Angle of Vanishing stability.

Or maybe all designs would merge into a characterless mediocrity, with no spark or interest, just a desperate chasing of the latest new fashion...

 

I don't believe that improved safety necessarily must equate to being bland. (Or Beige) as Billy Connoly would say.

Mychael

 

To get the CE mark, manufacturers have to get the STIX number in the process, but only a few of them publicize it for their models. So the question is why stability info is usually hidden or not properly explained, being it almost always thoroughly studied by designers....

As a collateral commentary: I still have my ration of doubts about the usefulness and meaning of this STIX number. I find it's a gathering of a lot of sub-stix parameters seeming to make quite a lot of sense by themselves, but the final number is not so clarifying to me...May the mixing could mean in fact nothing and, even worse, be dangerous, by hiding some undesirable characteristics? Should the manufacturers/designers provide the sub-stix parameters one by one, instead of mixing them in a final figure? Has this been discussed before in these forums or any others?

Here an spreadsheet for the STIX calculation, provided by SailDesign:
http://www.boatdesign.net/forums/showthread.php?t=7572&highlight=stix

Interesting page to get info on these matters:
http://cruisingresources.com/Hull_Shape_and_Performance

Most interesting, not only about stability:
http://www.oossanen.nl/pubs/VOA_MY_CEM.pdf

 

A brief news article in "Practical boat Owner" (english magazine).

"Death Trap dinghy Owner to Sue"
Two people died in a Bez 2 dinghy.Due to capsize. Investigation into the incident shows that the boat should never have been given the RCD Cat.C. It was misleading as to the vessels capability as tests showed it had a propensity to invert and lacked buoyancy.

Personaly I think the time must come when anything designed/produced and commercially sold must meet certain clearly defined safety criteria and make all the information available to buyers. Also these specifications must not just be on a designers say so but must be indepently tested before being marketed. You can still have something with racy handling, requiring skill to sail well, like a high performance car but it must also meet a certain standard in safety.

Mychael

 

This is exactly the situation that exists in Europe, with the exception of some very specific boats or those used in very sheltered waters. The problem with this boat wasn't a lack of regulation, but a failure of the human implimentation of those rules.

There was also a manifest failure by the operator to exercise either reasonable competence or even common sence. It's possible that the outcome of this accident would have been no less tragic even if the boat had been Cat C compliant. However, a change in some of the other contributing factors would have prevented it altogether. The son admits that if they had listened to the Inshore Waters Forecast, they would never had set out.

The interesting thing about the MIAB website is it gives an idea of all the incidents that occur in the commercial boating world, despite every aspect of their boat's design and operation being fully controlled by legislation. The RYA, I believe, is correct in maintaining that we have enough legislation and controls, and it's the effective implimentation of these together with adequate training, that will ensure acceptable safety standards.

 

It is still down to the designer to supply the appropriate stability information to the client. Most will be happy to supply it. To be honest, the AVS is not perhaps as important as the AGZmax (angle of maximum righting arm). Which is typically about 70 degrees. AVS is a good safety number, but AGZmax really specifies the maximum operational heel angle.

I'm not sure if a designer/broker has to supply stability information by law, but it is a good idea to supply the owner with two copies of the owner's manual and stability book. One for home, and one for the boat. These should (ideally) be hard-bound. The owner's manual and stability book may not contain all the information in the designer's report (not necessary by law, but useful) but it should detail everything that the owner is likely to need. The designer should also have a copy of the owner's manual and stability book as well as his own notes and report which are available to the owner (for viewing only) at the owner's request.

My thoughts on the Bez 2 incident are well publised on the "Dinghy Tradgedy" thread. I believe it was passed for Class C on the basis that it would be adjusted to incorporate the extra bouyancy. Evidentally this didn't happen. and therefore it is between the owners, importers, builders and designers to argue about who is liable.

Tim B.

 

To get a CE marking is mandatory the intervention of a Notified Body to perform stability tests, in almost all cases, and this NB checks all stability calculations after testifying tests, so it's not only a designers say so. But, as happened with the Bez 2 (case already discussed in these forums) even NB's may make gross mistakes, to say the least, as well as owners.

 

TimB, how does one calculate the AGZmax?

Additionally, is their a quick and dirty way to estimate the CG of a sailboat?