False bottom v on a flat bottom

Discussion in 'Boat Design' started by Juijitsufighter, May 4, 2015.

  1. Juijitsufighter
    Joined: May 2015
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    Juijitsufighter Junior Member

    Hello, my first post here. I've been planing on building a flat bottom garvey boat listed on spira int. for the middle Sacramento River . Here the river tends to be very wide and flat and alumaweld super vee type boats are the most common but way out of my price range. For cost and ease of building my first boat,the garvey looked like a good alternative. It's a 20x8 flat Bottom but I thought an easy to build upgrade might just be a simple glassed over, foam core ,false shallow v bottom toward the aft end to help an out board jet run better . Anyone have experience with this and could I then incorporate a pre jet intake tunnel to this plan? I'm looking for simple and economical to get me and my son on the river and family on the lake , I know a crazy cool aluminum jet boat is better but it doesn't do me any good cause I can't afford one. Also if I'm adding a false v bottom or delta pad would it be benificial to try and glass some reverse chine or is that dumb on a flat bottom . Would love any help thanks a lot , Tim
     
  2. tom kane
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    tom kane Senior Member

    Why have a flat bottom on any craft when a moderate v will serve just as well and have no deeper draft? V is stronger and probably no harder to build.
     
  3. Mr Efficiency
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    Mr Efficiency Senior Member

    No deeper draft ? How did you work that out ?
     
  4. tom kane
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    tom kane Senior Member

    The depth of draft is governed by the amount of buoyancy the boat has immersed in the water not the shape of the bottom of the hull.
     
    Last edited: May 4, 2015
  5. Mr Efficiency
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    Mr Efficiency Senior Member

    But a vee hull will draw more water than a flattie, all else being equal ?
     
  6. Rurudyne
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    Rurudyne Senior Member

    For some likely extremely silly reason I just imagined a displacement yacht equipped with a small blimp in order to reduce fuel consumption.

    ... we should see how much DC will give us to investigate this promising technology. Know any way to work beautiful bikini clad women into the project's mission statement?
     
  7. philSweet
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    philSweet Senior Member

    There are several good reasons to go with a file-bottom boat. In wood, you want it to drain when on a trailer (handy even if not wood). It really isn't any harder to build in ply. As long as you aren't trying to carry massive loads, the file bottom is very nearly as versatile as a flattie. It will probably be lighter as well, and lighter usually means lower material costs. What are you going to power it with?

    Scabbing a vee-bottom onto a flattie removes most of the benefits of both types. I'd build a file-bottom with frames every foot and use a drop-in floor grate made of 3.5 X 3/8 hardwood floorboards spaced to drain. (resaw oak 4 x 4 or similar for grate)
     
    Last edited: May 5, 2015
  8. tom kane
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    tom kane Senior Member

    False bottom v on flat bottom

    This subject would require a separate thread I am sure, it is not as simple as it may seem. Traditional practices are often wrong.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Welcome to the forum.

    Tom, as a rule, all V bottoms will draw more than a flat bottom of the same volume. It's pretty simple physics and not really a reasonable debate to the contrary, assuming the usual shapes employed in boats. The volume has to go somewhere, all else being equal, so if it's maximized on the centerline, rather than spread across the whole of the bottom plane, the answer is pretty obvious.

    To answer the original question, yep, you can do this, though it's not as easy at it might seem. The physical act of putting a V on a flat bottom is fairly straight forward, but where to place the V, how much, the transition areas, etc. can be a challenge for a novice designer. Firstly, you don't want to put a V on the bow, then transition to flat toward midship (assumed) just to transition to V again in the aft quarters. This will make an ill handling boat. If you're going the V bottom route, note that you'll be best advised, to stay very close to known types and shapes. This would been a monohedren or warped bottom hull form.

    If your speed requirements are less that 40 MPH, you'll do fine with a warped bottom, where the deadrise progressively flattens out, as it moves aft along the hull. If you want to go faster than this, consider a monohedren bottom, where you maintain constant deadrise from midship to the transom. A monohedren will draw just a bit more water than a warped bottom, for the same reason a V will draw more than a flat, immersed volume distribution.
     
  10. myark
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    myark Senior Member

    Flat bottom boats have an advantage with draught meaning less drag in the water for example the Myark 8.5 m self trailer barge has a slight kick ups in the front and back ends which when accelerates, it rocks the front up and because it floats in two inches of water it immediately gets up and plans.
    The picture shows two 25hp out boards which work well and can compete with smaller V hulls that have larger outboard motors.
    Better on gas, more people aboard and stable plus you can place your car on top if wanted and most of all land in shallow water, plus the plug in torsions suspension and wheels allows beach landings.
     

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  11. Mr Efficiency
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    Mr Efficiency Senior Member

    You did, huh ? That wouldn't be a disjointed thought process assisted by alcohol, would it ? :D "DC" really doesn't hand out money willy-nilly, does it ?
     
  12. PAR
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    PAR Yacht Designer/Builder

    This would be a very rare situation, as most flat bottom shapes employed have an excess of drag creation, because of their shape. It's only got three advantages; simple construction, efficiency in low and moderate plane mode speeds and of course it's shoal nature. As with all design decisions, it has its share of disadvantages as well.

    For the OP's needs, he's correct and likely does need some type of V bottom to handle the prevailing conditions, which wouldn't be well suited to a flat bottom.
     
  13. Rurudyne
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    Rurudyne Senior Member

    Actually, I can be deeply silly without any booze. It's probably for the best then that I can be incredibly cheap and tend not to drink. (I'm not a non-drinker, still, being cheap is a big part of why I lack "bad habits" that lured many of my peers in youth, I looked at the like and thought about how expensive they were ... The upside is now I have less to compete for resources with my desire for a cruising boat ... which are really expensive).
     
  14. tom kane
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    tom kane Senior Member

    Flat bottom vs v or round bottom for shallow draft is a complex discussion,needs a separate thread.
     

  15. Juijitsufighter
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    Juijitsufighter Junior Member

    Ok this seems to be a complex issue that I have laid down. Maybe too complex for a newbie to feel confident that I'm going to get a useful boat, but, to clear couple things up ... I wanted to build this flat bottom boat because it will be easy to build and has a high but flat front that people are able to make a "lander" of sorts . Where the front can open and a quad can be unloaded on a beach, not really possible with a v bow boat . Also being a flat bottom would be good for drifting not under power. I just thought a nifty addition might be simple foam core v or delta pad glassed on the bottom to cut the air before the jet Intake but looks like you guys are scared this might be too complicated or not work like I think it will and that's fine I can just run it a flatty
     
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