Engineering behind inboard to outboard 40-footer

Discussion in 'Boat Design' started by Swamplizard, Jun 8, 2017.

  1. Swamplizard
    Joined: Jan 2004
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    Location: Florida

    Swamplizard Senior Member

    Has anyone found decent discussions on converting twin inboard with BBC 8-cyl engines and outdrives to triple or quad outboards on an air tight bracket?

    Vessel is a deeep V with no steps in hull, 14k lbs 41x10 beam

    Assuming transom is solid, four 75 gallon fuel tanks are forward of the engines and gears ... is this a good candidate for conversion?

    Thanks!
     
  2. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    The transom may well be "solid", but not engineered for the purpose, that aspect will have to be a priority. I can't see any "in principle" reason you can't do it, not sure what you mean by an air-tight bracket, presumably you mean a closed pod(s) rather than a "frame" bracket that offers no buoyancy.
     
  3. Swamplizard
    Joined: Jan 2004
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    Swamplizard Senior Member

    Agreed on shoring up the transom either way - it is 4-inches thick with kevlar reinforced fiberglass ... but should be checked.

    Engine bracket would be the large aluminum hollow air tight one that adds some buoyancy. Figure the BBCs are 1000 each with gears and arneson drives weigh 150-200 each.....but then again 3 or 4 300hp outboards are heavy.
     
  4. gonzo
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    gonzo Senior Member

    The main problem with these types of changes is the shift of the center of gravity (CG). The dynamic behavior of the boat will change; not necessarily for the better.
     
  5. TANSL
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    TANSL Senior Member

    Not necessarily for the worse. To speak in these terms is like saying nothing.
    We must study the submerged hull as a whole and see what happens, because when changing the draft, both the vertical position and the longitudinal position of the center of buoyancy will change.
     
  6. Swamplizard
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    Swamplizard Senior Member

    It is basically a non-stepped Cigarette 38'hull and sits dead flat in the water at rest now full of fuel or empty. when under way at cruise there is little to no bow rise. I am not an engineer so for me to understanding how removing 3,500 pounds from the rearward third of the engine room and off the transom, adding a bouyant bracket and 4x 550 pound motors would effect a 40-foot cruiser that has 4 fuel tanks mounted center of vessel (75x4x8lbs per gallon is 2400 lbs) and 80 gallon water tank is forward of the fuel tanks (640lbs). Keep in mind this is a cruiser - births for 4, Genny and A/C forward of water tank, Head/ full galley ... not an empty race boat cabin-wise. Not sure if this is helpful info.
     
  7. gonzo
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    gonzo Senior Member

    The bracket will be out of the water at planing speeds, so its buoyancy is irrelevant at those speeds. Also, at planing speeds about half the forward hull is out of the water. In essence, the hull acts like a seesaw. The weight is shifting several feet aft, which necessarily changes the CG. That will affect the balance and the behavior of the boat.
     
  8. Swamplizard
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    Swamplizard Senior Member

    I could move the water tank under the front V-Birth if it is an issue but good points.
     
  9. TANSL
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    TANSL Senior Member

    I would not give anything for good or for bad without having some calculations before. The way a ship stands still and moving, can give many surprises.
    Be cautious before changing anything, it costs nothing to check if the opinions, sitting in a chair without more information than the one we read in this thread, may be right or wrong. A correct solution may not fix the problem completely but an erroneous solution can ruin everything. And you'll only find out when it's too late. Giving advice is cheap, fixing mistakes is not usually cheap.
     

  10. Mr Efficiency
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    Mr Efficiency Senior Member

    There seems likely to be a significant reduction in overall weight, which also changes the dynamics of the boat. What can be said in favour of swapping out inboards for outboards in a boat this size, is that the engine mass as a percentage of total weight, is less, compared to a much smaller boat, say 20-25 feet. Therefore, there is less chance of drastic behavioural changes, from engines changes.
     
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