Question about centerboard design on small sailing catamaran

Discussion in 'Boat Design' started by almoniyot, Sep 23, 2019.

  1. almoniyot
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    almoniyot Junior Member

    First time poster here but have been lurking for some time...

    I am building a 5 meter long catamaran sailboat that will use inflatable pontoons.

    The frame will be constructed out of aluminum (with trampolines aft and fore of the mast), and there will be a centerboard that retracts/pivots.

    I want to use pulleys to lift the centerboard but I am concerned I will not have enough leverage in the schematic that I made. The centerboard blade is made of aluminum - measures 1.2 meters long and weighs 3.3kg.

    I am including a picture of the centerboard on the Grabner Happy Cat Catamaran as well, which has inspired me.

    Any design suggestions would be welcomed.

    centerboardlocation.jpg
    centerboardsideprofile.jpg
    CenterboardHappyCat.jpg
     
    Last edited: Sep 23, 2019
  2. BlueBell
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    BlueBell Ahhhhh...

    So, what is the question?
     
  3. almoniyot
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    almoniyot Junior Member

    Question is whether there is enough leverage to lift the centerboard (with line shown in yellow) in my design. If not, how would one do it better?
     
  4. johneck
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    johneck Senior Member

    Looks like a 12:1 factor on the moment arm, so 12X the weight of the board with no other friction or resistance. Seems difficult at best. The solution is to get the pick point further from the center of rotation, or move the CG close to the center of rotation.
     
  5. BlueBell
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    BlueBell Ahhhhh...

    Almoniyot,

    I can't view either of the two last images in your original post.

    Keep in mind, if your Aluminum c-board is solid, it will only weigh 0.62 as much submerged.
    3.3 kg becomes 2.1 kg
    However, it's inertia remains constant and this is the first thing you must overcome to get it moving when raising.
     
  6. almoniyot
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    almoniyot Junior Member

    Thanks for the replies. I will take your advice into consideration.
     
  7. upchurchmr
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    upchurchmr Senior Member

    A flat plate as a centerboard will not work well.
    Bigger is better since the board will not generate sideforce well.
    Centerboards on the centerline of a cat don't work well, generally.

    You do realize that you need to place the board relative to the center of effort of the sails?
    Unless you have a large jib, the board at the crossbeam will be too far forward.

    I'd suggest you make a test part to see how much force is required to haul up the board. Better to find out for sure before you commit to something that "might" work.
     
  8. sharpii2
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    sharpii2 Senior Member

    Right now, it looks like the pull on the line needed to retract the center board is going to be roughly 50 kg or more. You could probably do better with two pulleys in the case. One could be aft the one you drew and both could be higher up in the case. The pennant could then attach to the trailing edge of the board just slightly aft of where it intersects with the aft end of the case when retracted. This would cut your mechanical disadvantage in half, so now you would be looking at about 25 kg of pull on the line.

    With this new set up, the pennant would go over the aft pulley, under the fore one, then up through the top.
     
  9. almoniyot
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    almoniyot Junior Member

    While it appears like that in the design I posted it isn't a flat plate.
    It is an aerofoil like profile measuring 18.6cm x 3.4cm
    https://i.ibb.co/cJxcWDV/centb.jpg



    The mainsail is roughly 7.5sqm, and there is also a jib measuring about 4sqm.
     

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  10. almoniyot
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    almoniyot Junior Member

    cbpulley.jpg
    Thanks that is helpful.

    Is this what you had in mind?:
    https://i.ibb.co/3RhHCsJ/cbpulley.jpg
     
  11. alan craig
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    alan craig Senior Member

  12. almoniyot
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    almoniyot Junior Member

    alan craig likes this.
  13. upchurchmr
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    upchurchmr Senior Member

    That is probably better than a flat plate, but it is certainly not an aerofoil.
    Is that something you can buy?
    It looks too fat compared to known and existing useful centerboards.
    With that much empty space in the foil, you are going to have to heavily weight the lower end to get it to go down, that will make it harder to pull up. Without a downhaul, you will never know just where the board will be when down, since water force will push it aft, affecting sailing.

    The two pulleys shown above is actually worse than a single pulley for the amount of force required to pull it up, due to friction.
    The only thing that matters is the horizontal distance from the pivot to the point of hoist attachment in your picture. Make the distance longer.

    The pivot should not be on the centerline of the section. That will cause the leading edge of the centerboard to twist off to leeward, ruining the force developed.
    The pivot should be somewhere around 30% aft of the leading edge.

    Can you provide a picture of the sail shapes? That is the only way you can determine where the centerboard should be.
     

  14. almoniyot
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    almoniyot Junior Member

    Thanks for your reply.

    Technically it is an aluminum shading profile. The 1.2 meter section I am using for the centerboard weights 3.3kg or 7.3lbs, although the weight is evenly distributed which I agree is not ideal (note we are talking about a fairly light boat here - maybe 70kg in total). I already purchased this profile which will be used for both my centerboard and rudder blade. I am going to fill the bottom with aluminum putty so there will be no water ingress. Is it perfect? Certainly not, but it should be durable, and was cost effective compared to what it would cost me to ship a proper blade to Israel.

    I have attached the approximate shape of the mainsail. It will have a large roach as many catamarans do. The luff is about 5.2 meters, and the foot roughly 1.75 meters. The jib (no drawing) will have a luff of about 4.6 meters and a foot of about 1.6 meters.

    "The only thing that matters is the horizontal distance from the pivot to the point of hoist attachment in your picture. Make the distance longer." - What exactly do you mean by this?

    Note while i have made some modifications, I can't claim this is a completely original design. For the location of the centerboard, I am simply copying what other manufacturers have been doing for many years. Is that necessarily correct? I cannot say.
     

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