Adding some V to a flat bottom hull

Discussion in 'Boat Design' started by johnnythefish, Apr 19, 2018.

  1. johnnythefish
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    johnnythefish Junior Member

    I have an 18 foot flat bottom dory that is built in stitch and tape plywood - it pounds a lot in our short chop.

    Would it be impossible to add some sort of V to it - even if only at the front to reduce this?

    I am rather fond of the rest of the boat and the lay out, but the pounding curtails my usage of it a little.
     
  2. PAR
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    PAR Yacht Designer/Builder

    This is the nature of flat bottom boats and though it may be possible to put some V in her, it's not as easy as you think to do a good job of it and she'll still likely pound in some condisions, as the back half would still be flat. Can you post a picture?
     
  3. johnnythefish
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    johnnythefish Junior Member

    Ok - so this is what the boat looks like at the bottom...

    [​IMG]

    There is a sole at the level of the second chine up.

    I was thinking one could make a V- hull with two stringers and some frames that you then glue to the bottom of the hull (after removing the skeg and cleaning the paint) of this existing hull.

    Tape all the way around and sheath with a cloth.

    Then remove the existing sole and stringers/ frames.

    You would have a deeper cockpit ( and yes a narrower sole to stand on - as this would be the bottom of the hull now). Boat may not be self bailing - but you will have a much deeper cockpit and it won't pound as much.

    Hull is very nice and stable as is and works great - just can't use it for what trolling into a head sea as it pounds so much.

    One could either join the added V- hull right along the chine (giving a similarity bottom to the PT skiff) - or, towards the transom end, could narrow the added V hull portion to leave the existing hull bottom as some chine flats.

    Not being a Naval architect, I have little idea of the pros and cons of any of these ideas - but I do like tinkering with things!!
     
  4. johnnythefish
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    johnnythefish Junior Member

    This was sort of what I had in mind... this one with chine flats to act as spray deflection as well - but I could do away with that... (I have only drawn the bottom of the hull - not the side panels)...

    [​IMG]


    [​IMG]
    [​IMG]
     
  5. Rurudyne
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    Rurudyne Senior Member

    Bolger and friends used a built up upside down hump to try to cope with slamming. Can't exactly recall what they called it.
     
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  6. PAR
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    PAR Yacht Designer/Builder

    Are you thinking of the slab sided box keel Bolger designs? These also slam, though not nearly as bad a flat bottoms.
     
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  7. johnnythefish
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    johnnythefish Junior Member

    Thanks Rurudyne - I don't know much about Bolger designs.

    Though I have thought of adding it only at the bow half of the hull... either tapering to the flat portion at the transom - or as a stepped hull; I think that just adding a V-hull to the bottom of the existing flat bottom hull - would be the simplest most predictable outcome.

    The PT skiff for example has multiple chines beneath the water line and seems to work ok!
     
  8. Rurudyne
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    Rurudyne Senior Member

    No. There was this upside down hump built up from plywood that some of those designs used that gave a slight deadrise to the underside of the bow. I can't remember exactly what they called it. I do seem to recall reading about it in at least one build thread hereabouts, but not which one. No helpful, I know, sorry.

    Not much for high speed, naturally, but was supposed to help in cruising conditions the boats were meant for. "Improvement" rather than solution seems to have been the idea.
     
  9. Wavewacker
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    Wavewacker Senior Member

    I believe Matt Layden used the "hump" or pad at the bow, simply layered wood then softened the edges smooth forming a v then epoxy/paint. Check out his site on the Enigma. Don't see why the same method couldn't follow back along the keel, keeping a shallow v flare. That's grinding, then lots of sanding.....

    Bolger had a false bow and the box keel was on his tri hull camp cruiser I believe. :)
     
  10. Wayne Grabow
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    Wayne Grabow Senior Member

    Yes, of course you can change it to a "V" bottom, but it will be a lot of work, it will change the displacement, and it will only lessen the pounding. I would use a constant angle deadrise for the aft approx. half of the hull then switch to a conic projection for the forward half of the hull. This will give you an increasing deadrise toward the bow to help lessen the pounding. I would probably pick a focus/center for the projection lateral and below the inverted hull (matching your selected aft deadrise angle), just where the beam at the chine starts to decrease. A laser or string projecting/connecting the focus to sequential points along the forward chine will give you the vertical offsets at the keel/midline that you need to define the altered forward keel. The result will be a developable surface that can easily be sheathed.
     
    Last edited: Jun 19, 2018
  11. Owly
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    Owly Senior Member

    Put a wood keel down the center, then cover the entire bottom with ordinary blue or pink XPS, and shape the blue foam into the V you want. Thick next to the keel, nothing at the chines. Then cover it with glass and epoxy. This will give you the V you want, of course change the displacement, and deaden the sound.

    H.W.
     
  12. W17 designer
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    W17 designer Senior Member

    As I am guessing that only the forward 3 ft or so slams in any serious way, I would suggest a quick, inexpensive fix that will certainly improve the situation, even though as others have said, you'll still have some. Here's my suggested addition to your original sketch. It's surprising what a simple bonded-on glassed-over foam Vee-block will do .... about 6" wide and say 4"deep only about 3-4ft long. The flat bottom is a very efficient planing shape and as I highly doubt it leaves the water aft, I'd just leave the shape as you have it. It will affect your steering a little though. Good luck.
    If you decide to try this simple fix, perhaps others would like to hear about the results so pls post them here.
    mike/
    www.smalltridesign.com
     

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  13. ReginaldChiswick
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    ReginaldChiswick New Member

    I had thought of something similar but then told myself it was a ridiculous idea. Now I feel more confident. May well give it a go...
     

  14. Squidly-Diddly
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    Squidly-Diddly Senior Member

    So its a low-speed only issue?
    Rather than change the hull, which might make this not-your-fave-boat in other ways, consider adding Phat-Sacks of ballast water.
    Origin Wakeboard Ballast Bag Fat Sac 2x 550 Lbs Plus Water Pump for sale online | eBay https://www.ebay.com/p/Origin-Wakeboard-Ballast-Bag-Fat-Sac-2x-550-Lbs-Plus-Water-Pump/1558226249?iid=331783771577

    Idea would be to push the hull into the water to "become the water", but would it instead put huge stress on hull and cause major failure in short order?
     
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