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

    Ok Grimalkin capsized during a fastnet race, that by itself does not make it a bad design. After that disaster the capsize number formula appeared. a number less than 2 (the lower the better) being accepted as good enough for offshore racing. In the book voyaging under power Beebe describes what is required of a boat to cross the Atlantic safely. His boat was 48-50ft. one of the best choices is the Nordhaven 46.

    Before the fastnet race other Golden Shamrock (30ft 7055lbs displacement) class boats were winning races. Now it is my opinion that any boat which is good enough to run the fastnet race is good enough to cross the Atlantic. The problem was the force 10 storm. There has never been a boat built which would leave me unconcerned in these conditions. NO ONE CHOOSES TO GO OUT IN SUCH CONDITIONS except rescue services. Any other year the Grimalkin would probably have made the distance without trouble.

    A quote from Masthead 03, "As a general rule serious seaboats require an Angle of Vanishing Stability of 120 degrees". Later it says 115 degrees or higher is ok when the capsize number is less than 1.7

    Grimalkins numbers are AVS = 113 degrees
    CN = 2.12
    NordHaven 46 numbers are AVS = 111 degrees
    CN = 1.69
    A laser dinghy has a CN = 2.71

    The Nordhaven was designed to be ocean going yet according to masthead 03 niether make the grade. What do you think.
     
  2. Tad
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    Tad Boat Designer

    I would say that's highly unlikely.

    There are many factors relating to seaworthiness and surviving a storm at sea. A boat's range of positive stability is just one.
     
  3. PAR
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    PAR Yacht Designer/Builder

    There are other issues associated with Grimalkin (Nicholson 30 ) had other issues to consider other then it's performance in the '79 Fastnet, to make less then desirable in heavy air off shore work. Generally the Holland design is well suited for near shore work and suffers from many of the early 70's IOR rules. She needs to be sailed flat or rounds hard, but this would be typical of boats shaped like she is. Grimalkin survived the '79 Fastnet (and to this day), though I think unfairly became the focus of much investigation. She wasn't a radical IOR for her era, but she was carefully examined and many changes to IOR and design in general came about. simply 30' boats in force 10 winds will have difficulty. I've owned early IOR's and they can be fine boats, given their "quirks". The Nicholson 30 is a solid, well built boat, though the 23 is a much better boat.
     
  4. Mik the stick
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    Mik the stick Senior Member

    Tad I calculated AVS to be 111degrees from Displacement = 48320lbs a draft of 5ft and ballast of 6600lbs from Beebe's book.
    Par I tryed to find out the requirements to race Fastnet now: an IRC rating of 0.85 the formula is a secret, and stix of 48. stix is so complicated I would be inclined to turn up for the race (with a new design perhaps) and say ok you rate it. AVS is the best stability rating I understand and can use. When Grimalkin raced at Fastnet nobody threw their hands in the air and said you cant take that boat out there.
     
  5. TeddyDiver
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    TeddyDiver Gollywobbler

    There are other factors too. Your "calculation" is an inaccurate estimate at best..
     
  6. Tad
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    Tad Boat Designer

    You cannot calculate AVS from a few numbers and someone's rule of thumb, it will be wildly inaccurate unless you are comparing like with like. Any rule of thumb will have been created for sailing boats with deeply immersed ballast and a relatively low CG. Powerboats are completely different. And at high heel angles all boats are completely different. The N46 will run out of stability between 80 and 90 degrees heel with full tanks. With empty tanks AVS will be below 80 degrees heel.

    The accurate calculation of AVS will require hull lines from which you create a computer model. You will also need accurate floatation measurements of the boat to ascertain real displacement. And you'll need to do an inclining to establish the real VCG. Run all this information through a hydrostatics program and you'll have a believable AVS. Then we can have a serious discussion.
     
  7. Mik the stick
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    Mik the stick Senior Member

    The calculation from Gerr is the standard Wolfson method times 97%. If this is inaccurate how can Wolfson and Gerr publish it. I admit inaccuracy is a possibility since a Trawler type yacht is not at all like the Grimalkin. I thought a NA would use this or similar formula to predict what was expected then adjust the prototype.
    Is there a formula/calculation with which I could rate a boats stability.
     
  8. TANSL
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    TANSL Senior Member

    One way to measure stability is to calculate the value of the righting lever (GZ) for various heeling of the boat.
    The formula for calculating the GZ is shown in the accompanying figure.
    GZ, depending on the type of ship, must meet certain conditions required by the stability criteria applicable to the ship.
     

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  9. PAR
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    PAR Yacht Designer/Builder

    Geer sites this calculation method as a "quick and fairly accurate" method, to roughly access a boat's AVS. My rough estimation of the AVS on the MKII Nicholson 30 is much closer to 120 degrees, though I haven't run a full curve through 180 degrees.
     
  10. bpw
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    bpw Senior Member

    Even if it had really good stability Grimalkin is not a particularly good design for anything except racing under the IOR rule.

    And I found a stability table showing less than 120 degree AVS and comparing it to a contessa 32. On my ipad so hard to post it but it shows up in a google image search for grimalkin fastnet.
     
  11. PAR
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    PAR Yacht Designer/Builder

    I got about 118 degrees (117.7) . . . The MKII wasn't that bad, compared to others in her class, but she's still what she is.
     
  12. Mik the stick
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    Mik the stick Senior Member

    I got ssv=34.97 from Tc=1.4ft Disp =7055lbs (400/34.97-10)+100 = 116
    116 times 97% =113 degrees
     
  13. Mik the stick
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    Mik the stick Senior Member

    This is the document mentioning contessa 32 i think
     

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  14. Mik the stick
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    Mik the stick Senior Member

    I wonder if you could race a Grimalkin at fastnet now. I looked at what was required a Stix rating of 48 or more I think. Now I like maths but stix is so complicated and requires you to know so much about your boat. I'm sure it is designed to frustrate Naval Achitects.
     

  15. TANSL
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    TANSL Senior Member

    Calculating STIX is not a complicated thing for NA, is something tedious. Rather it may frustrate those who are not NA, because for them it is not easy to carry out that calculation.
     
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