This in reference to a previous posting "sailing my 22' oday across the atlantic". A reply to that posting made reference to a "Capsize Screen" which as I understand is the initial resistance of a boat to capsize. Are numbers assigned to different types /brands of boats or is it a calculation on boat length, beam, lwl or what? Where would that information be available?
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M
mike
L
Lamar Sumerlin
Calculation of Capsize Screen
While I don't consider myself an expert in this area, it is my understanding the capsize screen provides a general comparison between boats. It is not a precise measurement. The formula that I have seen for calculating capsize screening is as follows: Capsize Screening Formula = (Beam - ft)/(Displacement - lbs/64)**1/3; a value less than 2 is relatively good while a value 2 or greater is relatively vulnerable to capsizing.
While I don't consider myself an expert in this area, it is my understanding the capsize screen provides a general comparison between boats. It is not a precise measurement. The formula that I have seen for calculating capsize screening is as follows: Capsize Screening Formula = (Beam - ft)/(Displacement - lbs/64)**1/3; a value less than 2 is relatively good while a value 2 or greater is relatively vulnerable to capsizing.
CSR
Stability is a dynamic event and is affected by quite a number of different vessel parameters. The Capsize Screen Ratio method has little value in the assessment of a yacht's dymamic stability. It's really addressing a concern that lightweight designs, with a wide beam, are suspect regarding their susceptibility to capsize. The cutoff number of “2" is based on the performance of boats in the '79 Fastnet, and is an arbitrary determination that takes no account of hull shape or ballast location, etc. STIX is the Stability Index, derived by the new International Standard for the stability and buoyancy of boats. STIX is calculated from the physical characteristics of each boat, and is a number generally in the range 5 to 50. Most data is derived from the GZ-curve**. A higher value suggests greater seaworthiness. The International Standard defines 4 Design Categories for boats broadly related to ocean (A), offshore (B), coastal (C) and local sailing (D). Each design category has a minimum qualifying STIX value, and also a minimum qualifying AVS* which is linked to the minimum sailing weight of the boat: * The “AVS” is the Angle of Vanishing Stability, also known as LPS, or Limit of Positive Stability. It is the angle beyond which a boat becomes unstable and will capsize to an inverted position. **The GZ-Curve is the Curve of Righting Arms. The Horizontal distance between the boat’s Centre of Gravity (CG) and a vertical line through it’s Centre of Bouyancy (CB). The curve is usually expressed in feet or meters along the vertical axis, and in degrees along the horizontal axis. With the vessel upright, the curve is at zero. As the boat heels, the center of buoyancy moves outboard, while the center of gravity remains stationary. This creates a Righting Arm or Righting Lever. It is the horizontal distance between the upward buoyant force and the downward force of gravity. The Righting Moment at any point on the curve is simply the product of the righting arm at that point (length) times the vessel's displacement (weight). In other words, Force times Distance equals Moment.
Stability is a dynamic event and is affected by quite a number of different vessel parameters. The Capsize Screen Ratio method has little value in the assessment of a yacht's dymamic stability. It's really addressing a concern that lightweight designs, with a wide beam, are suspect regarding their susceptibility to capsize. The cutoff number of “2" is based on the performance of boats in the '79 Fastnet, and is an arbitrary determination that takes no account of hull shape or ballast location, etc. STIX is the Stability Index, derived by the new International Standard for the stability and buoyancy of boats. STIX is calculated from the physical characteristics of each boat, and is a number generally in the range 5 to 50. Most data is derived from the GZ-curve**. A higher value suggests greater seaworthiness. The International Standard defines 4 Design Categories for boats broadly related to ocean (A), offshore (B), coastal (C) and local sailing (D). Each design category has a minimum qualifying STIX value, and also a minimum qualifying AVS* which is linked to the minimum sailing weight of the boat: * The “AVS” is the Angle of Vanishing Stability, also known as LPS, or Limit of Positive Stability. It is the angle beyond which a boat becomes unstable and will capsize to an inverted position. **The GZ-Curve is the Curve of Righting Arms. The Horizontal distance between the boat’s Centre of Gravity (CG) and a vertical line through it’s Centre of Bouyancy (CB). The curve is usually expressed in feet or meters along the vertical axis, and in degrees along the horizontal axis. With the vessel upright, the curve is at zero. As the boat heels, the center of buoyancy moves outboard, while the center of gravity remains stationary. This creates a Righting Arm or Righting Lever. It is the horizontal distance between the upward buoyant force and the downward force of gravity. The Righting Moment at any point on the curve is simply the product of the righting arm at that point (length) times the vessel's displacement (weight). In other words, Force times Distance equals Moment.
B
Bruce Trotter
T
tom
All boats will capsize
John Vigor wrote a couple of books on off shore boats. Vigor stresses that all boats will capsize under the right conditions especially if the seas are breaking. IMHO the main virtue of the CSF is that it is available!!! There is a site called the sail calculator that gives a lot of info on many boats. Given that all boats will capsize the tendency to stay upside down is most important. If a boat takes too long to right itself it will probably flood and sink!!!! Common sense would indicate that a beamy boat will tend to stay inverted longer than a narrow boat. The beamest boats with the lightest displacement will stay inverted forever.(catamaran)
John Vigor wrote a couple of books on off shore boats. Vigor stresses that all boats will capsize under the right conditions especially if the seas are breaking. IMHO the main virtue of the CSF is that it is available!!! There is a site called the sail calculator that gives a lot of info on many boats. Given that all boats will capsize the tendency to stay upside down is most important. If a boat takes too long to right itself it will probably flood and sink!!!! Common sense would indicate that a beamy boat will tend to stay inverted longer than a narrow boat. The beamest boats with the lightest displacement will stay inverted forever.(catamaran)
Thoughts on the capsize screen
Tom is right. Tank tests, and sea experience show that any boat can be capsized if struck on the beam by a breaking wave higher (trough to crest) than the beam of the boat. Design differences may make some difference in the size of wave needed, but it's a small effect. The capsize screen was developed after the Fastnet disaster to give an idea of the boat's ability to right itself after being capsized. Broad, light boats (with excellent initial stability, and great racing characteristics) will tend to stay inverted once they go over. Heavier, narrower boats (which gives a lower capsize screen) will easily pop back upright. Another, more precise measure of this capability is the AVS (Angle of vnishing stability). The Sydney-Hobart race requires an AVS of 120 degrees (if I remember). A boat with a capsize screen of 1.9 probably has an AVS over 140 degrees. If capsized, subsequent waves must rock the boat back to the AVS for the keel to take over, and roll the boat back on its feet. Boats that were unable to roll back to upright caused a number of deaths and losses in the Fastnet race, and in later Sydney-Hobart races. While this sounds extreme, the 10 to 13 foot waves on Lake Erie last night would easily roll a boat with a 10 foot beam if they broke on a boat the was not held at an angle to the waves. David Lady Lillie
Tom is right. Tank tests, and sea experience show that any boat can be capsized if struck on the beam by a breaking wave higher (trough to crest) than the beam of the boat. Design differences may make some difference in the size of wave needed, but it's a small effect. The capsize screen was developed after the Fastnet disaster to give an idea of the boat's ability to right itself after being capsized. Broad, light boats (with excellent initial stability, and great racing characteristics) will tend to stay inverted once they go over. Heavier, narrower boats (which gives a lower capsize screen) will easily pop back upright. Another, more precise measure of this capability is the AVS (Angle of vnishing stability). The Sydney-Hobart race requires an AVS of 120 degrees (if I remember). A boat with a capsize screen of 1.9 probably has an AVS over 140 degrees. If capsized, subsequent waves must rock the boat back to the AVS for the keel to take over, and roll the boat back on its feet. Boats that were unable to roll back to upright caused a number of deaths and losses in the Fastnet race, and in later Sydney-Hobart races. While this sounds extreme, the 10 to 13 foot waves on Lake Erie last night would easily roll a boat with a 10 foot beam if they broke on a boat the was not held at an angle to the waves. David Lady Lillie
W
WILLIAM SANDERS
WHAT IS THE CSR #
WHAT IS THE CSR FOR THE 26X AND IS THERE A DIFFERENCE WITH THE 26M? AND IS THE MACGREGOR STRUCTURAL STRENGTH TO WEAK FOR AN HAWAII CROSSING?
WHAT IS THE CSR FOR THE 26X AND IS THERE A DIFFERENCE WITH THE 26M? AND IS THE MACGREGOR STRUCTURAL STRENGTH TO WEAK FOR AN HAWAII CROSSING?
CSR (CSF) - Gotta agree to disagree
The capsize screening ratio or formula (CSR or CSF) IS a dynamic representation of how a boat reacts to a beam wave. While it looks simple, it's an empirical equation based on actual performance data. The AVS and GZC are static properties that don't apply in rolling seas. The CSR was developed for a particular class of boat but has value when comparing boats of similar size and weight. http://boatbuilding.com/content/ratios.html#CSF The link below is to Sail Calculator which has data for over 1,600 boats. http://www.image-ination.com/sailcalc.html By the way, the CSR for a Mac26X is a fairly respectable 1.93 Also, a catamaran may turn over but because it's not weighed down with ballast, it won't sink. Happy (not freezing in Florida!) sails *_/), MArk
The capsize screening ratio or formula (CSR or CSF) IS a dynamic representation of how a boat reacts to a beam wave. While it looks simple, it's an empirical equation based on actual performance data. The AVS and GZC are static properties that don't apply in rolling seas. The CSR was developed for a particular class of boat but has value when comparing boats of similar size and weight. http://boatbuilding.com/content/ratios.html#CSF The link below is to Sail Calculator which has data for over 1,600 boats. http://www.image-ination.com/sailcalc.html By the way, the CSR for a Mac26X is a fairly respectable 1.93 Also, a catamaran may turn over but because it's not weighed down with ballast, it won't sink. Happy (not freezing in Florida!) sails *_/), MArk
CSF is toosimplistic to be of much value ...
mArk: Quoting your link: “...The CCA characterizes the formula [CSF] as "rough". They go on to say that "While the capsize screening formula places a limit on excess beam, which is important for good stability range, it does not control for another main determinant, ballasting. With only simple data, this is as far as we can go." Naval Architect Robert Perry calls it,"...far too simplistic to be always accurate, but it is one of the currently popular ways of looking at a boat's offshore suitability." It’s really just a way of quantifying the observation that a boat is “beamy”, or not. Respectfully, Gord
mArk: Quoting your link: “...The CCA characterizes the formula [CSF] as "rough". They go on to say that "While the capsize screening formula places a limit on excess beam, which is important for good stability range, it does not control for another main determinant, ballasting. With only simple data, this is as far as we can go." Naval Architect Robert Perry calls it,"...far too simplistic to be always accurate, but it is one of the currently popular ways of looking at a boat's offshore suitability." It’s really just a way of quantifying the observation that a boat is “beamy”, or not. Respectfully, Gord
B
BILL
CSR
TO CARLYLE, ILL. YOU SAY THAT THE 26X IS 11.93 IS THAT THE CSR? SOUNDS IMPOSSIBLE, DOES ANYBODY KNOW WHAT THECSR FOR THE MAC IS?
TO CARLYLE, ILL. YOU SAY THAT THE 26X IS 11.93 IS THAT THE CSR? SOUNDS IMPOSSIBLE, DOES ANYBODY KNOW WHAT THECSR FOR THE MAC IS?
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