Converting a flat bottom pontoon boat to a tunnel hull

Discussion in 'Boat Design' started by mark herbert, Oct 21, 2018.

  1. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

    Hello, we have built a 6 X 14 ft small (square pontoons)pontoon boat for a small river boat. We have a 25 hp etec motor on the back which will plane off nicely. Our problem is we would like to tunnel hull the pontoons so that we could raise the motor up to allow for a shallow draft under power. Anyone got any suggestions on where I should start looking for info
     
  2. BlueBell
    Joined: May 2017
    Posts: 468
    Likes: 39, Points: 28
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Welcome to the forum.
    Google is an excellent place to start.
    Do you have any pictures or sketches of what you have done and what you want to do?
    Where in the world is "ms"?
     
  3. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

  4. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

    ms is mississippi. Pontoons are completely stuffed with styrofoam.
     
  5. fallguy
    Joined: Dec 2016
    Posts: 1,246
    Likes: 48, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    How much you drafting now and how much you want?

    Lifting strakes are a simple way to gain a bit; they are easy to add as well.

    Add a picture of the bottom so we can see the hull shape.
     
  6. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

  7. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

    they are flat. Filled with styrofoam. It drafts in 4 to 10 inches of water depending on the load. We have the prop below the bottom of the pontoon now so it hits in shallow water. We want to be able to raise the motor and prop higher so that it will not drag. Alot of the local flat bottom boats all have tunnel hulls built in them so that the prop is above the bottom of the boat. When we move the prop up in between the pontoons it will work at low speed but will cavitate when planed out. Someone surely has figured out how to funnel the water correctly in a pontoon. Maybe that is wishful thinking because most pontoons have round pontoons and really don't plane out.
     
  8. fallguy
    Joined: Dec 2016
    Posts: 1,246
    Likes: 48, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

  9. mark herbert
    Joined: Oct 2018
    Posts: 8
    Likes: 0, Points: 1
    Location: ms

    mark herbert Junior Member

    yes we know we need a jack plate . now we have a manual plate until we get everything figured out. The main problem is all the turbulence between the two pontoons causes the prop to lose grip or cavitate. I need someone with some pocket tunnel knowledge or cat knowledge that might shed some light on what they have done to help the problem.
     
  10. fallguy
    Joined: Dec 2016
    Posts: 1,246
    Likes: 48, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    okay, so you built cat hulls, but why?

    There are a couple solutions that sort of pop.

    Build the hulls out as a jon boat like monohull. This would eliminate wave interference and allow for green water at the prop. It would give you more reserve buoyancy for heavier loading. The cost is drag; the boat will be slower, but slower than what.

    You could try lifting strakes or you could try lifting strakes and a jack plate both. A little lift from the strakes and a little lower with the prop might be enough. The strakes get set so they are under the at rest waterline at the stern and at or above it at the bow. It is a little beyond me, but a lot of the flats skiffs on bateau.com use strakes for shallow water work.

    You could widen the toons spacing so the waves meet behind the motor. The amount of width can be calculated. It seems a little odd to widen them, but if it is easy; it is also a solution.

    A few ideas to ponder. I don't have any specifics. Your post title is rather confusing because pontoons don't convert to tunnels or at least I've not seen it.
     
  11. fallguy
    Joined: Dec 2016
    Posts: 1,246
    Likes: 48, Points: 48, Legacy Rep: 10
    Location: usa

    fallguy Senior Member

    The hulls can be modified to be asymetric and offer less interference, but not really very practical here because I don't think they are far enough apart for that mod and the mod would likely need to reduce the buoyancy and that is a no-no based on your info. I am not familiar with it enough to advise you much more than it won't likely work.

    Have you tried offsetting the motor? Theoretically, there would be less turbulence (not none) at the wave off one of the toons versus both waves in the middle.
     
  12. Mr Efficiency
    Joined: Oct 2010
    Posts: 6,774
    Likes: 178, Points: 63, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    What kind of prop are you running, alloy or stainless, number of blades ? This is crucial.
     
  13. dreamingbarrierreef
    Joined: Oct 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Colorado

    dreamingbarrierreef dreamingbarreef

    I see a catch-22 here. The flat bottoms is easy to plane at speed. So if you also want speed, then props have to stay low. changing to v-hull is any option at all? (both sides v-hull not mono-v-hull)
     
  14. PPRINT
    Joined: Dec 2016
    Posts: 12
    Likes: 2, Points: 3, Legacy Rep: 10
    Location: Texas, USA

    PPRINT RobT

    Start with a prop made for the application.

    Find a prop manufacturer that builds for the shallow/skinny water market and go from there.

    I personally use PowerTech Props on both of my shallow water boats.

    I have never met a boat owner that has ever said I wish I never used a properly spec'd prop.

    Rob
     

  15. dreamingbarrierreef
    Joined: Oct 2018
    Posts: 9
    Likes: 0, Points: 1
    Location: Colorado

    dreamingbarrierreef dreamingbarreef

    make it airboat instead might be another idea
    could also try negative rake
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.