basic multihull knoweledge

Discussion in 'Multihulls' started by neptunkryssare, Oct 23, 2012.

  1. bruceb
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    bruceb Senior Member

    buc float displacement

    Warwick, On my old buc 24 and other tris in that series, the floats were "about" 90-110 percent of assumed loaded displacement- lightly loaded you could almost/fly the main hull, loaded (or overloaded, as often the case), the float will bury when pushed too hard. My understanding is that they were intended to bury in a gust and the tri would round up much like a mono instead of capsizing. There are still lots of Bucs around, I guess it works most of the time;)
    I have added a-boards to my stock floats, and sailed some with them. My first impressions are that the lifting foils are an improvement (and certainly fast) in most conditions, but they don't make up for lack of float volume coming out of tacks and/or slowing down from punching a wave- I still think that at least 150% of real world sailing displacement would be better, even with the foils, at least on a moderate performance tri. B
     
  2. warwick
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    warwick Senior Member

    Thanks Bruce for your last post and comments.

    My problem was what load displacement point to work from, so I would think it would be best to work from a maximum pay load/ displacement. For it to be easily driven float would need to be about 150 %.

    As with most thing you need a starting point.
     
  3. neptunkryssare
    Joined: Sep 2009
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    neptunkryssare Peter

    That float volume you are talking about, is it volume per hull.?
    In that case I don't get it. Because in almost all tris I see, any sidehull is largeer than the mainhull.
     
  4. warwick
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    warwick Senior Member

    Hi Peter,

    I was referring to each float as individual percentage of the main hull displacement.

    In the past they were smaller around 100 percent, now the trend is to fly the main hull if and when possible.
     
  5. bruceb
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    bruceb Senior Member

    novice

    Warwick, I am a novice in tris, I can only tell you what I have observed with mine and a few others, but I do agree that starting around 150% floats will at least work. I have the specs on most of the current tris on the market, and the trend seems to be even larger floats , often in the 200% or more range, although only a few of them are using lifting foils in the floats. (Yet) B
     
  6. warwick
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    warwick Senior Member

    Thanks Bruce, we are all novices at some point, I think you are passing on relevant information gain from experience.

    From what I am finding with my own design that started out as a hobby idea, is that 150 percent of maximum payload is giving me about 180 percent in normal displacement.

    As to lifting foils, it could be a case of their benefits need to being proven.

    I am considering asymmetrical float dagger boards to free up main hull interior, then the next logical step would be combined with a lifting aspect latter on.
     
  7. bruceb
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    bruceb Senior Member

    Yes

    My asymmetrical float daggers are straight, set at about 24 degrees to vertical (tips inward) and I get quite a lot of lift from them. B
     
  8. frenette
    Joined: May 2011
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    frenette Junior Member

    I haven't seen a lot written about mast placement. The cross arm seems to be about 60% foreword (don't quote me on this it's based on dividers on pictures). The mast tends to be over the foreword cross arm for structural reasons. This puts the board(s) in the deepest part of the hull more or less. Look at the rake of the dagger board as this affects the effective cross section of the board presenting to the water. A thicker board with more rake present a thinner (read core to thickness) cross section. In most of the water I sail in kelp and other marine grass is having more effect on the outcome of the races.

    As far as flotation; water to air is somewhat less than 700:1. So dragging cross arms and nets through the water is SLOW. One notable designer thinks this is a safety feature by slowing the tri down when it's over pressed. I try to be polite.

    So if you go with these big bubble hulls out there you have wind-age the other 90% of sailing conditions. If you want to balance wind-age and weight vs light and wet then it comes down to how much wind you expect to be in. This is over simplification as you can always drag more sail up to mask the effect.

    The next element is the quality of the ride defined by the ama shapes with very wide and flat bottoms will be something with very snappy motion to and likely a less fun boat depending on what you're after. A boat that's just for around the cans can be a lot less comfortable than one you'll be on for weeks.

    You should also look at hulls that generate lift with some question. Boards generate lift with less drag for a given amount of lift based on what I've seen in real boats. Ama's need to be dead parallel to be fast and wings need some angle of attack to generate lift so you drift the boat sideways give that effect but is that something you really want to do?

    When in doubt get lighter. ALWAYS

    If this is your first boat do not go to the extremes unless you have a really strong motivation. Also keep a log of the stuff you actually use as you start with a lot of stuff you really don't need. Read pull out the drawers out and gear bags (you can't bring the stuff is there is no place to put it) until start winning races. Park about 10 minute walk to the boat with a walk back to the car for some questionable reason and offer to let the crew reduce what's in the bag. Boat weight isn't as much of a factor as the stuff you add to the boat. I've caught crew bring lead dive belts to Catalina races. You need to tell the crew you're back packing and get the tooth brush cut down as it's all weight if they want to win races. Some of the crew will still not get it aka the dive belt.

    Dan Frenette
     
  9. warwick
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    warwick Senior Member

    Thanks Dan for bring up the mast placement, apart from the float size.

    It is another part of a design process that needs to be considered. May be the will be more feed back to help others out there.

    My thought on the cross beam is to keep them as high as practically possible for wave clearance, distance back seams to vary. The scarabs 18 and 22 are about 38 percent back, with the mast on a support post. I think the F Boats the cross beams were about 33 Percent back. So may be the distance back could between 30 and 40 percent.
     
  10. bruceb
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    bruceb Senior Member

    mast placement

    I have modified my 24 - some would say grossly modified:p it with a taller mast and longer boom, plus a fat head full battened main. I only had to rake the mast a few inches less to compensate, and the helm is very well balanced. Mast and beam placement can be adjusted for with small changes in boards and mast placement as long as the coe of the rig is somewhere reasonable. Tris seem to be very forgiving. B
     
  11. cavalier mk2
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    cavalier mk2 Senior Member

    I'm going to make a post mentioning that sensible rigs are better for cruising when the long haul is more important than the first mark......Of course I'm still doodling a crab claw topmast/sail designed to fly off the boat in a hard gust;)
     
  12. warwick
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    warwick Senior Member

    I agree with Cavalier Mk2 that you have to consider the rig, to fit the purpose and level of experience.
     

  13. chunpei
    Joined: Jul 2012
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    chunpei Junior Member

    Yours paper is helpful ,can I have this complete book? thank you very much!
     
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