Has anyone seen a number for the range of positive stability for the Cape Dory 25D?
pjparker@village.uunet.be
CD 25D Range of positive stability
Moderator: Jim Walsh
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Bob Malinka
Re: CD 25D Range of positive stability
Check out http://www.image-ination.com/sailcalc.htmlPete Parker wrote: Has anyone seen a number for the range of positive stability for the Cape Dory 25D?
You will find the data on the CD25D and compare with other
makes and models. BTW the CD25D capsize ratio is 1.86.
Bob
s/v Ranger
CD25D #144
Ranger1442@hotmail.com
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Russell
Alas, that doesn't much about LPOS. Here's some other info.
The CD25 is a fairly small boat. How it is loaded will have a large impact on its righting moment.
The capsize ratio is just a measure of beam relative to displacement. You have to work pretty hard to infer anything from it about righting moment. You might reason that for boats with similar SA/D, drawn by competent designers, the skinnier boat has to have a lower CG, to make up for lack of beam in standing up to its sailplan. But that is a fairly long chain of assumptions, all of which are wrong in some cases.
To calculate a boat's righting moment at any degree of heel, you need three things. (1) The boat's displacement. (2) The boat's center of gravity. (3) The boat's hull shape. Run these through a computer program (or a naval architect), and out comes a stability curve. Importantly, "hull shape" means actual data on the shape of the hull and deck, not just beam and DWL. You have to be able to estimate the center of buoyancy for submerged portions of the hull. Capsize screen is not a substitute for this.
The good news is that Alberg clearly worked hard to keep center of gravity low. If you compare Alberg designs to other boats, they have low freeboard, minimal superstructure, dense ballast placed as low as the draft allows, and make use of their deep hull to place the interior and engines as low as possible. All of this adds up to good range of positive stability, and perhaps more importantly, low center of gravity.
IMS calculates a range of positive stability through a combination of hull measurements and incline tests. Their calculation is based on measurements of specific boats, as loaded, and ignores the buoyancy of the cabin trunk, resulting in numbers that typically are ten to twenty degrees less than what builders calculate. For example, Pacific Seacraft calculates an LPOS of 139* for the Crealock 37, but the IMS certificates show figures in the mid 120s. Relative to other boats this size, that is quite good. The Cape Dory 36 has an IMS stability of 129*. As I said, Alberg worked hard to keep weight low.
(A final parenthetical note. There is no one right way to calculate LPOS. Neither the IMS nor the computer programs used by builders are wrong. They are just different. Since these numbers are used to compare different designs, the important thing is to use the same method of calculation to generate the numbers you compare. No fair comparing a builder's number for one boat, to the IMS certificate of a second! "But," someone might ask, "what is the ACTUAL limit of positive stability?" At what angle of heel, when the actual boat is inverted in real seas, does it still try to right itself? If you think about this, it is a really thorny question. Is the buoyancy of the cabin trunk helping to right the boat? Maybe .. if all the ports and hatches are dogged, and the hatch boards are somehow locked in. Well .. at least the center of gravity remains where the IMS measurer estimated it, right? Again, maybe. When the boat inverted, how much stuff fell onto the ceiling, because it wasn't tied down or behind latched doors? Did your batteries come crashing through your lazarette lid? If so, it is now open, letting in water! And what about the sails? Were you flying a spinnaker before you capsized? Is it acting as a sea anchor? What is your ground tackle doing? It makes sense to compare the properties of different hulls, using consistent calculations. But capsizes are complex, real world events. Actual range of positive stability will be affected by the specific circumstances of the actual event.)
The capsize ratio is just a measure of beam relative to displacement. You have to work pretty hard to infer anything from it about righting moment. You might reason that for boats with similar SA/D, drawn by competent designers, the skinnier boat has to have a lower CG, to make up for lack of beam in standing up to its sailplan. But that is a fairly long chain of assumptions, all of which are wrong in some cases.
To calculate a boat's righting moment at any degree of heel, you need three things. (1) The boat's displacement. (2) The boat's center of gravity. (3) The boat's hull shape. Run these through a computer program (or a naval architect), and out comes a stability curve. Importantly, "hull shape" means actual data on the shape of the hull and deck, not just beam and DWL. You have to be able to estimate the center of buoyancy for submerged portions of the hull. Capsize screen is not a substitute for this.
The good news is that Alberg clearly worked hard to keep center of gravity low. If you compare Alberg designs to other boats, they have low freeboard, minimal superstructure, dense ballast placed as low as the draft allows, and make use of their deep hull to place the interior and engines as low as possible. All of this adds up to good range of positive stability, and perhaps more importantly, low center of gravity.
IMS calculates a range of positive stability through a combination of hull measurements and incline tests. Their calculation is based on measurements of specific boats, as loaded, and ignores the buoyancy of the cabin trunk, resulting in numbers that typically are ten to twenty degrees less than what builders calculate. For example, Pacific Seacraft calculates an LPOS of 139* for the Crealock 37, but the IMS certificates show figures in the mid 120s. Relative to other boats this size, that is quite good. The Cape Dory 36 has an IMS stability of 129*. As I said, Alberg worked hard to keep weight low.
(A final parenthetical note. There is no one right way to calculate LPOS. Neither the IMS nor the computer programs used by builders are wrong. They are just different. Since these numbers are used to compare different designs, the important thing is to use the same method of calculation to generate the numbers you compare. No fair comparing a builder's number for one boat, to the IMS certificate of a second! "But," someone might ask, "what is the ACTUAL limit of positive stability?" At what angle of heel, when the actual boat is inverted in real seas, does it still try to right itself? If you think about this, it is a really thorny question. Is the buoyancy of the cabin trunk helping to right the boat? Maybe .. if all the ports and hatches are dogged, and the hatch boards are somehow locked in. Well .. at least the center of gravity remains where the IMS measurer estimated it, right? Again, maybe. When the boat inverted, how much stuff fell onto the ceiling, because it wasn't tied down or behind latched doors? Did your batteries come crashing through your lazarette lid? If so, it is now open, letting in water! And what about the sails? Were you flying a spinnaker before you capsized? Is it acting as a sea anchor? What is your ground tackle doing? It makes sense to compare the properties of different hulls, using consistent calculations. But capsizes are complex, real world events. Actual range of positive stability will be affected by the specific circumstances of the actual event.)
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john
Re: Alas, that doesn't much about LPOS. Here's some other in
Russell's post just proves "the cleanleness of theory is no match for the mess of reality" Keep those hatches closed & latched. Would you rather be on a cape dory or some mass production boat that may break apart.Russell wrote: The CD25 is a fairly small boat. How it is loaded will have a large impact on its righting moment.
The capsize ratio is just a measure of beam relative to displacement. You have to work pretty hard to infer anything from it about righting moment. You might reason that for boats with similar SA/D, drawn by competent designers, the skinnier boat has to have a lower CG, to make up for lack of beam in standing up to its sailplan. But that is a fairly long chain of assumptions, all of which are wrong in some cases.
To calculate a boat's righting moment at any degree of heel, you need three things. (1) The boat's displacement. (2) The boat's center of gravity. (3) The boat's hull shape. Run these through a computer program (or a naval architect), and out comes a stability curve. Importantly, "hull shape" means actual data on the shape of the hull and deck, not just beam and DWL. You have to be able to estimate the center of buoyancy for submerged portions of the hull. Capsize screen is not a substitute for this.
The good news is that Alberg clearly worked hard to keep center of gravity low. If you compare Alberg designs to other boats, they have low freeboard, minimal superstructure, dense ballast placed as low as the draft allows, and make use of their deep hull to place the interior and engines as low as possible. All of this adds up to good range of positive stability, and perhaps more importantly, low center of gravity.
IMS calculates a range of positive stability through a combination of hull measurements and incline tests. Their calculation is based on measurements of specific boats, as loaded, and ignores the buoyancy of the cabin trunk, resulting in numbers that typically are ten to twenty degrees less than what builders calculate. For example, Pacific Seacraft calculates an LPOS of 139* for the Crealock 37, but the IMS certificates show figures in the mid 120s. Relative to other boats this size, that is quite good. The Cape Dory 36 has an IMS stability of 129*. As I said, Alberg worked hard to keep weight low.
(A final parenthetical note. There is no one right way to calculate LPOS. Neither the IMS nor the computer programs used by builders are wrong. They are just different. Since these numbers are used to compare different designs, the important thing is to use the same method of calculation to generate the numbers you compare. No fair comparing a builder's number for one boat, to the IMS certificate of a second! "But," someone might ask, "what is the ACTUAL limit of positive stability?" At what angle of heel, when the actual boat is inverted in real seas, does it still try to right itself? If you think about this, it is a really thorny question. Is the buoyancy of the cabin trunk helping to right the boat? Maybe .. if all the ports and hatches are dogged, and the hatch boards are somehow locked in. Well .. at least the center of gravity remains where the IMS measurer estimated it, right? Again, maybe. When the boat inverted, how much stuff fell onto the ceiling, because it wasn't tied down or behind latched doors? Did your batteries come crashing through your lazarette lid? If so, it is now open, letting in water! And what about the sails? Were you flying a spinnaker before you capsized? Is it acting as a sea anchor? What is your ground tackle doing? It makes sense to compare the properties of different hulls, using consistent calculations. But capsizes are complex, real world events. Actual range of positive stability will be affected by the specific circumstances of the actual event.)
John CD31 #18
redzeplin@yahoo.com
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Catherine Monaghan
Re: CD 25D Range of positive stability
Pete,
The CD25D has a capsize risk of 1.80 which is very good. The formula, in case you want to figure it out yourself, is:
CAPSIZE RISK = beam/(disp/(0.9*64))^.333
(You need a calculator that can handle exponents. The caret "^" means that the figure following it is an exponent. If it were ^3 that would mean cubed.)
If you haven't seen it already, I suggest you visit John Holtrop's website and read the <a href="http://www1.iwvisp.com/jholtrop/technica.htm">Technical Articles</a>, especially the following articles: <a href="http://www1.iwvisp.com/jholtrop/Article ... tm">Design Basics</a>, <a href="http://www1.iwvisp.com/jholtrop/Article ... htm">Fuzzy Logic</a>, and <a href="http://www1.iwvisp.com/jholtrop/Article ... Estimating Stability</a>.
And, Holtrop's website is accessible directly from the CCDOA's website from the "Cape Dorys" link in the navigation bar located at the top of every web page excluding the message board posts. From the Cape Dory's page click on "Specifications of Cape Dory Boats".
I'll e-mail you an Excel spreadsheet which includes all of the Cape Dory models and their "fuzzy logic" statistics. From the data in the spreadsheet and the information in the articles, you should be able to decide whether you can trust your CD25D or not.
You've got a good boat. There aren't many 25-footers as well built or as seaworthy as the little Cape Dorys.
catherine_monaghan@merck.com
CD32 <a href="http://www.hometown.aol.com/bcomet/real ... ization</a>, #3
Raritan Bay
Rahway, NJ
catherine_monaghan@merck.com
The CD25D has a capsize risk of 1.80 which is very good. The formula, in case you want to figure it out yourself, is:
CAPSIZE RISK = beam/(disp/(0.9*64))^.333
(You need a calculator that can handle exponents. The caret "^" means that the figure following it is an exponent. If it were ^3 that would mean cubed.)
If you haven't seen it already, I suggest you visit John Holtrop's website and read the <a href="http://www1.iwvisp.com/jholtrop/technica.htm">Technical Articles</a>, especially the following articles: <a href="http://www1.iwvisp.com/jholtrop/Article ... tm">Design Basics</a>, <a href="http://www1.iwvisp.com/jholtrop/Article ... htm">Fuzzy Logic</a>, and <a href="http://www1.iwvisp.com/jholtrop/Article ... Estimating Stability</a>.
And, Holtrop's website is accessible directly from the CCDOA's website from the "Cape Dorys" link in the navigation bar located at the top of every web page excluding the message board posts. From the Cape Dory's page click on "Specifications of Cape Dory Boats".
I'll e-mail you an Excel spreadsheet which includes all of the Cape Dory models and their "fuzzy logic" statistics. From the data in the spreadsheet and the information in the articles, you should be able to decide whether you can trust your CD25D or not.
You've got a good boat. There aren't many 25-footers as well built or as seaworthy as the little Cape Dorys.
catherine_monaghan@merck.com
CD32 <a href="http://www.hometown.aol.com/bcomet/real ... ization</a>, #3
Raritan Bay
Rahway, NJ
Pete Parker wrote: Has anyone seen a number for the range of positive stability for the Cape Dory 25D?
catherine_monaghan@merck.com
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Russell
The .9 does not appear in the usual capsize screen
> The formula, in case you want to figure it out yourself, is:
> CAPSIZE RISK = beam/(disp/(0.9*64))^.333
The usual formula for the capsize screen does NOT have that .9 in the denominator. It is just the ratio of the boat's beam to the side length of a cube of sea water equal to the boat's displacement. With beam measured in feet, and displacement in pounds, that is the formula above, with the .9 omitted. (Sea water weighs 64 pounds per foot cubed.) I don't know why Holtrop threw this extra factor into his formula, but it shifts the scale a bit from the usual capsize screen.
> CAPSIZE RISK = beam/(disp/(0.9*64))^.333
The usual formula for the capsize screen does NOT have that .9 in the denominator. It is just the ratio of the boat's beam to the side length of a cube of sea water equal to the boat's displacement. With beam measured in feet, and displacement in pounds, that is the formula above, with the .9 omitted. (Sea water weighs 64 pounds per foot cubed.) I don't know why Holtrop threw this extra factor into his formula, but it shifts the scale a bit from the usual capsize screen.
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Catherine Monaghan
Crunching numbers
Hi Russell,
John Holtop uses the formula derived by the United States Yacht Racing Union (now US Sailing) to calculate capsize risk. I think many leave out the .9 in the formula because it is so close to 1 -- they round up. But since we're dealing with a safety factor, maybe it's better to use the formula as is without rounding out any of the numbers -- that goes for entering the variables into the equation as well. But maybe it doesn't matter. Who knows?
And don't forget, when calculating the safefy or suitability of a sailing vessel. Capsize risk is not the only statistic to rely on.
Both static and dynamic stability issues are involved. Four key factors are:
Center of Gravity,
Center of Buoyancy,
Displacement, and
Moment of Inertia
all items mentioned by Russell in his previous post. And other "numbers" to consider are:
Disp. / LWL,
Sail Area / Disp.,
LOA / Beam,
Comfort Factor, and
Vmax / Vhull.
Everybody remembers their math, right?
catherine_monaghan@merck.com
CD32 <a href="http://www.hometown.aol.com/bcomet/real ... ization</a>, #3
Rahway, NJ
Raritan Bay
catherine_monaghan@merck.com
John Holtop uses the formula derived by the United States Yacht Racing Union (now US Sailing) to calculate capsize risk. I think many leave out the .9 in the formula because it is so close to 1 -- they round up. But since we're dealing with a safety factor, maybe it's better to use the formula as is without rounding out any of the numbers -- that goes for entering the variables into the equation as well. But maybe it doesn't matter. Who knows?
And don't forget, when calculating the safefy or suitability of a sailing vessel. Capsize risk is not the only statistic to rely on.
Both static and dynamic stability issues are involved. Four key factors are:
Center of Gravity,
Center of Buoyancy,
Displacement, and
Moment of Inertia
all items mentioned by Russell in his previous post. And other "numbers" to consider are:
Disp. / LWL,
Sail Area / Disp.,
LOA / Beam,
Comfort Factor, and
Vmax / Vhull.
Everybody remembers their math, right?
catherine_monaghan@merck.com
CD32 <a href="http://www.hometown.aol.com/bcomet/real ... ization</a>, #3
Rahway, NJ
Raritan Bay
Russell wrote: > The formula, in case you want to figure it out yourself, is:
> CAPSIZE RISK = beam/(disp/(0.9*64))^.333
The usual formula for the capsize screen does NOT have that .9 in the denominator. It is just the ratio of the boat's beam to the side length of a cube of sea water equal to the boat's displacement. With beam measured in feet, and displacement in pounds, that is the formula above, with the .9 omitted. (Sea water weighs 64 pounds per foot cubed.) I don't know why Holtrop threw this extra factor into his formula, but it shifts the scale a bit from the usual capsize screen.
catherine_monaghan@merck.com
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Russell
Catherine, do you have a USSailing report that gives it this
"John Holtop uses the formula derived by the United States Yacht Racing Union (now US Sailing) to calculate capsize risk."
I know that is what his web page says. What puzzles me is three things. (a) Holtrop's Web page is the *only* place I have ever seen that factor in the formula. (b) This includes, for example, fairly detailed description of the screen's origin and the research behind it. Kirkman and McCurdy write on the SNAME/USYRU capsize project in chapters 3 and 4 of "Desirable and Undesirable Characteristics of OffShore Yachts." They give the usual formula for the screen. I would be surprised if this is a short-cut, since they don't hesitate to give the more complex "capsize length formula." (c) The screen is a dimensionless ratio with obvious physical interpretation, and it just wouldn't make sense to define it with this stray factor.
But .. maybe it was defined with the .9 for some subtle reason, maybe Kirkman and McCurdy got lazy in writing about it, maybe everyone else followed their lead, and maybe Holtrop went back to the original. It doesn't make a lot of difference, since all this factor does is shift the scale 3%.
I'm just curious where this .9 appeared. Do you actually have something from USSailing that gives the formula with the .9 factor? If so, I'd be obliged for the reference. Just to satisfy my niggling curiosity.
BTW, if anyone's really interested in this stuff, here's the Amazon page on "Desirable and Undesirable Characteristics of Offshore Yachts." Chapters 2 through 4 are on stability. It has 17 chapters, covering lots of other topics. Nice book. Alas, not cheap:
http://www.amazon.com/exec/obidos/ASIN/ ... 24-7366526
I know that is what his web page says. What puzzles me is three things. (a) Holtrop's Web page is the *only* place I have ever seen that factor in the formula. (b) This includes, for example, fairly detailed description of the screen's origin and the research behind it. Kirkman and McCurdy write on the SNAME/USYRU capsize project in chapters 3 and 4 of "Desirable and Undesirable Characteristics of OffShore Yachts." They give the usual formula for the screen. I would be surprised if this is a short-cut, since they don't hesitate to give the more complex "capsize length formula." (c) The screen is a dimensionless ratio with obvious physical interpretation, and it just wouldn't make sense to define it with this stray factor.
But .. maybe it was defined with the .9 for some subtle reason, maybe Kirkman and McCurdy got lazy in writing about it, maybe everyone else followed their lead, and maybe Holtrop went back to the original. It doesn't make a lot of difference, since all this factor does is shift the scale 3%.
I'm just curious where this .9 appeared. Do you actually have something from USSailing that gives the formula with the .9 factor? If so, I'd be obliged for the reference. Just to satisfy my niggling curiosity.
BTW, if anyone's really interested in this stuff, here's the Amazon page on "Desirable and Undesirable Characteristics of Offshore Yachts." Chapters 2 through 4 are on stability. It has 17 chapters, covering lots of other topics. Nice book. Alas, not cheap:
http://www.amazon.com/exec/obidos/ASIN/ ... 24-7366526
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Catherine Monaghan
Re: Catherine, do you have a USSailing report that gives it
Russell,
Send me an e-mail and I'll send you the info I get from Holtrop and US Sailing regarding the "0.9" in the formula when I get it.
Thanks,
catherine_monaghan@merck.com
CD32 Realization, #3
Rahway, NJ
Raritan Bay
catherine_monaghan@merck.com
Send me an e-mail and I'll send you the info I get from Holtrop and US Sailing regarding the "0.9" in the formula when I get it.
Thanks,
catherine_monaghan@merck.com
CD32 Realization, #3
Rahway, NJ
Raritan Bay
Russell wrote: "John Holtop uses the formula derived by the United States Yacht Racing Union (now US Sailing) to calculate capsize risk."
I know that is what his web page says. What puzzles me is three things. (a) Holtrop's Web page is the *only* place I have ever seen that factor in the formula. (b) This includes, for example, fairly detailed description of the screen's origin and the research behind it. Kirkman and McCurdy write on the SNAME/USYRU capsize project in chapters 3 and 4 of "Desirable and Undesirable Characteristics of OffShore Yachts." They give the usual formula for the screen. I would be surprised if this is a short-cut, since they don't hesitate to give the more complex "capsize length formula." (c) The screen is a dimensionless ratio with obvious physical interpretation, and it just wouldn't make sense to define it with this stray factor.
But .. maybe it was defined with the .9 for some subtle reason, maybe Kirkman and McCurdy got lazy in writing about it, maybe everyone else followed their lead, and maybe Holtrop went back to the original. It doesn't make a lot of difference, since all this factor does is shift the scale 3%.
I'm just curious where this .9 appeared. Do you actually have something from USSailing that gives the formula with the .9 factor? If so, I'd be obliged for the reference. Just to satisfy my niggling curiosity.
BTW, if anyone's really interested in this stuff, here's the Amazon page on "Desirable and Undesirable Characteristics of Offshore Yachts." Chapters 2 through 4 are on stability. It has 17 chapters, covering lots of other topics. Nice book. Alas, not cheap:
http://www.amazon.com/exec/obidos/ASIN/ ... 24-7366526
catherine_monaghan@merck.com