Ballasted multihull thought experiment

Discussion in 'Multihulls' started by Clarkey, Oct 29, 2017.

  1. Clarkey
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    Clarkey Junior Member

    I was looking through some old books and AYRS papers the other day and was struck by the many attempts at creating a self-righting multihull, or at least a system to right one eventually. There were catamarans with ballasted keels and water bags on gin poles to right tri's end-over-end and all sorts of other strange stuff.

    At a certain point I started to think 'why does the boat have to roll upright in either direction?' could it be sufficient that the boat can only right in one direction?

    This pondering has led me to the idea of a tacking outrigger with a relatively lightweight ballasted keel on the vaka, enough to lift the ama back over even from a turtled position and put the boat on it's feet again. I wonder if this arrangement could provide an interesting combination of the self-rescue capability of a narrow keelboat and the stiffness and sail-carrying power of a multihull?

    I know the very idea of ballast on a multihull is anathema to many but wonder if this idea has come up before (there being very little truly new under the sun)?
     
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  2. Tom.151
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    Tom.151 Senior Member

    One way to think about the choices is to remember that adding ballast to an unballasted boat is adding unneeded ballast weight that you will carry and slow you 100% of the time - when in truth a water-bag with a gin-pole would do the job as well for the once in a lifetime or maybe never times you need it.

    It's a gamble in any case. But it's worthwhile to assess the probability of even using the righting method your choose, as part of the decision making.

    For smaller, 20 to 25 foot single outriggers, such as the Gary Dierking designs - they seem to need nothing to re-right if capsized, with crew body weight being sufficient on the safety ama side of the boat.
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Some of the fastest multihulls around have used ballast to go faster including Hydroptere. I think there is a lot more development that could be productive for ballasted multihulls.
    Just for inspiration: years ago when I suggested that a keelboat could foil there were a lot of people that ridiculed the idea-then all of a sudden Hugh Welbourn designed and built a foiling keelboat scow-the Quant 23. Never say never.
    Good Luck!
     
  4. Tom.151
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    Tom.151 Senior Member

    Help me out here Doug, the only imperfect recollection I have of Hydroptere ballast is water ballast tanks in the amas. But there have been so many iterations, that I certainly could have missed.

    Has Hydropytere tested fixed-high-density ballast?
     
  5. gonzo
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    gonzo Senior Member

    To start with, what size range are you talking about? I can right a Hobie Cat with my body weight.
     
  6. Clarkey
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    Clarkey Junior Member

    I suppose I am thinking about the awkward middle ground - something about 6-7m with a small cabin for solo coastal cruising. Too large to right easily singlehanded but without the stability of a 10m+ multi. Imagine something like a single Tiki 21 or 26 hull with an outrigger and a small ballasted bulb keel?
     
  7. Doug Lord
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    Doug Lord Flight Ready

    =======================
    Not to my knowledge, Tom. She used waterballast in the amas with an intake on the rudder(!) and had at least one tank in the main hull, I think.
     
  8. cavalier mk2
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    cavalier mk2 Senior Member

    Looking at the solid log ama of many traditional flying proas and their large crews I think it possible to submerge the solid ama with crew weight and right the vessel passing the ama underneath submerged. It's either got to go over or under after all. The solid ama functions as ballast on the lever arm under sail and gets some dynamic lift through shape as it skims the surface. The solid construction reduces the amount of buoyancy to over come when righting by submersion. Modern one-2 man crews are probably going to need negative buoyancy construction of the ama like solid glass skins and regulate the bouyancy from positive to negative with air/water tanks-flooding. Seems like Walter Green took this approach on the construction of the 35' cat he marketed for awhile with one hull having the solid skins for the above reasons.
     
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  9. ThomD
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    ThomD Senior Member

    Russ Brown type pacific proas are self-righting like a mono, they suffer a knockdown, the roll back up. Main problem is it is almost too willing to do so, but it is a proven concept (in larger versions).

    In Cats the main thing in that size range would be a G32. Jan was considering a 26" version shortly before his death. He felt all his other boats were too much trouble, that was before he launched Strings, and I have no idea if he changed his mind.

    In the real world, the 21 foot ply wharram is a toy. Admittedly one was sailed around the world. But the composite Wharrams have always been considerably different from the ply ones. Also, you are getting into the wearable boat range, so body size maters a lot. If you are too big, you will be in the wearable boat game.
     
  10. Clarkey
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    Clarkey Junior Member

    A big part of what I am speculating about is what such a ballasted outrigger would be like to sail, rather than being solely focused on the self-righting capability.

    With the ama to leeward I expect it would behave much like an even more exaggeratedly stable atlantic proa and with the ama to windward it could be an extremely forgiving pacific one. The design in my mind would have a very buoyant but lightweight ama - really the polar opposite of a dense log (though they have their own merits as has been described).

    Really, what I am pondering is if there may be a little-explored sweet spot for a relatively swift but forgiving small coastal cruiser that isn't tender and cramped like a small mono but is a bit tamer and more capable than a powered up small multi.
     

  11. cavalier mk2
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    cavalier mk2 Senior Member

    The solid ama is best for pacific proas. You want buoyancy to leeward for an Atlantic. Ballast keels on small cats have been done by the Brits years ago combined with a masthead float to keep it from flipping 180. For my money you'd be best served by a mono ULDB in that size range. We have a thread here somewhere on Golden Miller and Misty Miller, Mike Henderson's boats but here you go.
     
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