The specs for the Hunter 420 include a "capsize screen" of 2.03. Assuming it has to do with the capsizing potential, what does the "2.03" mean? What are the acceptable paramenters?
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Take a look at the second website for your boat!
2 really great websites: 1. http://www.tedbrewer.com/yachtdesign.html CAPSIZE SCREENING FORMULA (CSF): Some years ago the technical committe 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. try this one too: 2. http://image-ination.com/sailcalcv1.html hope this helps Bryan
2 really great websites: 1. http://www.tedbrewer.com/yachtdesign.html CAPSIZE SCREENING FORMULA (CSF): Some years ago the technical committe 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. try this one too: 2. http://image-ination.com/sailcalcv1.html hope this helps Bryan
Capsize screen
Actually, the capsize screen indicates the ability of the boat to recover from being capsized. That is, from the gross design dimensions, how likely/how quickly will it recover after a wave inverts it? Extensive tank tests have shown that there is little difference among designs if they are hit broadside by a breaking wave higher than the beam of the boat. They are likely to capsize in that case. This becomes a near certainty if the wave is 120% of the beam. Imagine being inverted in a keel boat with a lead keel of several tons over your head. the flat decks and cabin roof usually provide some stability in the inverted position. But following waves will rock the inverted boat, finally enough that the keel will take over and right it. The capsize screen says that the broader (flatter) a boat is for a given displacement, the harder it is for the following waves to tip it far enough for the keel to right it. The extreme case is a catamaran which is just as stable in the inverted position as upright. The measurement resulted from the statistics of losses in the Fastnet Race, and later racing weather disasters. Most of the losses come from inverted boats that do not recover. This is a constant issue, because beamy boats, with good form stability allow you to carry more sail in a given wind. The resulting flatter hull shape is also quicker to plane - both of these results make the boat faster. Cruising boats also use the increased beam to increase accommodation space, something the market clearly buys. If I were seriously contemplating blue water cruising, I would want to see the stability curve for the designs I was considering. The angle of vanishing stability will tell you how far the boat can roll before it will actually become stable in the inverted position. And how far it must roll in following waves to right itself. It is a more precise and accurate measure than the capsize screen - but it is often not available - thus the capsize screen if the best quick indicator we have. David Lady Lillie
Actually, the capsize screen indicates the ability of the boat to recover from being capsized. That is, from the gross design dimensions, how likely/how quickly will it recover after a wave inverts it? Extensive tank tests have shown that there is little difference among designs if they are hit broadside by a breaking wave higher than the beam of the boat. They are likely to capsize in that case. This becomes a near certainty if the wave is 120% of the beam. Imagine being inverted in a keel boat with a lead keel of several tons over your head. the flat decks and cabin roof usually provide some stability in the inverted position. But following waves will rock the inverted boat, finally enough that the keel will take over and right it. The capsize screen says that the broader (flatter) a boat is for a given displacement, the harder it is for the following waves to tip it far enough for the keel to right it. The extreme case is a catamaran which is just as stable in the inverted position as upright. The measurement resulted from the statistics of losses in the Fastnet Race, and later racing weather disasters. Most of the losses come from inverted boats that do not recover. This is a constant issue, because beamy boats, with good form stability allow you to carry more sail in a given wind. The resulting flatter hull shape is also quicker to plane - both of these results make the boat faster. Cruising boats also use the increased beam to increase accommodation space, something the market clearly buys. If I were seriously contemplating blue water cruising, I would want to see the stability curve for the designs I was considering. The angle of vanishing stability will tell you how far the boat can roll before it will actually become stable in the inverted position. And how far it must roll in following waves to right itself. It is a more precise and accurate measure than the capsize screen - but it is often not available - thus the capsize screen if the best quick indicator we have. David Lady Lillie
Bryan's right
The "screen" number is the second of 2 methods to calculate the range of stability or resistance to capsizing and not the "righting" ability once capsized or rolled over as David suggests. Most significant factors are beam, ballast location and mast weight. An excellent article can be found at Practical Sailing's library. "Buying and Selling" Just for info/comparisons, here are some screen nums for several popular boats: Westwight potter, 2.64 J/24, 2.45 Ranger 28, 2.04 Catalina 30, 1.94 Tartan 30, 1.87 Islander 36, 1.81
The "screen" number is the second of 2 methods to calculate the range of stability or resistance to capsizing and not the "righting" ability once capsized or rolled over as David suggests. Most significant factors are beam, ballast location and mast weight. An excellent article can be found at Practical Sailing's library. "Buying and Selling" Just for info/comparisons, here are some screen nums for several popular boats: Westwight potter, 2.64 J/24, 2.45 Ranger 28, 2.04 Catalina 30, 1.94 Tartan 30, 1.87 Islander 36, 1.81
absurd ratio
the "capsize screen" is nothing more than a ballast-to-beam ratio. It fails to consider such elements as: weight aloft, depth of balast, type of keel, hull shape, or chine shape. According to the capsize screen, a shoal-draft boat with a 65-foot furling mast and 11 foot beam running back to a pinched transom is more stable than a boat with a deep bulb keel of equal balast and a 60-foot carbon-fiber mast with 12 of beam carried well aft. Absurd. The only one who could possibly have use for such a ratio as a capsize screen would be a yacht broker trying to make a living selling boats designed in the first half of the twentieth century.
the "capsize screen" is nothing more than a ballast-to-beam ratio. It fails to consider such elements as: weight aloft, depth of balast, type of keel, hull shape, or chine shape. According to the capsize screen, a shoal-draft boat with a 65-foot furling mast and 11 foot beam running back to a pinched transom is more stable than a boat with a deep bulb keel of equal balast and a 60-foot carbon-fiber mast with 12 of beam carried well aft. Absurd. The only one who could possibly have use for such a ratio as a capsize screen would be a yacht broker trying to make a living selling boats designed in the first half of the twentieth century.
CSF is only one metric
When chosing a vessel intended for substancial offshore use, the CSF should be one of many metrics considered. My 35.5 has a CSF of 2 while a C&C 110 is calculated to be 2.15. This in spite of aluminum vs carbon fibre spar, 13K lbs vs 10.9K lbs displacement and 4.8K lbs vs 4.2K lbs ballast. Clearly the C&C has more counter weight energy yet scores more unfavorable in the standardized CSF.
When chosing a vessel intended for substancial offshore use, the CSF should be one of many metrics considered. My 35.5 has a CSF of 2 while a C&C 110 is calculated to be 2.15. This in spite of aluminum vs carbon fibre spar, 13K lbs vs 10.9K lbs displacement and 4.8K lbs vs 4.2K lbs ballast. Clearly the C&C has more counter weight energy yet scores more unfavorable in the standardized CSF.
Like the math?
If this kind of discussion is to your liking then see the current issue of Sail magazine entitled "How Stable Is Your Boat?". Another source I like(because my H37C comes in under 1.9) is in the Related Link. Have fun.
If this kind of discussion is to your liking then see the current issue of Sail magazine entitled "How Stable Is Your Boat?". Another source I like(because my H37C comes in under 1.9) is in the Related Link. Have fun.
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