40.5 Stability Index
- Thread starter SVPardonMyFrench
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Unlike the modern STIX stability index, the 'Capsize Screen' (or index) was created back before computers were used to design boats. It is an impossibly simple calculation used for generalizing boat performance in breaking seas. Even the CSA (who created it) describe it as 'rough'.
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This might be worth a look. I just need to determine distance between center of gravity and center of bouyancy (GZ) or find the actual stability curve and AMS/AVS for my Hunter. Any ideas how that's done or if the data is available from the old days?
http://www.wavetrain.net/boats-a-gear/471-modern-sailboat-design-quantifying-stability
http://www.wavetrain.net/boats-a-gear/471-modern-sailboat-design-quantifying-stability
Well that's a good treatment of how it works, but sadly it's not going to give you any clue as to how to generate the curves, or generate the CG and CB points. You really need some high-powered software, and a complete model of the boats parameters and characteristics to do that.
You can see how simple the capsize screen is, here's the formula:
CSF = Beam / (Displacement/64.2)1/3
Yeah, that's kind of the way I read the post, and of course complicates with modification, as Gunni has outlined. I found this to be very amusing, at least when comparing similar style boats and their relative Capsize Ratio. You guys have probably seen this already, but it's new to me...
http://tomdove.com/sailcalc/sailcalc.html
Courtesy of the amazing Dave Gerr, the Wolfson Formula. All you need is a calculator and some basic measurements of your actual boat.
Our Hunter 41 DS (Sapphire) has a stability index of 122.6 similar length and displacement but it is has a tab more beam than the 40.5.
41 DS ----- 40.5 Legend
LOA - 41.00 ----- 40.17
Rated L - 36.591 ----- 35.33
Bmax - 13.28 ----- 12.42
Disp - 20944 ----- 20000
Stab Index 122.6
41 DS ----- 40.5 Legend
LOA - 41.00 ----- 40.17
Rated L - 36.591 ----- 35.33
Bmax - 13.28 ----- 12.42
Disp - 20944 ----- 20000
Stab Index 122.6
That actually a nice find. And well written. But remember this is basically a reverse-fit into a curve created by 'standard' hull dimentions, it don't actually calculate for your boat. Gerr himself doesn't like the results, so he multiplies the result by an imperial constant! But I'm guessing it's probably going to be reasonably close, and probably as close as you can get without paying someone!
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Were you wondering for seaworthyness? It is a Category A. Of course, that is simply construction and doesn't address lots of other things. And, of course, any boat of this age is needing a full inspection. Or, were you just curious?
Interestingly enough, CE ratings are more about DIMENSIONS. The spec does not mention construction details. Its all about stability. As stability is a function of length, make it long enough and it will be Cat-A. It also why even VERY WELL BUILT boats under 30 feet have a big problem with making it to Cat-A. They are just too short.
I know my 376 doesn't have all that great of numbers but for the most part I have never had any problems.
I do remember one night in the middle of the Caribbean sea heading to Cartagena from Bahamas at about 3 am while I am sleeping in the cockpit on the low side being woken up with a massive amount of water coming in over the rail of the low side and soaking me like I just jumped into a bath tub. It wasn't splash, but more like a wave hitting the boat from the opposite direction. Weird I tell ya.
I still to this day do not know what happened. Conditions were about 25 gusting to 30 knots on the beam with 8-10 steep seas but she had been handling that just fine for a couple of days by then. I don't remember any excessive heel when being woken up but as you know it can be hard to judge heel when laying down on the low side.
One salty dude I was talking to in Panama said he believed it was my boat sliding sideways down a very steep wave creating a backwash. Another thought is that a wave just exploded out of the water just as I sailed by - waves popup all the time in the Caribbean sea but I have never seen one do that next to the boat.
I do remember one night in the middle of the Caribbean sea heading to Cartagena from Bahamas at about 3 am while I am sleeping in the cockpit on the low side being woken up with a massive amount of water coming in over the rail of the low side and soaking me like I just jumped into a bath tub. It wasn't splash, but more like a wave hitting the boat from the opposite direction. Weird I tell ya.
I still to this day do not know what happened. Conditions were about 25 gusting to 30 knots on the beam with 8-10 steep seas but she had been handling that just fine for a couple of days by then. I don't remember any excessive heel when being woken up but as you know it can be hard to judge heel when laying down on the low side.
One salty dude I was talking to in Panama said he believed it was my boat sliding sideways down a very steep wave creating a backwash. Another thought is that a wave just exploded out of the water just as I sailed by - waves popup all the time in the Caribbean sea but I have never seen one do that next to the boat.
Yeah, I hear ya! I reference Mr. Gerr's dissertation on vessel stability as a compilation of factors contributing to vessel stability more than an exercise in drawing an academic stability curve. Everything from the defined (ballast ratio) to equipment list and location to the value of practices like removing dorades and closing the companionway. Some of the things that really stand out are the contribution of weighty furling masts (-20 deg AVS!) and raising the CG with stuff like davits, dinks, gensets and batteries well above the waterline. Gerr simply makes the quantitative argument for all the cautions a savvy offshore sailor already knows about. And he illustrates how many of the things boaters add to their boats make them much less seaworthy. Especially when many modern sailboats ship with shallow hulls and low ballast weight.
OK, I'll bite on capsize risk one more time.
Exhaustive tank test of different hull designs were done after the Fastnet disaster. The results were clear: if any boat is hit on the beam by a breaking wave, it will capsize. There are no designs that reduce the certainty of this finding.
Survival after capsize depends on the boat righting itself from an inverted position. Boats with a small range of positive stability while inverted are most likely to recover from capsize in a reasonable time. Boats with hull shapes than enhance initial stability (form stability) are wider for the same displacement, and, therefore less likely to recover from a capsize. A catamaran is the extreme case. They have great stability while upright. But they also have great stability once they are capsized, and will stay inverted indefinitely.
The capsize screen screen does a reasonable job of differentiating between designs that are likely, or unlikely to recover from a capsize. I would also argue the a designer who designs a boat with a capsize screen above 2.00 is simply not designing a blue water cruiser!
Some modern hull designs carry their beam through the stern, provided spacious aft cabins, and high form stability to resist heeling in normal sailing conditions. This is an example of a design issue the the capsize screen will not detect.
Finally, the 40 foot Hunters of the early 90's have an excellent record of performance in blue water as reported on this site. Honestly, preparing yourself, and your boat for off shore cruising is a much bigger issue than the 40.5's design.
Here is an article with an excellent discussion of AVS (angle of positive stability) and STIX:http://www.yachtingmonthly.com/yacht-reviews/understand-boat-statistics-30154
What are your cruising plans for the boat?
Exhaustive tank test of different hull designs were done after the Fastnet disaster. The results were clear: if any boat is hit on the beam by a breaking wave, it will capsize. There are no designs that reduce the certainty of this finding.
Survival after capsize depends on the boat righting itself from an inverted position. Boats with a small range of positive stability while inverted are most likely to recover from capsize in a reasonable time. Boats with hull shapes than enhance initial stability (form stability) are wider for the same displacement, and, therefore less likely to recover from a capsize. A catamaran is the extreme case. They have great stability while upright. But they also have great stability once they are capsized, and will stay inverted indefinitely.
The capsize screen screen does a reasonable job of differentiating between designs that are likely, or unlikely to recover from a capsize. I would also argue the a designer who designs a boat with a capsize screen above 2.00 is simply not designing a blue water cruiser!
Some modern hull designs carry their beam through the stern, provided spacious aft cabins, and high form stability to resist heeling in normal sailing conditions. This is an example of a design issue the the capsize screen will not detect.
Finally, the 40 foot Hunters of the early 90's have an excellent record of performance in blue water as reported on this site. Honestly, preparing yourself, and your boat for off shore cruising is a much bigger issue than the 40.5's design.
Here is an article with an excellent discussion of AVS (angle of positive stability) and STIX:http://www.yachtingmonthly.com/yacht-reviews/understand-boat-statistics-30154
What are your cruising plans for the boat?
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Fascinating discussion.. learning a lot.
I was going through the safety requirement for the swiftsure that i will be participating in this spring and came across the following... hence my question:
"The boat shall comply with at least one of the following: demonstrate compliance with ISO 12217-2 design category A or higher, either by EC Recreational Craft Directive certification (having obtained the CE mark) or the designer’s declaration a minimum STIX value of 32 and AVS not less than 130 - 0.005*m (Where “m” is the mass of the boat in the minimum operating condition as defined by ISO 12217-2.); the stability index/AVS in ORC Rating System of not less than 103; or IRC SSS Base value of not less than 15"
the 40.5 is CE A but curious about the rest...
I was going through the safety requirement for the swiftsure that i will be participating in this spring and came across the following... hence my question:
"The boat shall comply with at least one of the following: demonstrate compliance with ISO 12217-2 design category A or higher, either by EC Recreational Craft Directive certification (having obtained the CE mark) or the designer’s declaration a minimum STIX value of 32 and AVS not less than 130 - 0.005*m (Where “m” is the mass of the boat in the minimum operating condition as defined by ISO 12217-2.); the stability index/AVS in ORC Rating System of not less than 103; or IRC SSS Base value of not less than 15"
the 40.5 is CE A but curious about the rest...