Outboard Platform or Cut-down Transom?

Discussion in 'Propulsion' started by CarbonFootprint, Oct 20, 2010.

  1. CarbonFootprint
    Joined: Nov 2009
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    Location: Northern Scotland

    CarbonFootprint Ill-Advised Boat Modifier

    Hello,

    I'm replacing the stern-drive in my boat and was intending to cut down the transom and build an outboard-well from marine ply and fibreglass. Somebody suggested I fit an outboard platform/transom extension instead - general opinion seems to be that this would give better performance/manoeuvrability and would avoid affecting the seaworthiness of the boat by cutting the transom down.

    I'd appreciate any opinions on this. Also, transom extensions seem to be engineered to varying standards, from purely GRP construction to cast aluminium to composite stainless steel/GRP. Is constructing one myself a viable option? I'd go for composite stainless/marine ply/fibreglass construction and extend the boat by following the waterline - is this sensible/practical/possible?

    Many thanks for any advice,

    Paul
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    If the boat was designed for an inboard, an outboard may be shifting the weight too much aft. With a platform, it would make it even worse. You may end up with a boat that porpoises.
     
  3. CarbonFootprint
    Joined: Nov 2009
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    CarbonFootprint Ill-Advised Boat Modifier

    Hello,

    I did wonder about that - the boat was designed for an outboard, though - the inboard setup was a factory option that was rarely taken up.

    If the transom extension was air-tight and offered some buoyancy, would that offset the weight-shift? I imagined I'd have to move the fuel-tank forward to compensate...

    Thanks for your thoughts :)

    Paul
     
  4. gonzo
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    gonzo Senior Member

    If what you are building is a hull extension, then it may work. The fuel tank should be moved aft. The center of gravitiy of the tank should be right on top of the center of floatation. Otherwise, as you use fuel, the trim of the boat will change.
     
  5. CarbonFootprint
    Joined: Nov 2009
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    CarbonFootprint Ill-Advised Boat Modifier

    Hello,

    thanks for that...I see what you mean about the fuel tank.

    Paul
     
  6. Submarine Tom

    Submarine Tom Previous Member

    I can't/wont comment without some pictures of the boat.

    Is this possible?

    -Tom
     
  7. CarbonFootprint
    Joined: Nov 2009
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    Location: Northern Scotland

    CarbonFootprint Ill-Advised Boat Modifier

    Hello,

    here's picture of the boat beam on...
    [​IMG]

    here's one of the transom with the sterndrive - there's just a hole now...
    [​IMG]

    and here's something along the lines of what I'm planning...
    [​IMG]

    The boat is 5.7m LOA and about 2.1m Beam...
    Thanks in advance for any thoughts :)

    Paul
     
  8. Submarine Tom

    Submarine Tom Previous Member

    Looks good to me Paul.

    Full hull extension, right?

    Single outboard, right?

    What size / weight?

    -Tom
     
  9. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    The vee in the hull is not too deep. Also, it doesn't sharpen until pretty far forward. I think that a hull like that is not excesivelly sensitive to trim changes. It should work, but you may end up needing trim tabs.
     
  10. CarbonFootprint
    Joined: Nov 2009
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    CarbonFootprint Ill-Advised Boat Modifier

    Hiya,

    I was thinking of a 3/4 height extension (to the top of the grey box in the diagram), but with the hull-sides extending down from the gunwales. Some sort of large stainless steel bracket central inside the extension to carry the outboard (just the one - about 100hp) and the rest of the extension built up from 6mm (or more?) marine ply joined with more stainless steel brackets, finished with a couple of layers of fibreglass.

    I'm currently thinking along the lines of a 0.5m extension, but it may have to be more, depending on the outboard dimensions (I'm just going by manufacturers spec-sheets at the moment). As you say, the V of the hull progressively deepens towards the bows - would it be best to follow that changing V profile aft or just match the profile at the transom for the whole of the extension?

    Thanks again for your help with this :)

    Paul
     
  11. Submarine Tom

    Submarine Tom Previous Member

    "- would it be best to follow that changing V profile aft or just match the profile at the transom for the whole of the extension?"

    Follow the V.
     
  12. mark775

    mark775 Guest

    Looks to me like it cud stand some more buoyancy aft.
     
  13. CarbonFootprint
    Joined: Nov 2009
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    CarbonFootprint Ill-Advised Boat Modifier

    Do you mean extend it further or change the profile of the extension?
     
  14. mydauphin
    Joined: Apr 2007
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    Location: Florida

    mydauphin Senior Member

    I can tell you guys have never done this. The outboard motor should be a lot lighter than the inboard. So boat may actually sit higher in water than with outboard. Use a two-stroke motor if you can to save even more weight. Other reason for bracket is that transom on inboard engine isnt design to take force from outboard so substantial beefying up is need. Easier and safer to make or buy bracket for engine. Boat will perform better with bracket. Use doelfin on outboard to reduce purposing. You can adjust engine for trim, trim tabs only needed for side to side trimming. Big problem with this installation is going to be walking. Dont move fuel tanks or batteries until you put boat in water. It looks kind of nose heavy anyway. I am not an engineer but I have done this at least ten times, to all kinds of smal boats.
     

  15. Ike
    Joined: Apr 2006
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    Ike Senior Member

    I have seen a lot of hull enxtensions go bad, mostly because they were not well engineered and well integrated into the existing hull. This results in stress fractures, hull stress from twisting, and so on. If you don't want to go to a lot of expense and get an engineering analysis, buy a bracket. The brackets are pre-engineered, and you bolt them on. They work very well. They also move the prop back to where it is in less disturbed water so they are more efficient.
     
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