how do you measure Capsize Screen? what is a good number and what is a bad number? this site says my boat has a 2.3??? what does that mean? is that good?
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AL
capsize screening formula...
"The Capsize Screening Formula was developed by the U.S. Sailing Association as a rough rule of thumb, helpful in comparing the initial resistance to capsize of various boats. Boats with a Capsize Screen factor of 2 or less will have a normal initial resistance to capsize and will in general be safer offshore boats than boats which exceed this value. The lower the value, the better, all other things being equal." taken from this site and clicking on "Performance Ratios"
"The Capsize Screening Formula was developed by the U.S. Sailing Association as a rough rule of thumb, helpful in comparing the initial resistance to capsize of various boats. Boats with a Capsize Screen factor of 2 or less will have a normal initial resistance to capsize and will in general be safer offshore boats than boats which exceed this value. The lower the value, the better, all other things being equal." taken from this site and clicking on "Performance Ratios"
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Clyde
Capsize Screening Formula
Al is right. The capsize screening formula is used for screening a boat design for offshore use. Note that this formula is very controversial and should be used as a screening test only and not on wherher or not a boat will capsize under adverse sea conditions. Boats with capsize ratios below two are said to be more suitable for offshore use. The calculation is Beam dimension in feet divided by the cube root of Displacement expressed in cubic feet of sea water (64 pounds per cubic foot). Capsize Ratio = Beam/(Displacement/64)^0.333 Any boat can capsize, some sooner than others. Fair Winds. Clyde
Al is right. The capsize screening formula is used for screening a boat design for offshore use. Note that this formula is very controversial and should be used as a screening test only and not on wherher or not a boat will capsize under adverse sea conditions. Boats with capsize ratios below two are said to be more suitable for offshore use. The calculation is Beam dimension in feet divided by the cube root of Displacement expressed in cubic feet of sea water (64 pounds per cubic foot). Capsize Ratio = Beam/(Displacement/64)^0.333 Any boat can capsize, some sooner than others. Fair Winds. Clyde
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Mike Linstrom
Interesting topic
I took a look at the performance ratios and verified Clyde's formula. But, I'm not sure I buy the principle. The formula indicates that a narrow beam boat is less likely to capsize than a wide beam. Even if I accept that there has to be more than 2 variables to the equation and draft has to be among them.
I took a look at the performance ratios and verified Clyde's formula. But, I'm not sure I buy the principle. The formula indicates that a narrow beam boat is less likely to capsize than a wide beam. Even if I accept that there has to be more than 2 variables to the equation and draft has to be among them.
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Doug T.
Relativity
The formula doesn't say that a narrower boat is less likely to capsize than a wider boat. It says that, for a given displacement, the narrower boat is less likely to capsize. That seems reasonable. I think the logic is that for a given displacement, the center of gravity (CG) will likely be higher if the boat is wider. If your CG is higher, you're more likely to capsize. The formula is certainly imperfect, though. Let's say you have a 7,400 lb displacement and 9' beam (like my boat). That gives me a capsize screen of 1.85. Sounds good. But what if that boat has a 2' draft, is 40' long, is constructed with a 100' tall solid lead mast and has a crows nest that comes equipped with a four-person hot tub? I bet I'd capsize a lot more easily than your Hunter! There are a zillion variables that should be considered...
The formula doesn't say that a narrower boat is less likely to capsize than a wider boat. It says that, for a given displacement, the narrower boat is less likely to capsize. That seems reasonable. I think the logic is that for a given displacement, the center of gravity (CG) will likely be higher if the boat is wider. If your CG is higher, you're more likely to capsize. The formula is certainly imperfect, though. Let's say you have a 7,400 lb displacement and 9' beam (like my boat). That gives me a capsize screen of 1.85. Sounds good. But what if that boat has a 2' draft, is 40' long, is constructed with a 100' tall solid lead mast and has a crows nest that comes equipped with a four-person hot tub? I bet I'd capsize a lot more easily than your Hunter! There are a zillion variables that should be considered...
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Clyde
Used For Offshore Comparison Only!
The capsize screening formula is based on a generic mathematical model of a generic offshore sailboat. After observing generic offshore sailboat models, which were based on beam and displacement, a general mathematical correlation was found as the various models capsized. This is used as a rule of thumb only. Most US recreational sailors are not offshore sailors. Most recreational sailing is done inshore or in sheltered waters. If you happen to be offshore in a storm or bad weather and adrift with another sailboat with a lower capsizing number than yours, then mathematical it is possible that you will capsize before the other sailboat. This possibility is not guaranteed since all real world parameters have not been factored in. Every sailboat can be capsized, some sooner than others. Be prepared when sailing offshore. Fair Winds. Clyde
The capsize screening formula is based on a generic mathematical model of a generic offshore sailboat. After observing generic offshore sailboat models, which were based on beam and displacement, a general mathematical correlation was found as the various models capsized. This is used as a rule of thumb only. Most US recreational sailors are not offshore sailors. Most recreational sailing is done inshore or in sheltered waters. If you happen to be offshore in a storm or bad weather and adrift with another sailboat with a lower capsizing number than yours, then mathematical it is possible that you will capsize before the other sailboat. This possibility is not guaranteed since all real world parameters have not been factored in. Every sailboat can be capsized, some sooner than others. Be prepared when sailing offshore. Fair Winds. Clyde
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David Foster
Recovery from capsize
I understand that the capsize screen is a general indicator of the likelihood that a boat will recover from a capsize. Boats with high form stability - that means wide -, and low ballast weight are more likely to stay upside down once they get there. Catamarans are the best example of the idea. Open 60s are tested for their righting capability, because this type of design - light and wide - is an efficient racing shape. Since the most likely cause of a capsize is a large, breaking wave, any boat can be rolled. The desired offshore boat will recover in good time. A capsize screen of 2,30 says that your boat is designed for inshore or coastal waters, can carry lots of sail due to form stability, and will plane quickly. Good performance, and probably at a good price, because heavy ballast will drive sturdier construction - also desirable for offshore. On the other hand your boat was not designed for offshore work. Thats a good trade-off for the way most of us use our boats. David Lady Lillie
I understand that the capsize screen is a general indicator of the likelihood that a boat will recover from a capsize. Boats with high form stability - that means wide -, and low ballast weight are more likely to stay upside down once they get there. Catamarans are the best example of the idea. Open 60s are tested for their righting capability, because this type of design - light and wide - is an efficient racing shape. Since the most likely cause of a capsize is a large, breaking wave, any boat can be rolled. The desired offshore boat will recover in good time. A capsize screen of 2,30 says that your boat is designed for inshore or coastal waters, can carry lots of sail due to form stability, and will plane quickly. Good performance, and probably at a good price, because heavy ballast will drive sturdier construction - also desirable for offshore. On the other hand your boat was not designed for offshore work. Thats a good trade-off for the way most of us use our boats. David Lady Lillie
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Ken Cobb
Ditto
The screen calculation is about recovery; I agree. The great blue-water boats have high displacement and relatively narrow beams, making them pop up like corks when knocked down. That was true of the boat that survived the perfect storm (it's brand name escapes me, but it was only a 32). The new, beamy "dockominiums" that are popular as inshore boats today might be slow to come up again if turtled, if they come up at all.
The screen calculation is about recovery; I agree. The great blue-water boats have high displacement and relatively narrow beams, making them pop up like corks when knocked down. That was true of the boat that survived the perfect storm (it's brand name escapes me, but it was only a 32). The new, beamy "dockominiums" that are popular as inshore boats today might be slow to come up again if turtled, if they come up at all.
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