Hard v round bilge and leeway

Discussion in 'Multihulls' started by Lachie, Mar 23, 2021.

  1. Lachie
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    Lachie Junior Member

    A lot of discussion on the forum has been on the relative merits of hard v round hulls and little reference to sideways motion. I noticed years ago that that the Woods Gypsy , a chine boat, I built seemed to go well to windward, and at the time felt it was due to the inability of the water underneath to flow from leeward to windward due to the turbulence the chine created. I remember discussing this with Richard and at the time he felt the same. I am also aware that I have very little scientific knowledge to say one way or the other. I have just finished a Kohler Duo 480 c which has assymetric hulls, the outsides of which have perpendicular sides and thus a very hard chine. You see this with the Paradox mono and a number of small catamarans. I am wondering whether this very hard chine may be largely contributing to its windward ability or not, thanks.
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    It sounds like swings and roundabouts, you get an advantage at certain points of sailing, but lose on others.
  3. redreuben
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    redreuben redreuben

    I thought the windward ability of the Kohler hulls was the lift effect of the asymmetry ?
  4. Lachie
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    Lachie Junior Member

    Yes but I wonder how much the perpendicular sides of the assymetric hull are preventing leeway. The Paradox also has perpendicular sides and it uses chine runners as well. The Micromegas 5 also had slab sides and no boards or LARs. As I said earlier I have little knowledge in this area, only observations for what that's worth.
  5. redreuben
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    redreuben redreuben

    The only way to do that is to do the science. Kohlers vortex thingys are also claimed to stop or disrupt the flow but nobody to my knowledge has done the experiments so all we have is here say and some say it made a major difference and some say it does nothing. Take a look at any boat designed to take windward work seriously and it has daggerboards.
    Cruisers use LAR keels for convenience and dependability. Asymmetric hulls might work a bit but have design limitations due to the shape and high aspect ratio required.
    You might want to read some of Phil Bolgers work.
  6. jamez
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    jamez Senior Member

    I think that chines on a dory or single chined hull give some dynamic lift that is absent from a round bilge configuration. How this could be measured in a realistic fashion is beyond me. Real world examples: My tri has single chined floats and with the daggerboard up will sail to windward surprisingly well although obviously not pointing as high as with the board down. When I rebuilt my Wharram Hinemoa years ago I replaced the worm eaten Kohler style winglet/grounding skids with a piece of 40mm H3 tanalised pine. These were slightly narrower than the flat on the bottom of the hulls and other than a chamfer front and rear they were not faired into the hulls in any way, leaving a square edge both sides. During a subsequent haul out I had a conversation with a guy who had previously owned a Hinemoa in the UK. I mentioned that I was thinking of fairing in the edges of the grounding skids to make them look a bit tidier. He said he had done the same on his boat but doing so had adversely affected its windward ability (which in a Wharram is saying something). His advice was to leave them as is - which I did.
  7. Gary Baigent
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    Gary Baigent Senior Member

    Asymmetric and very fine hulls on CSK catamarans of the 1960s and '70s had a reputation of being okay to windward designs - until they lined up with board boats of the day. I remember Jim Young talking about the large CSK cat design he built and then launched off Takapuna beach (they bulldozed a trench for the truck) and he was quite angry with the sloppy steering (because of the very shallow draft rudders) and resulting poor windward performance because of pitching when it was delivered to Noumea.
  8. patzefran
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    patzefran patzefran

    Asymmetric Hobie cat hulled definetely rely on their rudders to provide lift when going to windward. They use plenty of aft mast rake to load the rudders and the rudders are balanced to keep a light helm ! contribution of hulls is negligible at low leeway angle. Hobie Alter designed good boats with wrong ideas !
  9. Clarkey
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    Clarkey Senior Member

    Yes, I think this is also true of the Paradox. Traditional Venetian sailing boats have operated like this for centuries.
    Last edited: Apr 12, 2021

  10. guzzis3
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    guzzis3 Senior Member

    Just a quick comment on hobies. I have a seat of old (early) H14 rudders in the shed. It never ceaces to amaze me how big they are. They are just a tad smaller than the rudders and dagger boards specified for sango and other similar size modern catamarans. Hung off a 14' beach cat they are absolutely HUGE!
    JimMath likes this.
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