Single-person keelboat stability

Discussion in 'Stability' started by Hernandiz, Feb 9, 2009.

  1. Hernandiz
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    Hernandiz Junior Member

    Hi,
    I'm spending some time on a 18' single-person keelboat design.
    considering that the skipper don't move transversally the righting moment shall be greater...
    Based on Skene's Elements of yacht design I assume that the WPC might be around 1.5 or a Dallenbaugh angle of aprox. 10deg.

    does it look right?
    prelim hull form.JPG
     
  2. alan white
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    alan white Senior Member

    No pivoting keel? A design like that almost demands some mechanism to keep the boat upright. If there's a wide stern to encourage planing, without crew weight to windward, she'll sail on her ear and develop a bad weather helm due to an assymetrical waterplane.
    I think this kind of concept works well with a displacement speed-limited boat with canoe-like waterlines which remain balanced as the boat heels---- which it will without a pivoting keel, which is okay if it is designed to do so.
    It will also work well as a planing boat using a hinged keel and a reasonably wide stern.
     
  3. PAR
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    PAR Yacht Designer/Builder

    On boats that size and shape, you have to expect the crew to provide some "live" ballast, or make the initial stability very high to stand against her area aloft. Considering how light your design appears to be (18', very fine entry, firm yet modest width bilges, shallow hull, etc.), you'll have little choice but to move the crew or ballast the appendage(s). You could divide the rig up into smaller, lower aspect pieces, but then some elements of your performance envelope will suffer.
     
  4. Hernandiz
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    Hernandiz Junior Member

    I will reshape the hull to be wider ( about 6') but preliminary stability looking really bad! I think I will have to use a pivoting keel.
    My first idea was a simple affordable ant exiting boat but do those 3 words can fit?
     
  5. alan white
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    alan white Senior Member

    One further option, and one that isn't so expensive to do, is a water ballast that shifts between two tanks located at the greatest beam (and that area could be extended somewhat like a sponson)-----it's perfect for foot pump operation (bicycle crank chain-geared to diaqphragm pump, etc.)
    100 kg wouldn't take too long to shift (and an electric pump isn't out of the question). That would be enough to tame the beast.
     
  6. Hernandiz
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    Hernandiz Junior Member

    That might be a good idea...
    for the moment (if I'm right) the WPC=1.46 with a 6ft wide hull and pivoting keel.
    Not a good as I expect but on the right way.
    I don't think a water ballast could be as much efficient but look interesting.
    minimiV4.JPG
     
  7. alan white
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    alan white Senior Member

    The movable water ballast is effective with a centerboard, which can be raised. This would substantially increase speed when running, where the deep bulb becomes a parasitic appendage. In such a case, the water can be discharged to lighten the boat allowing easy planing.
    Trailering would become possible, something you might enjoy in a practical design.
    I don't know if the engineering and construction, and day to day costs of a pivoting keel would be worth it. It depends on how much money you've got to play around with.
    The concept of an airplane-like cockpit is interesting though, where one has all controls led to one place. Blondie Hasler's Jester, though larger, was set up that way. Lighter boats have to shift ballast somehow.
     
  8. Doug Lord

    Doug Lord Guest

    Attached Files:

  9. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Doug, that would be a fair representation of a performance dayboat of the general size desired, except for the hiking requirement.

    A canting keel will likely also require twin daggers, or some other form of lateral area to offer some resistance to leeward skid.
     
  10. Doug Lord

    Doug Lord Guest

    I came up with the idea to use a retactable foil within the bulb of a canting keel in order to get rid of the requirement of daggerboards or twin foils(CBTF).
    Here are two renderings of the K FOIL:
     

    Attached Files:

  11. Paul B

    Paul B Previous Member


    Don't listen to these "old style" thinkers who want every boat to be a wineglass hull with a yawl rig.

    The SKUD 18 is similar to what you are proposing, only it is a 2 person boat. It is used for paralympic sailing (folks with paralysis, amputations, etc) and the sailors sit on centerline. It is a skiff shape, yet somehow does not exhibit the problems the "old school thinkers" always attribute to boats that are not of the 1950s style written about in books from the 1960s.
     

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  12. Paul B

    Paul B Previous Member

    Of course this guy isn't telling you that for his foils to be big enough, the bulb that they would "retract into" would have to be so oversized and draggy that it would be a joke. Ask him to see a technical drawing and see where that gets you.
     
  13. Doug Lord

    Doug Lord Guest

  14. Hernandiz
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    Hernandiz Junior Member

    Hi Paul,
    The skud is a quite good example of what I'm looking for. did you tried one? Or have some extended spec. I would like to compare with my design.
     

  15. Paul B

    Paul B Previous Member


    You can see the class website:

    http://www.skud.org/?Page=18384


    No, I have not sailed one. I have seen them up close. The design was by Julian Bethwaite, same person who penned the Olympic Class 49er.

    Safe and easy enough for handicapped (handicapable?) people to compete in.

    Therefore claims by people who only know what they read in 40 year old books about 60 year old designs should be ignored. I don't drive a 60 year-old car and claim it is better performing and safer than current design, why would I believe it when someone makes the same claim about sail boat design?
     
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