Grimalkin IS a good design

Discussion in 'Stability' started by Mik the stick, Sep 9, 2014.

  1. Mik the stick
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    Mik the stick Senior Member

    An interesting document but begs the question how do boats of about 40ft get entered for racing if UK regs judge them unsuitable. I include the previously mentioned document for comparison
     

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  2. sharpii2
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    sharpii2 Senior Member

    A very good point.

    But I do believe that the '79 Fastnet storm created some highly unusual local conditions, even by off shore standards. Same too with the '98 SH storm.

    I read that one boat got out unscathed simply because the skipper sailed it off the course. The winds were the same but the waves were less chaotic.
     
  3. TeddyDiver
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    TeddyDiver Gollywobbler

    Not unusual, just not predicted at the time being so strong. Today they have the most poverfull computers computing weather patterns and they are 50% of time wrong, anyway in high latitudes. Long term the odds are against survival with unseaworhy vessel..
     
  4. MikeJohns
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    MikeJohns Senior Member


    It's pertinent that the CYC report into the 98 S.H was considered a whitewash of the real issue which was predictably poor offshore boat design. Ironic really that they blamed the unpredictable nature of maritime conditions ! That belies the fact that many of the boats had predictably poor design for the conditions they found themselves in and that survival conditions can occur at any time through the year.


    Bass Strait is renown for severe localised poorly forecast weather events that are a hazard to smaller vessels. It's a rough piece of water. The severity and frequency of adverse weather is double that of the English Channel region.The 98 S-H fleet provided only a handful of the myriad of sailing craft that have come to grief in that region over the years.

    The MCA guidelines adopted for commercial operations ( In survey) that I posted before, are a good guide. But they are less stringent than Southampton initially suggested. The initial proposal was for an even safer standard.
     
  5. MikeJohns
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    MikeJohns Senior Member


    Wave tank testing has shown that all boats regardless of design can be knocked to 90 degrees by a significant beam on breaking wave. A knockdown isn't a capsize but whether it carries on rolling or recovers correlates reasonably well with the area of the GZ curve between 90 degrees and the Limit of positive stabilty.

    The energy (area under) indicated by the GZ curve (multiplied by the vessels mass) between 90 and LPS is a good comparison of capsize resistance of different craft.
     
    Last edited: Sep 20, 2014
  6. Mik the stick
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    Mik the stick Senior Member

    Thank you I' beginning to learn a bit more about /safety and stability.
     
  7. sharpii2
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    sharpii2 Senior Member

    Generally, I'm inclined to agree with you.

    But after reading a book about the storm and the devastating damage it did to some of the boats, I'm inclined to believe the only safe vessel would have been a nuclear submarine.

    All boats are a compromise between performance, livability, and safety.

    The sea can destroy anything made by man.

    Boats that are fast have to be designed to minimum safety standards, so they can perform their major design function, which is to go fast.

    I don't know how you feel a bout the so called "open 60" boats, but I see them as being even more extreme than boats such as "Grimalkin". The major difference being that they are designed to carry massive spreads of sail, while Grimalkin was designed to make the optimum use of limited SA.

    Many Open 60's have completed their voyages in some of the roughest conditions known to man. This is despite the fact, that until recently, they had an AVS that was even lower than most IOR era boats, including Grimalkin. I doubt that Slocum's Spray had an AVS even close to that, but it sailed around the world and even rounded the horn, as have several copies.

    I recently downloaded a "STIX" spreadsheet and put in the data of a design of mine intended to be extremely seaworthy. It did well with the numbers and earned an "A" rating. Even so, I doubt it would be able to survive toppling 50 ft breakers.

    By the way, I think I understand the STIX system and agree with it.
     
  8. TeddyDiver
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    TeddyDiver Gollywobbler

    The main difference between those two is the size which makes the Open60 much more stable.
     
  9. MikeJohns
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    MikeJohns Senior Member

    The UK Coastguard (MCA) stability requirements were produced by Wolfston, Southampton after a very thorough investigation. They make a very good guide, not just to designers but also to boat operators. Introduced circa 1990 it well predicts that the boats rolled in the 98 SH were at risk. Had we the data we could produce a more accurate predictive tool adjusted on the area of the GZ curve from 90 to LPS ( multiplied by the crafts mass). That now appears to be the best single indication of capsize resistance for ballasted monohulls.

    As for open 60's. A sensible LPS goes down as the boat gets bigger. For example the MCA requires a 25m sailboat have an LPS of over 90 degrees, While at 10m length it's over 140 degrees. You might also find it interesting that the open 50's were not adequate for Southern latitudes and had a high rate of inversion. The open 60's just by virtue of the extra size fared much better.

    As for yacht racing: Yes smaller boats sacrifice safety for performance, generally the odds are stacked in your favour through good weather forecasting and prompt close rescue services. That's not a good paradigm for a cruising boat, which is why you should choose for example, the C32 over Grimalken. That is if you sail offshore, outside of the benign weather areas of the world. As Marchaj spent so much effort showing in his book about Seaworthiness.

    We can make sailing craft reasonably invulnerable which means stacking the heavy weather odds in favour of not rolling. It can still happen though much less likely.
    You'll note that the predictably safer boats in the 98-SH were not casualties of wave inversion. The riskier boats are easily identified and skippers should know the risks and their crafts vulnerabilities. Few do, and many don't want to know, blaming bad luck or the unpredictable nature of the ocean on the disasters.

    Here's Grimalken and a similarly sized Contessa plotted on the MCA requiremnt and their respective GZ curves. Grimalken was a predictable csaualty and the C32 predictably not a casualty.
     

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    Last edited: Sep 20, 2014
  10. MikeJohns
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    MikeJohns Senior Member

    And a high roll gyradius from a bulb on the end of a 15 foot keel arm.
     
  11. bpw
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    bpw Senior Member

    And the best sailors in the world doing everything humanly possible to keep the boat on its feet.

    The Open 60 has a pretty good safety record (especially if you ignore structural failures) but I suspect it would be a different story if they where being sailed by the average weekend racer. These are after all boats in which a single mistake can drop the rig over the side.
     
  12. CT 249
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    CT 249 Senior Member

    The S&S 34 and Cole 43 were among the boats that inverted in the 98 S-H, therefore they must not be among the "predictably safer boats" according to the above classification.

    The S&S 34 appears to be quite similar to the Contessa 32 in general design terms, as does the Cole 43. What makes them designs that were "at risk" and the Contessa 32 "predictably safe"?

    Given the record of both boats, particularly the S&S 34 which has completed several nonstop solo circumnavigations, is it fair to classify them as risky?
     
  13. MikeJohns
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    MikeJohns Senior Member

    Chris

    Well, risk is based on a statistical likelihood. It doesn't say that it cannot happen. Whether it's likely that a boat will be rolled past it's LPS or not is surprisingly well indicated by this method. It's not about completely eliminating risk but reducing risk to an acceptable level. Wolfston looked at a lot of inversions of craft to obtain their data set.

    I posted before that more recent tank testing shows that all boats are similarly vulnerable to knockdown to 90 degrees from the same relative sized breaking wave encountered beam on. Whether a knockdown proceeds to a full inversion is the significant factor. Not that the boat is simply knocked down.

    Current refinement of the inversion model for ballasted monohulls shows the biggest factor in the boats favour is the area under the righting moment arm from 90 degrees to LPS. There are other factors that make a difference but they are all much less significant.

    There is another boat not mentioned before and that was Loki. Another predictable casualty by the Wolfston/MCA screening method.

    Another issue that became clear in 98 and that was that ABS ORY minimum fore deck scantlings for GRP craft were deficient.

    Can you give me the names of the two boats you mentioned and were they knocked down or rolled ? There is a very important disticnction between the two but it is often confused .
     
  14. CT 249
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    CT 249 Senior Member

    The Cole 43 Solo Globe Challenger (LPS 130+) was one. Incidentally, her sister rolled in the 1970 S-H. SGC's very experienced owner said that she may not have survived another wave like the one that rolled her. The same boat and owner have since done a solo non-stop circumnavigation via the Southern Ocean.

    Solandra (S&S 34 with a very experienced crew and LPS of 130.3) rolled, at to 180 or close to it and probably 360; I haven't gone through the police reports slowly recently to confirm whether it was 180 or 360.

    What is the a very important distinction between a deep roll and a full inversion? Boats that rolled deeply have lost more people than those that inverted.

    Re "risk is based on a statistical likelihood. It doesn't say that it cannot happen."

    Yes, agree 100%. Which is why arguably the use of Grimalkin and similar screening values may (IMHO) be over-rated.

    Nothing is perfectly safe. We can keep up increasing safety levels but there is a cost; not just financial but also in whether people get driven out and do other activities (which may or may not be safer) or cut costs (and hence often reduce safety factors) in other areas so that they can afford to keep on doing the sport they love. Pushing people into buying larger boats, as some of the current rules do, does not come without costs, and not all of them are financial. IMHO that should not be done without a clear case that in the real world, more people are dying because of boats that are more likely to invert and that cause may not have been shown yet.

    The other thing that appears to be often ignored is the fact that all else being equal, a faster boat can be considered to spend less time at sea during a given passage and therefore will spend less time at risk of extreme conditions. Consider a case where boat A is 4% faster than boat B and therefore A spends 4% less time at sea if they both do the same race. Surely it would be reasonable to say that all else being equal, boat A has 4% less chance of being exposed to conditions that could knock her down.

    Very rough and quick sums indicate that there is something like a one in a thousand chance of being inverted or nearly inverted in a Fastnet or Hobart. Therefore if we increase the chances of such a roll turning into a dangerously long inversion even by (say) 33% by sailing boats with a low LPS, the probability of such an event remains extremely low. The indications are that sailing a low LPS boat is much less dangerous than sailing a boat that is 4% slower and therefore remains exposed to more storms.

    It would appear to be arguably safer to have a faster boat that is at slightly more risk in the extremely unlikely event of a serious knockdown than to have a boat that is slightly safer in this extremely unlikely event, but which spends a significantly greater time at risk during a passage.
     

  15. MikeJohns
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    MikeJohns Senior Member

    Low probability but extreme risk if survival conditions are encountered. That's the crux. Racing fleets can stack the odds more favourably for any design through detailed weather analysis and close rescue services. I said before that's not a good paradigm for a cruising boat. Racing boats also have large able crews.
    The opening post is exploring whether a Grimalkin equates to a good cruising boat for him to cross the Atlantic in.

    Kim Taylor pointed out that the boats Dovell considered as having inverted in his report to the coroner were predictably likely to invert when compared with MCA guidelines. But the MCA stability guidelines do predict those casualties quite well.

    To argue that being knocked down is somehow more dangerous than inversion based on fatalities in the S-H is creative, but boats that are inverted invariably lose their rigs, are partially flooded and usually disabled, they are are also at a considerably greater risk of being rolled again. Prompt rescue changes the outcomes, and what occurs close to helicopter rescue service is quite a different scenario to mid ocean.

    A storm is pretty unlikely to be encountered twice in a passage or even a race, If it occured close to the finish your 4% faster boat will be in port. But then the slower boat will also have had the benefit of an advance warning and it's getting close to shore too. If it's a long passage and the survival conditions occur mid passage then the more seaworthy the boat the better.

    The inversion lottery requires the boat to encounter a beam on breaking wave. If the wave is not breaking it won't invert a boat. Broaching beam on to a steep wave is a dangerous pre-cursor to inversion as the steep wave is more likely to break. As is a pitchpole.

    Without good forcasting the fast boat can run into bad weather that abates later or moves away from the course sailed. With good advanced forecasting either boat can plan ahead and alter course if you want. It's a pretty open ended scenario. I wouldn't consider it significant until the boat can average around 17 knots, then it can effectively weather route to great advantage.

    The knocknown to 90 degees for all hull types came initially from Wolfston by Andy Claughton at the dept of ship science researching the Fastnet casualties. Various other experiments with models have been conducted and validaded his findings. Van Oosanen is also very interested in this and has his own analysis and is worth contacting if you are interested. He was part of the working group that put the STIX system together for ISO. There have also been detailed studies in Japan but the studies have not been translated. Claughton actually suggested the stability between 110 degrees and LPS as a good capsize screening indicator !

    Casualty analysis is pretty stock standard way of setting requirements or recommendations. I'll post below one of Peter Van Oossanen's 'casualty' based plots as an example of statistical data and fitting a reasonable indicator. Note that this is his intellectual property. But it gives an idea of how casualty analysis is used.
     

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