. Sensible distribution of baslast is important (simple rule - the lower the better). Having said all that, good seamanship is probably the single most important factor in preventing knockdowns.

[Pblais]

Many IOR designed boats ended up having to put additional lead in their keels to meet the new righting requirements.

But the capsize ratio takes no account for lead in the keel. The number in question is afterall not the number we are debating. There are simple numbers and complicated numbers. It's to say one is more significant, but racing rules have over time been given to great amounts of change.

I tend to agree that seamanship includes more than numbers can account for.

[Alan Wheeler]

Yes and part and parcel to the lead in the keel is, thus the center of gravity is not in the same plane as the waterline. And as the boat rolls, so does the position of center of gravity change. Hence an actual stability curve is not a perfect curve. Although there are many other resons why it is not a perfect curve to add into the mix as well. So I really can't see how that formulae gives an accurate enough indication to be usuable.

[GordMay]

RORC Stability and Safety Screening Indices – SSSN and STIX
http://rorcrating.com/ir2000/irc/stix.pdf
And:
http://www.rorc.org/content/view/121/95/
And:
http://www.rya.org.uk/NR/rdonlyres/4...ilityIntro.pdf

[Jeff H]

I know that I have explained this on this forum before but here it is again, both of the capsize screen formula and motion comfort index 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 are very similar but are totally useless and misleading in most cases.

Neither formula contains almost any of the real factors that control motion comfort or stability. Neither formula contains such factors as the vertical center of gravity or buoyancy, 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, nor does max beam, which in this formula is measured at a single point on the deck.

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

[Alan Wheeler]

Great lesson Jeff. It's kinda what I had in my head but had no idea how to explain it in the excellent way you did. Thanks.

[kevlarpirate]

This formuls has great value and should be taken with a dose of salt!
The quote by Ted Brewer, a Naval Architect with over 270 designs to his credit is accurate. The CSF is a simple formula but one that yeilds a first order answer. It was kept simple for a reason. It basically says that a heavier narrow boat has substantially more area under the positive side of the stability curve than the negative side, That helps hugely in the static world. In the dynamic world, narrower beam is less tripped by big beam-on waves and more mass increases roll inertia. all good things. Motion comfort is also greatly enhanced. I have experienced this many times. Most people in flimsy
wide beam, low ballast boats cant admit this. As an anicdote, no boat with ovet a 3:1 length to beam ratio was rolled in Fastnet 79. Again the formula applies to most boats from Farr's and J-35's (worst offenders) to 12 meters on the other end. Just about every production boat known falls between these two extremes. Yes the formula can be corrupted by hanging 500 lbs from the masthead, but that does not in any way render it useless in the real world.

[Jeff H]

I cannot see how anyone can see any utility in the CSF. At least according to an earlier post where someone showed him a quote similar to mine above, allegedly even Brewer has disowned it.

But to make my point even more graphically I suggest that we compare two comparatively modern boats that numerically have a lot in common. I suggest that we compare the Farr 38 and the Catalina 380. Under the CSF the Catalina would seem to be substantially the more stable, and yet if you compare full volume analysis of LPS the Farr has an LPS something over 120 (not to be mistaken for an IMS LPS which is only 107 for the Farr) vs the Catalina which I have seen quoted as being is down around 115 degrees. But the LPS does not tell the whole story. The Farr has a significantly higher ballast ratio, shallower canoe body, and a narrower beam than the Catalina, all of which should give the Farr a much higher stability advantage. Yet there is that old CSF saying the Catalina should be more stable. If someone bought the Catalina 380 over the Farr 38 expecting greater stability, they would have been sorely mislead.

If you actually read the various post storm disaster studies, you would find that beam and displacement are comparatively small factors in actual capsizes. The single common factor that all of the studies show seems to be waterline length. Ballast ratios, (more specifically vertical center of gravities relative to vertical center of buoyancy), and dampening being the only identified secondary factors that seem to impact capsize.

None of these, except length are actually in the capsize screen formula.

Respectfully,
Jeff