1) The angle of vanishing stability for a Hunter 37C is 133.71 (2'draft) or 160.53(3' draft). For the calucation the draft is defined as the draft of the hull not including the keel. Does anyone have anything different? 2) I calculated the capsize ratio to be 1.81, yet the HOW cites 1.91. (Capsize Ration = BEAM/((DISP/64)^.3333)). Can anyone explain the difference? thanks guys/tj
angle of vanishing stability
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I agree
My calculation also gives 1.81. Phil should be able to verify and change this. David Lady Lillie
My calculation also gives 1.81. Phil should be able to verify and change this. David Lady Lillie
The "beam" number
The term "beam" is not something you can go out and measure. Just like when calculating the hull speed the coefficient to the square root takes into account the "fineness" of the hull the beam term takes into account the "broadness" of the boat. Double enders are less broad than boats with a wide transom. Not that that means that a double ender is more likely to capsize than a boat with the same physical beam that has a wide transom. The capsize ratio is really only a relative term at best as the skill of the sailor is MUCH MORE IMPORTANT to not having a capsize than any ratio the boat may have.
The term "beam" is not something you can go out and measure. Just like when calculating the hull speed the coefficient to the square root takes into account the "fineness" of the hull the beam term takes into account the "broadness" of the boat. Double enders are less broad than boats with a wide transom. Not that that means that a double ender is more likely to capsize than a boat with the same physical beam that has a wide transom. The capsize ratio is really only a relative term at best as the skill of the sailor is MUCH MORE IMPORTANT to not having a capsize than any ratio the boat may have.
still ... vanishing stability
I agree and have no argument that one's sailing skill is one's best insurance. Still ... both formulas (capsize and AVS) were developed by experts in the field for safety purposes and as a result of sailing mishaps. The capsize ratio of 1.81 and 1.91 may be only a ten-one-hundredths of a separation (the IOR rule for oceangoing sailboats is 2.0), I suspect that it has a geometric progression. I was just wondering why the difference in the results. To me, the capsize ration has the same relativity as taking a cross-country road trip in a car with bad brakes. The fact that you discount it concerns me. Do you know something I don't? More importantly, I would love some feedback on the angle of vanishing stability, which also comes with a recommendation of not less than 120 degrees for ocean going sailboats (the lower the number the longer the boat is likely to stay inverted). Additionally, I have been unable to find a definition of the "screening value." The screening value is part of the calculation that is used in determining the angle of vanishing stability. The powers that be, use it to qualify vessels for races and other offshore activities, I'm not sure what the number means though. It seems on several calculation I've done that as the AVS increases the screening value decreases. Does anybody know anything about this? thanks tj
I agree and have no argument that one's sailing skill is one's best insurance. Still ... both formulas (capsize and AVS) were developed by experts in the field for safety purposes and as a result of sailing mishaps. The capsize ratio of 1.81 and 1.91 may be only a ten-one-hundredths of a separation (the IOR rule for oceangoing sailboats is 2.0), I suspect that it has a geometric progression. I was just wondering why the difference in the results. To me, the capsize ration has the same relativity as taking a cross-country road trip in a car with bad brakes. The fact that you discount it concerns me. Do you know something I don't? More importantly, I would love some feedback on the angle of vanishing stability, which also comes with a recommendation of not less than 120 degrees for ocean going sailboats (the lower the number the longer the boat is likely to stay inverted). Additionally, I have been unable to find a definition of the "screening value." The screening value is part of the calculation that is used in determining the angle of vanishing stability. The powers that be, use it to qualify vessels for races and other offshore activities, I'm not sure what the number means though. It seems on several calculation I've done that as the AVS increases the screening value decreases. Does anybody know anything about this? thanks tj
tj if you can get a copy of Sail
magazine from Sept. and Oct. 2004, Dave Gerr wrote a very technical two part article on stability. I'm not a math guy so it was a little over my head but the essense of various stability characteristics were with in my grasp. The AVS in simplest terms is the highest angle of heel that a boat can tolerate and still right itself when the heeling force is removed. The capsize screen has more to do with the amount of force required to get the vessel to the point of capsize and its tendency to stay or right itself. AS I recall, among the factors are the displacement to beam ratios, ie heavy narrow hull does not like to be upside down while wide, light hull may not right itself. As the two factors compare, a lower AVS can be acceptable if the capsize screen is a bit higher; and vice versa. If you can't find the articles, Email me at kjazzdt@alltel.net and I'll try to get you a copy. To the contrary of other commentators, I agree that the STATS are valuable in evaluating a design if you understand how they relate. Dennis
magazine from Sept. and Oct. 2004, Dave Gerr wrote a very technical two part article on stability. I'm not a math guy so it was a little over my head but the essense of various stability characteristics were with in my grasp. The AVS in simplest terms is the highest angle of heel that a boat can tolerate and still right itself when the heeling force is removed. The capsize screen has more to do with the amount of force required to get the vessel to the point of capsize and its tendency to stay or right itself. AS I recall, among the factors are the displacement to beam ratios, ie heavy narrow hull does not like to be upside down while wide, light hull may not right itself. As the two factors compare, a lower AVS can be acceptable if the capsize screen is a bit higher; and vice versa. If you can't find the articles, Email me at kjazzdt@alltel.net and I'll try to get you a copy. To the contrary of other commentators, I agree that the STATS are valuable in evaluating a design if you understand how they relate. Dennis
Basis of the Capsize screen
After the cruel losses of over 20 boats, and a number of lives in the Fastnet race storm years ago. extensive tank test were conducted to discover safe design characteristics of boats in bad weather. There were two conclusions of this testing and analysis: 1. A wave as tall as the beam of a boat that is breaking broadside to the boat will capsize it. The result is not sensitive to boat design or other parameters. 2. Once capsized, some boats will return to an upright orientation in a timely way, while others will trap the crew in an inverted position for an extended (and dangerous) period. To their suprise, a simple, dimensionless calculation of displacement and beam turned out to be a good predictor of the boats ability to right itself after being capsized. Like the first result, this one is not sensitive very sensitive to boat design or other parameters. Boats with capsize screens below 2.0 were found to be likely to self-right themselves in a timely way. In both cases, the beam is the maximum width of the boat. A capsize screen of 1.91 indicates a boat that will make a timely recovery. A capsize screen of 1.81 indicates a a most seaworthy boat with respect to capsize recovery. People died in the Fastnet race (and in later Hobart races) in boats with capsize screens over 2.0. I think it is worth having this ratio right on our website. Of course, none of the performance ratios we use tells the whole story. But since designers are much more aware of the meaning of capsize screen than we are, I take it as given that a design with a capsize screen over 2.0 is not intended to take care of its crew in extreme conditions for extended periods of time. David Lady Lillie
After the cruel losses of over 20 boats, and a number of lives in the Fastnet race storm years ago. extensive tank test were conducted to discover safe design characteristics of boats in bad weather. There were two conclusions of this testing and analysis: 1. A wave as tall as the beam of a boat that is breaking broadside to the boat will capsize it. The result is not sensitive to boat design or other parameters. 2. Once capsized, some boats will return to an upright orientation in a timely way, while others will trap the crew in an inverted position for an extended (and dangerous) period. To their suprise, a simple, dimensionless calculation of displacement and beam turned out to be a good predictor of the boats ability to right itself after being capsized. Like the first result, this one is not sensitive very sensitive to boat design or other parameters. Boats with capsize screens below 2.0 were found to be likely to self-right themselves in a timely way. In both cases, the beam is the maximum width of the boat. A capsize screen of 1.91 indicates a boat that will make a timely recovery. A capsize screen of 1.81 indicates a a most seaworthy boat with respect to capsize recovery. People died in the Fastnet race (and in later Hobart races) in boats with capsize screens over 2.0. I think it is worth having this ratio right on our website. Of course, none of the performance ratios we use tells the whole story. But since designers are much more aware of the meaning of capsize screen than we are, I take it as given that a design with a capsize screen over 2.0 is not intended to take care of its crew in extreme conditions for extended periods of time. David Lady Lillie
Are you saying that a 22' boat with a capsize
ratio of 1.90 is actually safer than a 40' boat with a capsize ratio of 2.00. Think again.
ratio of 1.90 is actually safer than a 40' boat with a capsize ratio of 2.00. Think again.
Ooops, got the capsize screen numbers backwards,
David is correct, The lower Capsise Screen number indicates the higher stability. Still the relationship is the same to the AVS, ie; if one no. shows a little more stability the other may be acceptable showing a little less. For Example: Dave Gerr states in his article that a capsize screen of any number less than 2.0 is acceptable for offshore work. If your planning to round the Horn or cruise Antarctica, 1.7 or less is recommended. However;for ordinary ocean cruising with a capsize screen of 1.7 or less an AVS of only 115 degrees(vs 120)is acceptable for boats under 75 feet LOA and an AVS of 105 degrees is acceptable for boats over 75 feet LOA. Again, these numbers reference Dave Gerr's articles in Sail Magazine September and October 2004.
David is correct, The lower Capsise Screen number indicates the higher stability. Still the relationship is the same to the AVS, ie; if one no. shows a little more stability the other may be acceptable showing a little less. For Example: Dave Gerr states in his article that a capsize screen of any number less than 2.0 is acceptable for offshore work. If your planning to round the Horn or cruise Antarctica, 1.7 or less is recommended. However;for ordinary ocean cruising with a capsize screen of 1.7 or less an AVS of only 115 degrees(vs 120)is acceptable for boats under 75 feet LOA and an AVS of 105 degrees is acceptable for boats over 75 feet LOA. Again, these numbers reference Dave Gerr's articles in Sail Magazine September and October 2004.
Further to David's good comment
Imagine your boat is already upside down. The wider the beam and the less the weight the more likley it is to stay that way. The wider the beam and the less the weight the higher the capsize ratio will be. Some boats were designed with very wide beams to make them stiff (whilst right way up) but a tendency to be stable when inverted. Some are even made with escape hatches in the stern. The angle of vanishing stability gives a measure of how near to "perfectly upside down" the boat must be before it will stay upside down. If this was 150 degrees then the implication is that if it is more than 30 degrees off vertical (=180-150) then it would self right. None of this is what you want to contemplate on a Sunday afternoon sail with the Admiral and family, but it is nice to know that you don't have a fundamentally unsound boat.
Imagine your boat is already upside down. The wider the beam and the less the weight the more likley it is to stay that way. The wider the beam and the less the weight the higher the capsize ratio will be. Some boats were designed with very wide beams to make them stiff (whilst right way up) but a tendency to be stable when inverted. Some are even made with escape hatches in the stern. The angle of vanishing stability gives a measure of how near to "perfectly upside down" the boat must be before it will stay upside down. If this was 150 degrees then the implication is that if it is more than 30 degrees off vertical (=180-150) then it would self right. None of this is what you want to contemplate on a Sunday afternoon sail with the Admiral and family, but it is nice to know that you don't have a fundamentally unsound boat.
what i noticed was that the mearsurements are off
The 37c did not come in 2 or 3 foot draft. they are 4 or so and 5'1 feet of draft?
The 37c did not come in 2 or 3 foot draft. they are 4 or so and 5'1 feet of draft?
screening value not capsize ratio
OK, OK, OK... 1) My calculation of the H37-C's capsize ratio is 1.81. The published capsize ratio is 1.91. Can anyone explain why my ratio is different than the published one? Capsize Ratio = (Beam/Displacement/64)1/3) = 11.83/(17,800/64) cubed. You guys do the math and see what you come up with. 2) When I talk about the screening value, I'm really talking about the Screening Stability Value (SSV), not the capsize formula (I apologize for the confusion). The SSV is used in determining the Angle of Vanishing Stability (AVS; AVS = 110+(400/(SSV-10). The SSV must be fairly important on its own as event organizers, use it to qualify vessels for offshore events. I'm not sure what the number means though, because as the AVS increases the SSV decreases. For example with an AVS of 131.71 the SSV is 26.87; but with a AVS of 160.53 (a severe angle of heel) the SSV is 17.92. I don’t get it. Do any of your? 3) Do you know the meaning of the letters I, J, P and E in the sail plan measurements? I understand what the letters correlate to; I = Foretriangle Height; J = Foretriangle Base; P = Mainsail Hoist; and, E = Mainsail Foot. thanks for the input ... tj
OK, OK, OK... 1) My calculation of the H37-C's capsize ratio is 1.81. The published capsize ratio is 1.91. Can anyone explain why my ratio is different than the published one? Capsize Ratio = (Beam/Displacement/64)1/3) = 11.83/(17,800/64) cubed. You guys do the math and see what you come up with. 2) When I talk about the screening value, I'm really talking about the Screening Stability Value (SSV), not the capsize formula (I apologize for the confusion). The SSV is used in determining the Angle of Vanishing Stability (AVS; AVS = 110+(400/(SSV-10). The SSV must be fairly important on its own as event organizers, use it to qualify vessels for offshore events. I'm not sure what the number means though, because as the AVS increases the SSV decreases. For example with an AVS of 131.71 the SSV is 26.87; but with a AVS of 160.53 (a severe angle of heel) the SSV is 17.92. I don’t get it. Do any of your? 3) Do you know the meaning of the letters I, J, P and E in the sail plan measurements? I understand what the letters correlate to; I = Foretriangle Height; J = Foretriangle Base; P = Mainsail Hoist; and, E = Mainsail Foot. thanks for the input ... tj
Sunshine
"Safer" is a much broader question than safety in the contingency that you have capsized. Your 22 footer will probably return to upright quicker than your 40 footer. It's design will make it easier to roll back, and its smaller size will make it easier for wave action to initiate that process. But size is not a particularly good indicator of safety. There are a number of boats in the 24 to 30 foot range that are safer on most counts than are most 40 foot production boats today. Further, I challenge you to find a 22 foot design with a capsize screen of 1.9. Most boats of 22 feet are designed for day sailing, or pocket cruising, where the assumption is that you will return to port in severe weather. Their capsize screens tend to 2.2 or higher. David Lady Lillie
"Safer" is a much broader question than safety in the contingency that you have capsized. Your 22 footer will probably return to upright quicker than your 40 footer. It's design will make it easier to roll back, and its smaller size will make it easier for wave action to initiate that process. But size is not a particularly good indicator of safety. There are a number of boats in the 24 to 30 foot range that are safer on most counts than are most 40 foot production boats today. Further, I challenge you to find a 22 foot design with a capsize screen of 1.9. Most boats of 22 feet are designed for day sailing, or pocket cruising, where the assumption is that you will return to port in severe weather. Their capsize screens tend to 2.2 or higher. David Lady Lillie
Hey Phil!
The capsize screen for the Hunter 37c should be 1.81 - can you change it? Thanks, David Lady Lillie
The capsize screen for the Hunter 37c should be 1.81 - can you change it? Thanks, David Lady Lillie
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