The perils of edgy design offshore

Discussion in 'Sailboats' started by CutOnce, Jul 18, 2011.

  1. Paul B

    Paul B Previous Member

    Up until 2005 the Mac Race was run as a Category 2 event. To qualify for Category 2 a boat is required to have a minimum SI of 110.

    This brings up some questions. What happened in 2005 that caused the RC to change the requirement from Cat 2? Did the race get shorter, or closer to shore? If the course did not change, why was the requirement changed?

    Now the RC is changing the requirement again. Why are they not going back to the calling the race a Cat 2? Instead of doing that they are making up an arbitrary SI requirement, based on what?
     
  2. quequen
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    quequen Senior Member

    Sorry, but how is this 103 STIX index related with ISO 12217-2? What kind of sailboats reach that number? :confused:
     
  3. Paul B

    Paul B Previous Member

    Here is the SI calculation from the ORR book. You can read more in the ORR book available on the internet.

    Most boats easily meet the >110 requirement. I am attaching a spreadsheet of the 2011 ORR list with the boat types, ratings, and the SI (in the last column).
     

    Attached Files:

    1 person likes this.
  4. Stumble
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    Stumble Senior Member

    I can't speak for the numbers themselves, but both the J-30 and the J-35 will no longer qualify for the Mac. At least according to some class reps I talked too.
     
  5. Paul B

    Paul B Previous Member

    Not true. One of your class reps need to do some reading.

    The attached spreadsheet shows the J-35 averages around 111. That easily meets the 103, and would even meet the 110 requirement for Class 2 events. I didn't see any outliers that would fail the 103, but I didn't care enough to look at every one of them.

    The J 30 does seem to be one of the very few boats that does not qualify.
     
  6. quequen
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    quequen Senior Member

    Thanks Paul B!
    As an example
    J105: ISO STIX 33 - ORR STIX 112
    J120: ISO STIX 43 - ORR STIX 120 to 125

    Attached the RYA Stability List
     

    Attached Files:

  7. Paul B

    Paul B Previous Member

    I thought it would be interesting to plug in some numbers and see why the J-30 has a problem.

    Using the actual beam for MB we'll plug in 11.18.
    For DSPM we will use the listed 7000 pounds from J Boats literature.
    For LSM0 we'll use the published DWL of 25.0.

    With those points in the fomula it tells us the AVS of the J-30 is around 106 degrees. That's not very healthy for a boat going off into a Cat2 race.

    In the formula AVS is by far the most powerful variable. MB is also quite powerful, and we are lucky to have a pretty accurate number for that. It seems if you shaved the beam from 11.18 to 10.0 the SI would go up from 100 to 103.5. So the "pumpkinspeed plan" shape of the early J-Boats isn't doing them any favors under this calculation.

    DSPM and LSM0 are not very powerful and big changes to them make little difference in the SI. So not having the exact numbers here is insignificant.

    It would be interesting to see an actual certificate for one of the boats that have been measured.

    The J-29 has similar numbers to the J-30. That is not surprising, since they share the same hull (less a small change in freeboard) and both have keels listed at 2100 lbs. The 29 keel is probably about 4 inches deeper.

    If you look at the J-30, the low AVS is probably due to having a keel that is only 30% of the total displacement and somewhat shallow draft (5.25').

    The shape of those old J Boat keels also had a rather high CG compared to some other keels (fat up high, with a lot of taper).
     
  8. ancient kayaker
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    ancient kayaker aka Terry Haines

    Interesting thread. A pity there’s so much repetition and insults which make it a long and tiresome thread; but it seems to have settled down. Because of its length I haven’t had time to read the report by Hawley et al so forgive me if my comments below miss some facts.

    Extreme weather conditions are often highly localized; and it seems to me that, had it passed over another boat we could have been reading a different story, possibly just as tragic. There also seems to have been some bad luck - or bad planning - involved. I’m not defending the design of the boat BTW: perhaps the boat was unsuited for the conditions it encountered, but how many of the boats out there were? It wasn’t by any means the only boat laid on its ear. With multiple severe storm systems forecast before the start perhaps the race should have been postponed or cancelled? Certainly no place for children IMHO.

    A point that doesn’t seem to have had much discussion is the fact that the wings would slow but not prevent self-righting unless they have significant buoyancy positioned well out from the centerline. In the image of post #222 it looks like the majority of the volume in the wings is close to the hull so it should have little impact on self-righting, less if the wings can flood when immersed for a while. They would have a dampening effect on the usual violent motion following righting.

    Another unaddressed factor is the boat’s flooded state, obvious in the image of the upright boat in post #23. The boat is so completely swamped that all stability calculations for that boat - or any other in a similar state - are irrelevant. I am surprised she came back up again in that condition even if the wings flooded; few boats that low in the water would have done that.
     
  9. sharpii2
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    sharpii2 Senior Member

    That the volume of the wings was concentrated near the the hull has little to do with it's inverted initial stability. All that is needed is SUFFICIENT volume out at the ends of the wings. Think of it like a trimaran. The further the floats are from the main hull, the less volume they need to provide sufficient righting moment. Only a few inches of thickness at the tips would do the trick.

    The wing on the low side probably slowly filled with water until the water got close to the tip. That would explain why it took some time to right itself. When keel boats are swamped in the inverted condition, they usually lose the initial stability that kept them inverted, long before they are filled with enough water to sink. Several of the boats abandoned in the '79 Fastnet race were recovered after being found right side up and half full of water, long after the storm had passed.

    An inverted keel boat is like a top heavy boat with a wide flat bottom.

    Once water gets in, it sloshes to the lowest side until enough gets in to negate the buoyancy shift that keeps the boat inverted, the boat becomes unstable in its inverted condition and rights itself.
     
  10. Paul B

    Paul B Previous Member

    One more data point.

    Found this in a magazine from 1985. Seems a Kiwi 35 was rolled back then (twice), but looks like it came up both times.

    After the second roll it had to be rescued.
     

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  11. sharpii2
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    sharpii2 Senior Member

    If this one had water ballast, that would explain why it could right itself. The water could move to the low side, negating its buoyancy.

    I find it strange that, with the jib halyard gone, he could not get the boat pointed into the wind.

    It would seem that with the jib gone, the main would weathercock the boat into the wind.

    I'm just wondering about the sailing knowledge of the person who wrote this article.
     
  12. Paul B

    Paul B Previous Member

    If the boat rolled 180 degrees there is no low side, except maybe the top of the mast. Transferring ballast could help, but I don't think that's what occurred.


    Have you considered maybe he had been hove to? If you lose the jib halyard when doing so...


    I think the person who wrote the article was probably a pretty good sailor, probably better than many internet denizens.
     
  13. sharpii2
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    sharpii2 Senior Member



    That's my story. And I'm sticking to it.
     
  14. Paul B

    Paul B Previous Member

    Seems the boat this thread was originally about did a 360 (or a 180 and back) with no one pumping any water ballast anywhere.


    Seriously? You think this is a solution to keeping the bow into the wind in storm conditions?

    I wonder why no one else has ever figured this out? Maybe there is a good reason?


    If you tell us what type of boat it was maybe someone here can tell you what you were doing wrong.
     

  15. sharpii2
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    sharpii2 Senior Member

    It was a sailing raft of my own design (when I was 17). I put the boards too far forward. Due to the nature of the design, moving them aft was not practical.
     
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