Originally Posted by daviid
I have been trying to get my head around boat safety for some time now. More specifically this includes a study of the particular boats stability curve and obtaining the STIX rating. This has not been an easy task to be honest. Manufacturers have asked me to contact the dealer who has asked me to contact the maufacturer and so we go. When I have managed to get some numbers, they have not been the same as those I have found from various boat reports including the ones published by Practical Boat Owner and Yachting Monthly. I am also aware that there seems to be no universal standard for determining the stability curve as far as the manufacturers go; they seem to rely on a computer program which in turn is reliant on a whole host of different variables for each manufacturer. The net result is that making comparison of each boats stability characteristices and in particular their published AVS is pretty pointless unless there is a standard for determining how these numbers need to be determined. I am also aware the the ORC determine their own AVS on account of these defficiencies by subjecting each boat to an incline test and then running their own computer program. Even if there test isn't better, at least this way each boat can be compared which brings me to my 2 questions:
1. An AVS of 118 seems to be the magic number certainly if one wants to do the Sydney to Hobart race. Whose AVS is this, the boat maufacturers or the ORC? If this is the right number, then this would exclude the X38c, the Salona 37 and the Dufour 40e but would include the Beneteau Oceanis 37 and the Jeanneau 36i. mmmm, I don't think so ... What is the right number? Comments?
2. If there is no way of getting meaningful numbers that are measured in a similar way for the stability curve and for the AVS, then what boat design characteritics are necessary to ensure that a particlar boat is safe? L/B, B/D , the area below the x-axis on the stability curve spring to mind. What else should be considered?
Sometimes it feels like the proverbial never ending rabbit hole when it comes to assessing stability and therefore safety.
You are absolutely correct and I never brought it here because it is really a dirty and complex business and I don't want to discuss it on this thread because it goes away from its subject. You can open a thread about it but I really think it is a too technical and polemic subject to be discussed on an open forum.
But I can give you my opinion about it:
The stability curves are made by the designers and as you say they are made using different programs and probably even different parameters. The only one that is equal for all is the one for ORC, that is also the only one based in real inclining experiments, and therefore the only one that provides comparable results. But as you know not all boats race in ORC so you cannot have those stability curves for all boats.
The ones you have for all boats are the ones used to certify the boats and these ones are clearly not comparable. Some show results that are very close to the ones from the ORC curves others results that are much better.
You don't know also if the curves that are used by most designers (and that give better results than the ORC stability curves) are closer to reality than ORC curves.
Also you have to know that there are two stability curves, one with minimum load another with maximum load. With maximum load you will have a bigger stability but a lower AVS. They used to certify the boats only the minimum load curve, than the two and now I think just a mix of the two (it is not the curves that are used but the data taken from the curves).
I completely disregard STIX number. It would give me a lot of trouble to explain why but I would say that I consider that some factors that have importance on its determination don't make sense. For instance the sail area is a factor that diminish STIX. If I am in bad weather if I have a third reef and a stay sail I would be in much better shape than another boat that has an overall smaller sail area but (as most) has only a second reef (with more area than the third reef from the other boat) and a furled genoa. The sail area counts big time for the Stix number.
If I don't have ORC stability curves to compare, between two boats with similar hull characteristics, similarly sized cabins, with similar draft and similar bulbed keels I assume the one with more Ballast/weight ratio would be the one with a better stability curve and a better AVS.
Regarding boats with different hull shapes, a many year's interest for the subject and after analyzing hundreds of different stability curves I have a pretty good idea of the differences in stability curves between different types of hulls, to give me an approximated idea how a beamy hull can compare with a narrow hull.
The issue is so polemic that my favorite magazine, the German "Yacht" that I believe it was the first to publish stability curves, stop publishing them five or six year's ago. I still like to see the stability curves provided by the designer on the British magazines but I really think they should warn the readers about what is going on. They also publish the curves and after completely disregard the provided data on the boat tests, or confound GZ curves with RM curves. I have already call their attention to the subject and posted on their forum about it with no avail. I guess it is a too hot subject
Regarding stability is convenient not to forget that we are only talking about static stability and it has been shown that dynamic stability is a more important factor to seaworthiness than static stability. I have my opinion about the subject (I will not discuss it here) but experience shows for instance that a boat like the OVNI 435, one of the boats more extensively used offshore and made in large numbers has a lousy static stability curve and an unblemished seaworthiness record and that the mini ocean racers that have a very small RM curve (because they are very small and light) have an impressive safety record taking into account the number of transats and even a circumnavigation.