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  1. #1

    Default Misleading stability

    Some moths back I had wrote a letter to Yachting Montlhy magazine and as I believe the subject is important I would like to share it and have your comments and opinions about the issues at stake.

    That's the letter that was sent by email:

    "Before I had a boat I was already a Yachting Monthly reader (I had my first boat 30 years ago) and occasional subscriber and I am grateful for the several articles on boat stability that were published along the years and the inclusion of boat stability data in your boat tests.

    This information has led me to take an interest in the subject.

    In my opinion Yachting Monthly have contributed to give its readers a better comprehension of boat stability: initial stability (for carrying sail), reserve and final stability (for safety). That information as well as the AVS and the inverted stability are important criteria that any boat buyer would have interest in knowing and understanding before choosing a boat.

    However I would like that on your boat tests the information on reserve stability would not be resumed only to the printing of a stability curve. The boat testers can only experience (while sailing) the initial stability, the one that is used for sailing and they do so, commenting on the boat performance (carrying sail ability, stiffness and power), but they normally remain silent in what regards safety stability. They cannot experience it (for example the capacity of recovering from a heavy knock down or capsizing) but they have the data to comment on that. So, why remain silent? Why not make that assessment as a part of all boat tests, instead of only printing the stability curve that many readers cannot understand?

    I would also like to call Yachting Monthly attention for the discrepancies on the stability curves that are provided by the manufacturers.

    I explain:

    When you tested the Hanse 430 I was amazed by the bad stability curve that was printed. I thought that there was a mistake because the boat had a better than average Ballast/Displacement ratio and a deep keel with a bulb and it could not have an AVS under 110º, a lot worse than all French boats that have a lower Ball/Displ and similar characteristics.

    I was interested in the boat so I talked with the boat designers on a boat show. The Naval Architect who talked to me said that the Hanse numbers where correct and that the French boats could not have the AVS (and stability curves) that were published and served to certify the boats. I thought that he was kidding. Everybody was wrong and he was right?

    This summer I visited a boat factory and talked with the resident Naval Architect. I wanted to know why the boat I was interested in had such a good Ball/Displ ratio and only an average AVS (117º). Well, he basically said to me the same as the other naval architect. Basically he said that the stability curves that were submitted for the boat certification were not supervised by anybody and that the different programs used for calculating stability could give completely different stability curves because they didn’t use the same parameters and that many curves where “artificial”.

    I was skeptical. It looked too bad to be true.

    But then he showed me a lot of ORC AVS (LPS), the ones taken from stability curves that are used for rating the boats for offshore racing … and I was finally convinced. Those stability curves are all calculated the same way (for all the boats), in a credible manner.

    http://www.orc.org/rules/Stability%2...ion%202010.pdf.

    After all, his boat had a good stability curve, if compared for instance with a Dufour 40e, that suddenly lost about 10º on the AVS to show just a poor 110,7º.

    (http://www.ffvoile.org/ffv/public/ha...pdf/10065a.pdf).

    And that is not the most worrying. The Dufour 40e is a performance boat and has a not so bad Ball/Displ ratio (32%). What would be the AVS of an Oceanis 43, if calculated by the ORC rules, knowing that the boat has a Ball/Displ of only 27%? It would be probably a lot worse than the 107º that Hanse claims for the 430.

    It seems that something is very wrong with the stability data that are used to certify the boats and that are given to the public. It seems that some boat builders (or their designers) use the correct data while others use programs that give inflated results. Anyway it is completely inacceptable that the data that serves to certify boats, specifically the stability curves, are not obtained the same way in all boats, as it is the one that is used on the stability curves that are used for rating the boats for Offshore Racing (ORC).

    I felt cheated and I believe that after reading this, I will not be the only one.
    I leave a suggestion for Yachting Monthly: Compare the LPS (AVS) that is on the ORC rating files with the one they use to certify the boats and ask the manufacturers why the difference. Let us know what they say and please tell us in what manufacturers the difference is more substantial. "


    Regards

    Manuel de Carvalho

  2. #2
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    One thing which strikes me straight away, and I don't know how or if it is included in AVS curves, is coachroof buoyancy.

    If you recall, the classic case of the Fastnet 1979 found flush deck, wide beam racing boats, probably with poor ballast ratios too, had stayed inverted for quite some time after B2 knockdowns.

    Some cruising boats designed using the Fastnet Enquiry results, the Anderson 26 is one example which springs to mind, therefore adopted high buoyancy coachroofs, so that if the boat became inverted the buoyancy of the coachroof would assist righting; exactly the same thinking as with modern lifeboats.

    It would be interesting to know if this is factored into AVS curves; I had always, perhaps lazily judging by what the OP says, thought it was ?

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    Think you are attacking the wrong people here. It is unreasonable to expect a magazine to carry out the tests you suggest.

    If there is a problem with the data provided by designers/builders, or the variability of methodologies, then you should draw this to the attention of the regulators and certifying bodies.

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    But perhaps the magazines could well play a part in raising a noise?

    There are other factors too, besides the coachroof bouyancy mentioned by Seajet, such as Bridgedeck height to prevent downflooding....

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    A while ago the ex-M.D. of Andersons and self were seriously thinking of putting the Anderson back into production; one of the things to sort was RCD clearance, I got the very strong impression we'd have to pull a test boat over to at least 90 degrees, in fact I discussed this with the RYA and they have a place to do this.

    I suppose a complete inversion as one needs for a full AVS curve would be difficult, and expensive in several ways ( I'm saying I think it was only a 90 degree test, memory fades and we had a sample boat we could do anything with anyway ) - one might think a computer program could estimate the inverted part, though as the OP says, it would be jolly handy and fair if everyone used the same software / formula.

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    If YM (or PBO etc.) got their teeth it a debate like this it might make it an interesting read.

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    It might lead to the loss of some advertising revenue too though...

  8. #8

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    Quote Originally Posted by Tranona View Post
    Think you are attacking the wrong people here. It is unreasonable to expect a magazine to carry out the tests you suggest.

    If there is a problem with the data provided by designers/builders, or the variability of methodologies, then you should draw this to the attention of the regulators and certifying bodies.
    I am not attacking nobody. I like Yachting Monthly and I am a subscriber.

    The reason I have posted it here was not for attacking anybody but just to attract the attention of the right people to call the attention of of the regulators and certifying bodies.

    I don't believe that the certified bodies ignore what is happening. The boat designers certainly know about that. It is not only necessary to call the attention but to join the necessary muscle to make them do something about it, against the interest of the majority of boat manufacturers and believe me, those have a lot of weight.

    Regards

    Manuel

  9. #9

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    Quote Originally Posted by Seajet View Post
    One thing which strikes me straight away, and I don't know how or if it is included in AVS curves, is coachroof buoyancy.

    If you recall, the classic case of the Fastnet 1979 found flush deck, wide beam racing boats, probably with poor ballast ratios too, had stayed inverted for quite some time after B2 knockdowns.

    ....

    It would be interesting to know if this is factored into AVS curves; I had always, perhaps lazily judging by what the OP says, thought it was ?
    No, this is not anything to do with coachroof buoyancy that is normally included on the stability curves.

    If you look at the links on the letter you will see that the AVS are completely different. No what happens is that most calculations for certifying a boat are based on the stability curve. The stability curve is provided by the manufacturer through the boat designer, that assumes the responsibility for its accuracy. What happens is that they use different softwares to make the stability curve, based on slight different criteria and the results are very different.

    It would not be too bad if the German boats were not always penalized and if the results that come from the comparison of the designer stability curve and the stability curve needed to certify the boat for ORC racing did not show always a difference on the French boats (and others) of 10º or more on the AVS. The German boats normally don't show a significant difference between the two AVS, meaning that the stability curves are close enough.

    The criteria for the stability curve used for ORC certification is well defined and mandatory to all boats.

    Those 10º difference in AVS would turn in Class B a lot of boats that actually are certified as Class A boats.

    Regards

    Manuel

  10. #10

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    Quote Originally Posted by Seajet View Post
    A while ago the ex-M.D. of Andersons and self were seriously thinking of putting the Anderson back into production; one of the things to sort was RCD clearance, I got the very strong impression we'd have to pull a test boat over to at least 90 degrees, in fact I discussed this with the RYA and they have a place to do this.

    I suppose a complete inversion as one needs for a full AVS curve would be difficult, and expensive in several ways ( I'm saying I think it was only a 90 degree test, memory fades and we had a sample boat we could do anything with anyway ) - one might think a computer program could estimate the inverted part, though as the OP says, it would be jolly handy and fair if everyone used the same software / formula.
    That is not needed. You only need paperwork (stability curve and all the calculations made by a program on the computer) and a cerified designer to provide the signatures.

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