Capsize Ratio

[fred4936]

A fascinating discussion! Our boat has a ratio of 2.02, which by the numbers makes it borderline. Having sailed her in 30 kts, but nothing higher yet, I am pretty confident is her stability. This is also bay sailing. Offshore would be a different experience altogether. I do have an interest in doing the Marion- Bermuda Race; their requirements call for a stability index number that I see as a costly exercise to obtain. They do allow sistership's certificates, but Pearson only built 35 or so of the 34-2, so the odds of finding one are slim. So all this remains to be seen. But being a numbers oriented person, I find all this very interesting! I could always just sail to Bermuda and not worry too much about it, and make sure I have all the appropriate gear and that everything on board is ready.

[MJH]

Originally Posted by Lexi22

When I look at the Capsize Ratios of sailboats, the findings are strange. Catalina 315, for example, way over 2.0. How shoud I read this particular ratio, with what other ratios being taking into consideration?
Ex: Oceanis 30.1 and Dehler 29 score better than Cat 315, both way under 2.0, but does this make them more seaworthy than 315? What else should I consider here?

Being an informed buyer is always the preferred position to be in. It is wise to do the calculations when selecting your boat...you will better understand what you have bought. It is far superior to buying the sailboat with the salon arrangement that your spouse likes alone.

In doing so you should look at all the relevant data and ratios and review them in conjunction to your intended use and waters to be sailed...no boat does all things well. It is a good place to start but other things are important as well such as tankage volumes, material, and their location; keel type, material, and depth; rig type (sloop, cutter, ketch, masthead or fractional); etc.

The availability of saildata.com and sailcalculator Pro makes that job less tasking these days. In my day I built a data base of 3,000+ sailboats in different sailing configurations (sloop, cutter, ketch, yawl where applicable) and computed the SA/DISP, DISP/LWL, COMFORT, CAPSIZE RISK, V HULL, and a couple of other ratios to consider. A comparison of the boats you are interested in is the important part...that is where the tradeoffs come into consideration...are you willing to trade some speed for a more comfortable and safer crossing...will you have to add additional tankage (usually on deck) because the boat's tanks are inadequate for the planned voyage...these are just some of the questions you should ask yourself.

Good Luck.

~ ~ _/) ~ ~ MJH

[Lexi22]

Originally Posted by Mike OReilly

Sorry Lexi, I missed the "my" part -- My bad .

All good. No need for apologies; I was giving you a hard time.

[laila23]

https://sailboatdata.com/sailboat/contessa-35
I've compared many similar new sailboats to Contessa 35, and all of them were lack of at least one of the ratio levels. What's the secret ratio that makes this boat great?

[Adelie]

Originally Posted by Mike OReilly

That's cool. I wonder how they do it. It looked, and stayed, perfectly stable upside down. And then it finally started to go over. The keel didn't seem to move to my eye. Was there some other mechanism being used?

Standard monohulls with capsize ratios below 2.0 are inherently unstable when upside down, so they can't remain in that position. At least that's the theory.

Capsize ratio is NOT about inverted stability and how long it would take to right itself.

Capsize ratio approximates roll moment of inertia which resists capsize in the first place and beam which helps drive the capsize.

Tank testing after Fastnet ‘79 showed that a hull without a mast was more likely to capsize in a breaking wave than the same hull with its mast even though the hull with mast had less righting moment and a smaller AVS. This was counter intuitive: more stability does not make the boat more capsize resistant, they are not the same thing. This was the point where they realized that stability is a static phenomenon and capsize is a dynamic one.

All sailboats will have a little bit of stability in the inverted position, none are inherently unstable inverted. AVS is a good predictor of how long a boat might take to right itself assuming the wave that capsized the boat wasn’t a complete rouge and there are plenty of waves slightly smaller to disturb the boat.

The EU STIX method is bogus. It uses stability curves which are based on static conditions. Their argument is that the more area under the curve the more energy it takes to roll the boat over. In static conditions that is correct. And under normal sailing conditions the loads vary but can be treated as approximately static. A boat with a wider beam will have a lot more initial stability and a lot more area under the curve. And the boat will be more prone to capsize than a similar boat, same length, displacement and roll moment of inertia but narrower beam.

If you want to read up on this:

https://www.amazon.com/Desirable-Und.../dp/0393033112

https://www.amazon.com/Fastnet-Force.../dp/0393308650

https://www.amazon.com/Seaworthiness...s=books&sr=1-4

[Lexi22]

Originally Posted by Adelie

Capsize ratio approximates roll moment of inertia which resists capsize in the first place and beam which helps drive the capsize.

Yes, some of the replies have been confusing me. Capsize ratio is about before the capsize happens, my understanding. Seems to me as how good a sailboat will be able to resist it, lower the ratio, better it gets - of course considering other factors as well... I hope I'm on the right page!?

[sandy stone]

I completely agree that capsizing must be treated as a dynamic event, as shown by the dismasted boat example. That immediately makes the situation so complex that any mathematical analysis can only be an approximation. I think it's a little presumptuous to claim the STIX number, developed over years of study by smart people with more expertise than any of us, is bogus. It obviously isn't perfect, but I think it's about the best we can do at this point in time. (And way, way better than the Capsize ratio)

[Mike OReilly]

Originally Posted by Adelie

...If you want to read up on this...

Thanks Adelie, interesting stuff. It's beyond my pay grade to assess the accuracy of what you say. I don't really want to read up on it, so I'll take your word for it .

[Adelie]

Originally Posted by sandy stone

I completely agree that capsizing must be treated as a dynamic event, as shown by the dismasted boat example. That immediately makes the situation so complex that any mathematical analysis can only be an approximation. I think it's a little presumptuous to claim the STIX number, developed over years of study by smart people with more expertise than any of us, is bogus. It obviously isn't perfect, but I think it's about the best we can do at this point in time. (And way, way better than the Capsize ratio)

The capsize ratio addresses 1 particular aspect of seaworthiness, there are whole bunch of others.

STIX attempts combine a bunch of aspects of seaworthiness into one convenient number. I've worked my way thru the formula. It is very much formula by committee with a heavy influence by commercial boat builders.

The basic form seems to be a weighted average of the various pet ideas of the designers involved. Individually some of the ideas have merit but this is a really poor approach. An almost fatal flaw in one area can be made up by "good" characteristics elsewhere. As an example, lets say the boat down-floods and sinks at 80* heel but has such a "good" stability curve it still gets the A rating for offshore sailing. Weighting all the aspects is the wrong approach, the lowest rated aspect should be the rating, that is the one most likely to kill you.

In reviewing what goes into the STIX formula, nowhere could I find anything that evaluated roll moment of inertia. It might be in there, but I looked hard for it. And the research after the Fastnet '79 catastrophe made it clear that roll inertia was very important to capsize resistance. I get that it is hard to calculate and hard to measure, so is the stability curve of a boat but that is included.

Regarding commercial influence on the STIX rating, let's start with the fact that area under the righting moment curve is a significant component. How is this important? First stability is a static phenomenon, so why is it included in a a discussion of seaworthiness? Stability has to do with how much sail can be carried for a given wind speed and when you need to reef. More stability means you can have more sail up longer and go faster in general. To get more area under the stability curve you can put more ballast in the boat to make the peak higher and extend the AVS which costs a fair amount and which doesn't help sell boats because heavier means less speed and higher cost to build. Or your can make the boat wider which probably has similar speed to what the boat was originally and is probably costs the same as adding ballast but hey, there's a lot more room inside which makes the boat more inviting to buyers. Does the beam help capsize resistance? No it hurts it, increased beam creates a longer lever arm for breaking waves to act on. Why include the area under the righting moment curve? Because it sounds good and scientific and obscures that stability really doesn't equate with capsize resistance. What does it equate with? Wider boats that sell better.

Don't take my word for it. Read this by the European Naval Arch Guillermo at https://www.boatdesign.net/threads/s...-ratios.13569/
Who cites other major NAs with reservations about STIX.


I work with building codes, parts of it work very well, business and trades input creates something that isn't perfectly safe but pretty safe and is economical to implement and there are parts of it that don't follow normal mathematical rules but can't be changed because "that's the way it's always been done". I see how codes are created, it's like making sausage. STIX is the same.

Am I being presumptuous in dismissing STIX? Don't care, I've looked at what goes into the number and can see huge fatal flaws in it and I have an educated opinion about this. So what if smart people were involved, smart people with an agenda create things that serve their own interests all the time. It was presumptuous of the committee that came up with it to think that one number can adequately reflect seaworthiness. I get that people want one simple number. Sorry, sailing vessels operating in real world conditions is a very complex system and reducing everything to a single number seems like hubris to me.

[Pandor]

Adelie,

I’ve looked for a source of SI and can’t seem to find. I’m particularly interested in the SI of the Oceanis 390 (recall Ocean Madam). Of course I would need other vessel’s SI’s for comparison. Do you or others have a good reference source?

[proudsailor]

I enjoyed the hell out of this thread! Learned a lot and will consider these things in future boat buys. Also enjoyed comments that showed what a hilarious bunch we are so vested in our passion (take Mike for an example) that we've got graphs and charts and videos!

Would think that CR would also be a consideration only if my cruising took me way off shore or crossing oceans. I've had two Catalinas that I've sailed in some slop both on the coast and in the Chesapeake. Perfect for the sailing I was doing. Great boats that pointed well and held up to rough weather.

Was in another new production boat (no longer being made) coming down the coast of New England in crappy weather and it was so poorly constructed that had standing rigging issues, keel issues leading to significant water intrusion, overheating issues. Had greater fear of keeping her from falling apart and sinking underneath us.

I guess how much consideration you give to CR and other seaworthy measures is dependent on how you use the boat. Could put a motorhome on large pontoons on a lake and never worry. Oh wait, some already do that....

[Adelie]

Originally Posted by Pandor

Adelie,

I’ve looked for a source of SI and can’t seem to find. I’m particularly interested in the SI of the Oceanis 390 (recall Ocean Madam). Of course I would need other vessel’s SI’s for comparison. Do you or others have a good reference source?

Oddly enough, I don’t know many, just the 2 listed in Guillermo’s post at boat design.net above. I believe that post or another one by Guillermo is where I got the excel spreadsheet with the STIX formula. This was several years ago.

[ohthetrees]

Originally Posted by thomm225

Unfortunately, these days big boat factories are pouring out sailing cruisers that are neither
directionally stable nor have balanced hulls. Still they are being sold as ocean cruisers since
their static swimming-pool stability has been found (or calculated) to be good enough. You’ll
easily recognise these boats by their sharp, vertical bows, wide sterns and huge steering
wheels to cope with the weather helm. In a pinch these floating caravans could be used for
marina-hopping, mostly in sheltered waters - and always with an eye on the weather
forecast. You have been warned.

I don't think it is as bad as all that. My Hanse 505 has 33000 nm on it, been from the Med to the South Pacific, back to the Pacific NW, currently in Mexico. Unless you call that Marina Hopping!

[Sailing Hanse37]

Originally Posted by tenchiki

On the other hand, there aren't too many boats that have to prove that they can right themselves from inverted before acceptance: the new Hugo Boss:

Without mast ? Is this representative ?

[thomm225]

Originally Posted by ohthetrees

I don't think it is as bad as all that. My Hanse 505 has 33000 nm on it, been from the Med to the South Pacific, back to the Pacific NW, currently in Mexico. Unless you call that Marina Hopping!

I don't call it anything.

I figured I'd just do a search for CR after there seemed to be differing opinions posted and that is one of the quotes that came up from a sailor from Norway I believe.

https://www.junkrigassociation.org/R...%20cruiser.pdf

Hall of Fame - Arne Kverneland - PDF Free Download

Plus you can sail thousands of miles on most any old sailboat as long as you are blessed with good weather and the boat is in decent shape. (and the Skipper has some experience)

Steven Callahan completed an 1800 mile voyage in 76 days in his Avon 6 man life raft after his 21' boat sank about 400 miles East of the Canary Islands.

https://www.amazon.com/Adrift-Sevent.../dp/0618257322