Measuring an existing keel

Discussion in 'Sailboats' started by Marc Andrea Klimaschewski, Nov 20, 2021.

  1. Marc Andrea Klimaschewski
    Joined: Nov 2021
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    Location: Seattle

    Marc Andrea Klimaschewski New Member

    Hi all,

    I'm new to this forum so thanks in advance for your help. I would like to figure out if the keel on Moonshine fits with any kind of foil shape. The boat is custom so there are no templates available. I was curious if there was a good way to manually measure an existing keel.

    The thing I'm struggling with is how to determine the thickness of the keel in different positions. I could imagine how to measure max draft with two straight edges and I could also figure out where the max draft is. However, I don't know what kind of tool to use to measure the thickness at other points - other than having the keel LIDAR scanned which is out of the budget for now.

    Thanks
    Marc
     
  2. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

  3. jehardiman
    Joined: Aug 2004
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    jehardiman Senior Member

    Like TANsl showed; sticks and ticks been around since forever and is very accurate.

     
  4. wet feet
    Joined: Nov 2004
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    wet feet Senior Member

    There are probably as many ways to determine the shape of the keel as there are keels,which is to say many.Of those keels,some will actually be the same shape on both sides but I wouldn't assume that all are.Which is why I would recommend taking the shape of both sides or at least trying the template for one side on the opposite side for verification of symmetry.You don't actually need a Lidar scan and I have never actually seen that technology around a used boat.You could find somebody with a Faro arm and a laser scanner if you would rather not spend a winter day beneath a boat and they would be done in less than an hour.Even a Faro arm picking up points along a line would give a fairly useful amount of information.
    Alternatively you could go low tech and get a couple of step ladders and some plywood and take the shapes directly.The tick stick works,so does a spiling block and there is always the old standby of pencil compasses to translate the shape laterally so you can cut it out for another and more refined line.In all cases you will need a few reference points,things such as centreline,height of the top surface and a longitudinal reference to establish the rake of the leading edge.It wouldn't be particularly wise to leave the keel dangling unsupported so don't get too eager to document the precise nature of the bottom face.

    Having determined the shape(s) at various points,whats the plan?Is it just curiosity or would you contemplate major revisions to arrive at a known foil section?
     
  5. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Even if the keel is perfectly symmetrical, it is always advisable to take each measurement on both sides, and calculate the average value, to avoid possible errors due to incorrect positioning of the measuring template.
     
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  6. Marc Andrea Klimaschewski
    Joined: Nov 2021
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    Location: Seattle

    Marc Andrea Klimaschewski New Member

    Thanks a lot for all the good advice.

    As for what to do with the measurements: Right now, I am mostly curious about what the shape is. The boat is being raced and I'm noticing that we are really fast downwind but struggle pointing upwind. When looking at the keel, it feels like it does not have a lot of draft. I think the measuring project would be a success if after measuring I could say something along the lines of "that behavior is expected based on the foil shape" or even "that behavior is probably not caused by the keel, keep looking elsewhere".

    Down the road, a new keel may be in the boat's future. I would certainly let a pro design that but I'd like to understand the current state to give a better design brief.
     

  7. Rumars
    Joined: Mar 2013
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    Rumars Senior Member

    The fast version of the tick stick is the profile or contour gauge, either store bought or selfmade.
    This is a simple version, I recommend glueing the cardboard to something stiffer, like a piece of wood.

     
    Sam C and TANSL like this.
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