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Old 14-03-2012, 15:37   #1
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Capsize Ratio's

I have a general Question regarding the importance of Capsize Ratios? Are they really important and have any real meaning? I ask this question because after reviewing 27 foot sailboats, I have noticed they vary greatly. Example: the Catalina 27 varied from 1.79 to 2.1... The Norsea 27 seemed to have the best ratio of 1.59. I did not see a relationship between beam or keel type (perhaps I missed it). The MacGregor 26M was 1.9 while the MacGregor 26S was 2.24... A Tartan 27 is 1.77 where as my Dana 24 is 1.72. A Cheory Lee Offshore 26 is 2.14..

The website I obtained these numbers states under 2.0 is considered safe in rough conditions- then states the ratio is controversial LOL.. Just wondering how valid this ratio is and why it varies so greatly?

Thanks
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Old 14-03-2012, 16:06   #2
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Re: Capsize Ratio's

Yes it's controversial. There's unfortunately no single formula for seaworthiness and such. BR Teddy
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Old 14-03-2012, 16:32   #3
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Re: Capsize Ratio's

Thanks Teddy, I should have done a search bc I found some threads here on this topic. I never really understood these numbers. Thanks Again
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Old 14-03-2012, 16:51   #4
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For an excellent discussion of capsize ratios look at Steve Dashew's 'Cruising Encyclopedia'.
Go to setsail.com to see all books and dvd's
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Old 15-03-2012, 19:26   #5
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Re: Capsize Ratio's

The capsize ratio you are looking at is pretty meaningless. All it goes by are beam and displacement. You can find it for any boat you know these numbers for here Capsize Formula
To describe how useless it is if you take any boat and add 1000 lbs on deck, or high up the mast for instance, the number gets lower!

A much more useful discussion of stability can be found here Sailboat Design and Stability
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Old 15-03-2012, 22:10   #6
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Re: Capsize Ratio's

You might find this site helpful...
Ted Brewer Yacht Design
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Old 15-03-2012, 22:26   #7
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Re: Capsize Ratio's

useful? really?
dude, if the wind blows hard, your boat will lean over away from the wind. if it's only the wind you will be ok because the wind at best will push you about 87 degrees away from it's direction.
If your boat design is (and i only speak about mono hulls here) faulty you will drown, the boat will sink and someone on this forum will talk about how much worse it could/would have been if you were in a cat or tri....
If on the other hand you stuffed too much crap down below OR your boat has 'issues' with heeling then you will go swimming.
cheers.
bluetriguy.
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Old 15-03-2012, 22:28   #8
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Re: Capsize Ratio's

useful? really?
dude, if the wind blows hard, your boat will lean over away from the wind. if it's only the wind you will be ok because the wind at best will push you about 87 degrees away from it's direction.
If your boat design is (and i only speak about mono hulls here) faulty you will drown, the boat will sink and someone on this forum will talk about how much worse it could/would have been if you were in a cat or tri....
If on the other hand you stuffed too much crap down below OR your boat has 'issues' with heeling then you will go swimming.
cheers.
bluetriguy.

p.s.
Or they will go on and on about 'capsize ratio' leaving out the part how on my tri or betty on her cat will capsize but NOT sink and be severely dehydrated but alive after living in an upside down hull for a couple of weeks.
really,
cheers
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Old 15-03-2012, 22:30   #9
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Re: Capsize Ratio's

uuhhhh that was a bit severe. but basically truthful if you actually read this forum's posts....
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Old 16-03-2012, 04:33   #10
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Re: Capsize Ratio's

A much better parameter is the angle of positive stability. It is the angle that a boat can heel to and still have a positive righting moment. After that, the boat turns turtle.

Good blue water boats have about 140 degrees of stability: Pacific Seacrafts, Cabo Ricos, etc. Coastal cruising boats are in the 120 degree range.

David
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Old 16-03-2012, 05:20   #11
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Re: Capsize Ratio's

Wind will very rarely capsize a mono, it can happen , but would need to be a huge monster of a freak gust to get enough momenta to completely roll a mono, a cat is different, they should be reefed very early.

Waves are what you need to watch and it is breaking waves!

Capsize from breaking waves is directly proportional to length and beam, ie bigger is always better. The Stability curves don't matter a hoo ha to a breaking wave, it is immaterial how stable the boat is , a breaking wave of x size will capsize a boat of y size regardless of its stability curve, However a boat with a very small area under the negative part of the stability curve will self right much quicker than one with a large area under the curve.

There are videos kicking about form an english university tank model test, where they tested various cats dagger boards up /down, various beams, various monos and they found all that was really important was lenght and beam, also dagger boards could trip cats when abeam to the swell.

having a boat with a good positive stability will not stop you being rolled by breaking waves , but you will spend a lot less time upside down in one and take on much less water.
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Old 17-03-2012, 15:56   #12
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Re: Capsize Ratio's

The Capsize Screening Formula is a quick and dirty formula for indicating whether a naval architect should do more analysis of a boat's capsize resistance.

The more involved analysis looks at the roll moment of inertia of a boat to determine its susceptibility to capsize due to wave action. The CSF uses displacement and weight together as a proxy for roll moment of inertia.

Stevensuf's statement that 'bigger is always better' in regards to length and beam is incorrect. Consider a Westsail 32 vs a Santa Cruz 37, the SC is longer and wider, but which is more likely to roll over in a storm? Or a WS32 and an SC40? Or a Pearson Triton 28 and a Kiwi 35?
WESTSAIL 32 sailboat specifications and details on sailboatdata.com
SANTA CRUZ 37 sailboat specifications and details on sailboatdata.com
SANTA CRUZ 40 sailboat specifications and details on sailboatdata.com

TRITON (PEARSON) sailboat specifications and details on sailboatdata.com
KIWI 35 sailboat specifications and details on sailboatdata.com

Mitiempo tries to use an absurdity to try to make his point. The formula is intended to be used on a normally ballasted monohull in sailing condition. An extra 1000lb up the mast of most boats less than 45'-50' would roll them over at the dock, that's hardly sailing condition. This would be akin to saying a Toyota MR2 sportscar is bad car because it doesn't have the ground clearance to go off-roading. In reality it is a fine car for going fast on paved roads. Likewise if you use the formula in a manner outside what was intended you get bogus answers.

By way of comparison, consider the alternative: remove the mast. The removal of that weight would increase the peak righting moment of the boat and increase the angle of vanishing stability. Physical testing has shown that boats without their masts are more likely to capsize in breaking waves contrary to the intuitive expectation.

So then the question becomes what is the appropriate amount of weight aloft for capsize resistance? 1000lb is obviously absurd, but no weight aloft is not that good either. The answer is complex and involves tradeoffs between capsize resistance and ability to carry sail.

I have read that in centuries gone by hoisting small to moderate weights into the rigging was a way to prevent capsize and calm the motions of boats. I do not recall where I read this, I do not know if this is apocryphal. The only mention I can find currently is in Richard Henderson's book on single handing where he indicates on pg 166 that Voss and Waller used anchors hoisted aloft to ease their boats' motions.

Once again, the CSF should be used as a pointer as to whether more in depth investigation is in order, not as a be all and end all evaluation of capsize resistance.

Generally the best way to use the formula for the lightship weight with engine and sails aboard but no water, fuel or other gear. This should give you the most conservative number. Generally as you load the boat you will keep the weight low so roll moment of inertia increases with little or no decrease in general stability. If you are not loading the boat this way, you should give some thought to doing so.

I would not expect the CSF to work with the Macgregor boats, they rely on water ballast and the assumptions about ballast location and roll moment of interia that went into the CSF do not apply.

There are actually 2 distinct 27' Catalina hulls, the Cat27 and the Cat270. The 270 is wider and lighter which would give a higher CSF number.
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Old 17-03-2012, 17:39   #13
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Re: Capsize Ratio's

Yes, the capsize ratio is very quick and dirty. But if you think about it, turning turtle is not the worst thing that can happen. The worst thing is not rolling back up.

You want a boat that's stable when the mast is sticking straight up in the air, but very unstable when the keel is sticking stright up in the air. That's why cats become expensive life rafts when inverted, and why those modern monohulls with big fat (and flat) arses scare me.
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Old 17-03-2012, 18:01   #14
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Re: Capsize Ratio's

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. But while that shows the ability to recover from a wave induced capsize it has no real bearing on capsize from a breaking wave in the first place if the wave is large enough.

And size does make a difference. From US Sailing:
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Old 17-03-2012, 21:56   #15
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Re: Capsize Ratio's

Quote:
Originally Posted by mitiempo View Post
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.

As previously stated the CSF is a pointer for a more detailed analysis, not authoritatively indicative on it's own.

For many of the lighter boats with bulb keels we see today it is entirely useless.

I would expect the CSF to still be reasonably useful with bulb keel boats unless the bulb was on an exceptionally long fin. The change in center of mass of the ballast relative to the center of mass of the whole boat does not change that much compared to a normal boat.

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.

The Pogo-10.50 does have a fairly poor CSF, but having a very deep keel (9+ ft) with a bulb, it likely has a roll moment of inertia higher than most boats of similar length and displacement. I would not consider this boat to be typical.

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.

The formula is intended to be used to evaluate the boat as designed, not as it is being used or misused.

An actual righting curve showing the angle of vanishing stability is a lot more meaningful. But while that shows the ability to recover from a wave induced capsize it has no real bearing on capsize from a breaking wave in the first place if the wave is large enough.

And size does make a difference.

Yes size does usually make a difference, it is normally a good predictor of increasing roll moment of inertia.

From US Sailing:
Normally I have a fair amount of respect for USSailing, but in this case there are a number of problems with the article.

First the article conflates stability and capsize resistance. Stability is really a large topic dealing with hull shape and center of mass and generally represented by a static stability curve. Capsize resistance is a very specific topic related to dynamic properties. While aspects of the two are related, they are not the same topic.

Also the article states that the 1998 Sydney-Hobart storm was a Typhoon. Not so.

Given the obvious inaccuracies in the article, I am disinclined to trust the rest of it.

One of these days I will get hold of the Alard Coles book, I would like to track down the research cited and read it in the original.
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