Materials for keel weights

Discussion in 'Materials' started by SamC, Sep 13, 2019.

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  1. SamC
    Joined: Sep 2019
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    Location: Fulton county Ga

    SamC Junior Member

    Hello,
    Firts of all I am new to boat building so please forgive my lack of knowledge and use of boat terms. I just started building my second boat out of plywood and fiberglass and I am wondering what materials I can use to as weights. I know lead is commonly used but it's very expensive especially since I want it to sit low in the water and it's 12' long and am already spending on lead for bullet casting. Also, when I refer to the "keel" i just mean the center. The Bottom of the boat is flat and there is no natural place to put weights.
    Any advise would be great
    Thanks, Sam
     
  2. Ike
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    Ike Senior Member

    If these will be inside the boat, old railroad rails make excellent weights. Scrap yards should have them and be able to cut them to whatever length you need. Do not use anything that absorbs water such as bricks or concrete. Or what about water ballast? Water weighs 8.34 lbs per gallon. bladders full of water have been used as ballast. They have to fasten down some way though.
     
  3. SamC
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    SamC Junior Member

    Thank, I'll probably experiment with railroad rails as weights. Would putting equal weight on each side of the bottom inside of the boat make it unstable or cause some other problem?
     
  4. Ike
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    Ike Senior Member

    no.
     
  5. kapnD
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    kapnD Senior Member

    Could you expand on your answer?
    I’m no engineer, just curious.
    Wouldn’t equal weights on the outside edges of a flat bottom have the same effect as none at all, other than increasing displacement?
    I was under the impression that adding weight for stability depended on sort of a pendulum effect, and had to be carried below cg .
     
  6. Ike
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    Ike Senior Member

    The Center of Gravity doesn't move when a boat heels. When the boat is upright both the vertical and longitudinal CG and CB are on the centerline of the boat. As the boat heels the CB moves, because the immersed shape of the hull changes as the boat heels. The CB moves outward away from the centerline.

    Adding weight will always increase displacement. What you are trying to do here though is increase the righting arm, that is the amount of force tending to turn the boat up right. You want to have equal weights on both sides of the boat. Putting the weight near to the center line accomplishes that. As a boat heels to the side, the CB of the immersed part of the hull shifts outward, increasing the the righting arm because the distance between the CG and the CB increases. If the weights are moved out ward on both sides the distance from the CB to one of the weights increases, but the distance to the weight on the side the boat is heeling towards, decreases (if the weight is outboard of the CB). I suppose I made that clears as mud. See http://newboatbuilders.com/docs/stability.pdf It may make it a bit clearer. Basically what I am saying is put the weights on or near the centerline of the boat.
     
  7. kapnD
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    kapnD Senior Member

    Thanks, Ike
    Maybe a little clearer now, I am continuing to read up on the subject.
    I suppose my real concern is that the OP has a 12’ flatbottom skiff, which in my experience should not need ballast, and could be easily overloaded.
     
  8. Ike
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    Ike Senior Member

    Frankly I can't see why you would need ballast in a boat that small. The largest weight in the boat will be you. Just shifting your weight around will keep it stable. I have a 12 foot flat bottom rowboat and it handles two of us with no problems and is very stable as long as we are sitting. Any boat that size will be less stable if you stand up because by standing you raise your center of gravity about 3 feet. Sitting keeps your CG about at the same level as the boats cg.
     

  9. SamC
    Joined: Sep 2019
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    Location: Fulton county Ga

    SamC Junior Member

    Thanks for all the help. I will try it without ballast first once built and make sure not to overload it.
     
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