Bridgedeck centreboard why don't they work???

Discussion in 'Multihulls' started by valery gaulin, Jan 10, 2017.

  1. brian eiland
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    brian eiland Senior Member

    Why enough??

    There may be more ideas out there that have yet to come to light....like the ones most recently posted.

    I just don't understand why some feel this subject thread should be abandoned, or halted?? (closed minds?).
    Just ignore it if you've had enough.
     
  2. UpOnStands
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    UpOnStands Senior Member

    put some designs on paper to encourage comment.
    say 10 m catamaran with 70-80cm bridge deck clearance, speeds of 14 knots?
    sail area of 62-64 sq m.
    how much extra "stuff is needed"?
    replacing 2 boards with center one means 2 sets of cases can be deleted and hulls are smooth, good. Also banging in the cases, gone. in-haul and out-haul systems gone. No problem with fouling of case internals when sitting at anchor or on the beach.
    Cavitation can be countered by slightly increasing length.

    how much extra support structure is needed? loading of support structure? reliability of support structure? PFTE side bearings are sufficient? fore-aft placement of board matches mast bulkhead? if forward, by how much?
     
  3. Doug Lord
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    Doug Lord Flight Ready

    Bridgedeck Centerboard

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    It's also been pointed out how well a system like this can work with the most recent real world example yesterday. The problems have been solved in a number of real world designs that seem to be ignored by some who question the viability of a bridgedeck board-that's unfortunate.
     
  4. valery gaulin
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    valery gaulin Junior Member

    Maybe the center board could be mounted between two large heavy duty swivel plate in the bridgedeck! This would be for sure really strong configuration. Here is a photo of the tyoe of swivel plates I am talking about. I am sure they make them even bigger and in stainless steel or aluminum.
     

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  5. brian eiland
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    brian eiland Senior Member

    Valery, that's the general idea, but it can be more simply than that.

    Sorry I'm not computer graphics capable, and I am not presently in a place where I can draw it out and scan same (on holiday in Thailand).

    Let me first go back to this quote from a previous posting
    So this nacelle plate and the boards appear something like this...
    [​IMG]
    [​IMG]



    One of the asymmetric boards looks something like this..
    [​IMG]

    Now imagine that 'rod' shown there made a good 1 foot in diameter, and simply made of UHMWPE (ultra high molecular weight polyethylene)
    ....and its cheap and can be worked with ordinary woodworking tools.

    So now we have a simply 'cheap bearing pin' that has a nice big size to it to help 'up-down-up' rotation of the boards, and with the considerable bending loads the two asymmetric boards are going to exert on the flat plate nacelle.

    Now we insert a thin piece of either teflon sheet or UHWP sheet between the flat surfaces of the asymmetric boards and the flat plate nacelle. This further reduces our friction between the boards and their mounting plate, and provides for even a greater total surface area to resist the twisting of the centerboards (flat against flat)

    PS: I have worked with this UHMWPE before. I made kick-up rudder castings out of it for my Firefly tri design (in lieu of a SS fixture). It provide a nice 'casting' that was cut out by bandsaw, then drilled for rudder shaft bearings and kick-up bearings all in one homogeneous piece of plastic,... that was also virtually sea fouling proof.
     
  6. Doug Lord
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    Doug Lord Flight Ready

  7. UpOnStands
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    UpOnStands Senior Member

    diameter of pin is not so critical, its the bending forces that are the killer.
    Think of the normal spade rudder -- now remove the top bearing and try to control rudder loads with just one bearing.
    Slack or deformation in the support/bearing will alter the angle of the board in the water triggering strong changes in loading.
    This arrangement uses large lateral faces with thin sheets of UHMWPE on both bearing faces of both boards to restrain movement. There are steel plates on the outsides of the boards -- the plates are retained.restrained by a thru bolt/nut. The boards rotate on a UHMWPE pin -- yellow in the figure. However, this is considerably more difficult to fabricate. Easy to draw:D, difficult to fab.
     

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  8. Stumble
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    Stumble Senior Member

    Brian,

    Your design is interesting but the material choice is disasterous. UHWP has zero structural rigidity and deforms locally under even moderate loads. So much so that you can't use it for deck planking because it isn't self supporting over any distance. You could bend a 5' long UHMP board into a pretzel with your hands.

    The engineering is beyond me, but to get a reasonable projected area and decent aspect ratio you will need to go to a very thick fiberglass/core, plywood, or carbon/core. Again just guessing, but I would bet if you run the numbers that a solid carbon section is pretty price competitive when you take into account how much thinner it can be made.

    But instead of using a massive pin, just invest in a couple of ring bearings. They are far easier to fabricate the board to fit and they already exist in the marine industry (used for stockless rudders).

    As for the nacell. Meh, this seems like a lot of construction to avoid a daggerboard, would probably be at least as expensive and a lot heavier than just making the board stiff enough to handle th unsupported length.


    I guess I am just not sure what problem we are trying to solve with center mounted boards. It's a poor engineering choice compared to retractable dagger boards, and using curved board in the hulls you don't even have to take up that much space. A little creative design and the board can just follow the curve of the hul and be placed inches from the outerhull. The trunk then also becomes a structural member taking the place of internal framing.
     
  9. Doug Lord
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    Doug Lord Flight Ready

    Bridgedeck Centerboard

    -----------------
    I completely disagree with this. The bridgedeck board is clear of the water-no growth in the trunk. One board and its trunk and any associated handling problems are eliminated. No way dual ,retractable, daggerboards are a better "engineering choice" than a pivoting centerboard that kicks up completely clear of the water. Read Rob Denney's post 20, page 2.
     
  10. DennisRB
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    DennisRB Senior Member

    Agreed Doug. Like I already said, the idea needs to be incorporated into the whole design. The center mounted outboard and fuel tanks and perhaps even the longeron for the headsay (no forebeam) will all be complimentary in the design. Perhaps the nacell could even give a lowered walkway into the bridgedeck / bimini meaning it could be lowered.

    I find it odd some members appear to want the discussion finished, even declaring there is nothing to talk about in spite of the obvious benefits (with detriments too of course) plus the now numerous examples of the concept working quite well.
     
  11. Doug Lord
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    Doug Lord Flight Ready

    Bridgedeck Centerboard

    Just found this Aussie cat with twin rudders and a bridgedeck board-with a hydrofoil on it!

    [​IMG]
     
  12. brian eiland
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    brian eiland Senior Member

    A single gybing board might well work, but a couple of things concern me.
    1) Trying to keep a fairly long board that is supported at it's peak end from twisting in relationship to the boat (not enough good flat areas to work against one another)
    2) This might be OK on smaller vessels, but I don't think I would want it on a larger vessel.
     
  13. DennisRB
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    DennisRB Senior Member

    A local sail maker, switch sails, races that design. I even think it was on my school uniform!
     
  14. brian eiland
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    brian eiland Senior Member

    I agree totally, and that is why I chose to utilize as large of a diameter bearing as I might.
    And I was looking for a bearing idea that would have VERY little 'slop'. A firm fitting solid material bearing might well eliminate the small tolerances that are found in conventional ball / roller bearings. Why do you think they still prefer bush/plain bearings on engine crankshafts, etc.
    My UHMWP bearing was to be a 'bush/plain bearing'.

    Nice illustration of what I had in mind.
    [​IMG]

    I did NOT intend to utilize any steel plates in my design, and the backbone structure of the central board(s) was to be the flat plate nacelle running down the center of the vessel that was also built to act as a 'backbone rib' to the vessel as a whole.
    I had not worked out what I was going to utilize on the outer 2 sides of the twin boards. Figured that could wait until a client for the idea arose. And just possibly if that big dia 'no slop bearing' in combo with the large flat face of the inner sides of the boards might well be enough to not require any sophisticated support on the outer surfaces of the twin boards.
     

  15. redreuben
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    redreuben redreuben

    Dennis I agree also as I put in post #83 I'm thinking about boats like the Woods 6-8m cats where he uses a drop down floor, the nacelle would provide the footwell for the central cuddy.

    Doug;
    So the hydrofoil is an inverted "T" ?

    What happens when that kicks up ?

    Similar problem to inverted "T" rudders, when they kick up massive braking (and breaking) effect.

    There is/was a design on the web somewhere where the horizontal foils pivot, but thats more complexity which is what we're avoiding right ?

    By the way if anyone has a link to that design I'd still like to find it again !
     
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