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Go Back   SailNet Community > General Interest > General Discussion (sailing related)
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  #1  
Old 01-19-2007
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Capsize Ratio

I was reading a thread early on today about capsize ratio and it made me thank. What is a good to average capsize ratio? My cat. 22 has an 2.26256 capsize ratio. Is that good or is that average or is that really bad and I need to thank about a new boat for the family.

Any thoughts on this.
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  #2  
Old 01-19-2007
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It really depends on where you'll be sailing the boat, and under what conditions. A capsize ratio of 2.26 is probably just fine if all you're doing is daysailing and taking short coastal trips on relatively protected waters, where you can get out of bad weather quickly. For offshore use, the number is too high, as it is generally recommended that you have a CR of less than 2.0 for bluewater use, and the lower the better.
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  #3  
Old 01-19-2007
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Lake Sailing

For now we are just doing lake sailing. As we advance in our sailing the wife and will be looking to some bluewater sailing(It has been a dream of my for a long time). Right now we are get her(SunnySideUp) in ship shape for the season. Get a few years out of her and move to bigger bluewater style boat. I have been looking at the Tartan yachts for the bluewater adventures. I was wondering about some other sailboat manufacturer of coastal cruisers.

Any thought on this.
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  #4  
Old 01-19-2007
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The capsize screen ratio tells you absolutely nothing about the stability of a Catalina 22. I know that I have explained this on this board many times before but here it is again. You hear a lot of talk about the capsize screen ratio and Motion Comfort Index but neither the capsize screen ratio and Motion Comfort Index provide any useful information about stability or motion comfort. Both of these surrogate 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 except in comparing boats that in almost all ways 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 which collectively really are the major factors that control motion comfort or likelihood of capsize.


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 1000 pound weight at the top of the mast. (Yes, I know that no one would install a 1000 lb weight at the top of the mast.) 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, especially so in your case where you are dealing with a moderately heavy 22 footer with minimal stability.

Respectfully,
Jeff
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Old 01-19-2007
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Old 01-19-2007
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Capsize ration is simplistic and not very efficatious.

Jeff H. is absolutely right. This formula does not take into account hull form or center or mass of center of effort. Mathematically it is a complete waste of time to screw with.

One could write a formula that would give good ideas about stability, but one would have to input discriptive formulas for hull lines which would be very difficult to obtain or calculate.

Last edited by Chaunclm; 01-19-2007 at 10:26 AM.
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For an explanation of the formulas, go to this page and scroll to the bottom of the page.
http://www.tedbrewer.com/yachtdesign.html
The last two explanations will answer your question. They are brief, to-the-point, and make use of the term "relative" in discussing boat performance. "The intention is to provide a means to compare the motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch." People seem to forget that point. It should be noted that I do not dismiss Jeff's example of lead weight at the end of a mast, but while that isn't gonna happen, some cruisers can get into trouble by counting on a given capsize ratio, only to upset this number by stacking their boat high and deep with stuff over time. This will raise the center-of-gravity and affect the boat performance. For example, the greater weight may actually help motion comfort, but it could also cause the boat to wallow to excess when it should be recovering. This can set up other problems. Balance is important. As Mr. Brewer states, these formulas are relative (not absolute), but they do help with general comparisons.
Also, to be fair, Jeff's boat (Farr 38, isn't it?...and I'm not dissing it) comes in at 2.19 capsize ratio, and 17.81 motion comfort. :-P

Still, gotta take issue with the thousand pounds at the top of the mast. It would improve motion comfort. As I touched on this already, the weight would slow the boat reaction to the sea, causing a slow wallowing recovery. While not favorable to survival, it would still act within the motion comfort idea, which is a measure of damping of directional change...in a relative sense it would be more comfortable, til it flipped over or was swamped due to excessive wallowing. As stated, balance is important. However, the formulas were developed with the understanding that would take into account the normal basic layout of a sailboat. Having 1,000lbs at the top of the mast is not reasonable, nor realistic. That example is akin to comparing the speed capabilities of two boats, but having one drag anchor on the bottom, and the other is not. Point being, any formula can be discredited when unrealistic data is plugged in.
Similarly, the capsize screen is simply a formula used to compare beam and displacement (nothing else), and how the two interact to dictate characteristics of a boat in a seaway. Again, nobody considers the handling characteristics of a boat with a top-heavy mast. Why should they? The formula simply is used to get an idea of how a given amount of beam to displacement will affect handling. Too much here, too little there and you get stablility issues, or more stability than needed at the cost of speed/efficiency. Whether those stability issues are controllable (in a given situation...coastal vs bluewater), perhaps even beneficial, is something that has to be fleshed out. That's when you plug in other factors. Look at boat design. They all start out the same way....hull first, and nowhere do they have a sign hanging saying to factor in a top-heavy mast. You start with the basics, then adjust for an intended use, and compensate (or should I say compromise..."All boats are a compromise..." for excessive good/bad characteristics as they creep in.
It is safe to say that no formula can fully predict boat characteristics, but is is wrong to say these formulas, and others do not offer useful information.

Last edited by seabreeze_97; 01-19-2007 at 03:35 PM.
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Old 01-19-2007
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I agree with Jeff. My CS34 is over 2 on the CR. But it doesn't take into account the weight bulb at the end of the 6'3" keel. Sometimes you need to think in terms of basics physics instead of big formulas. These ratios and formulas do not take into account the numerous variables encountered at sea. They are just guidelines.
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I absolutely agree with bestfriend's comment above. Just looking at the numbers a Ford pick up truck has many similar numbers to a Mercedes S-class car. Pure numbers are only significant at the margins, when comparing similar vessels.
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Well, actually, it isn't a big formula, and it does deal with basic physics. Isn't that the major complaint? It only considers two basic components, and not other factors like 1,000 lbs 6ft below (or 40 ft above) the hull? Again, the formula addresses the basic stability of a given hull form. What is done afterwards, and how/where the weight is distributed, as well as how these issues improve or detract from the basic design bluewater stability/capability is a wide-open issue. Good designers know how to exploit the strengths and minimize the weaknesses of a given design.
Take the CS 34. By the formula, borderline for bluewater, but a 6ft bulb is probably the best for that design to maximize potential stability, while minimizing a drag penalty. It's a nice boat, no doubt about it. The formula just says it's not the best for, say, Fastnet '79 conditions as far as the boat is concerned. However, in the hands of a good, experienced crew, it'd have a better chance than something with a 1.6 CR and a less capable crew.
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