Do you really need a short shaft on an inflatable?

Discussion in 'Propulsion' started by roatan diver, Sep 24, 2010.

  1. roatan diver
    Joined: Sep 2010
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    roatan diver New Member

    I have an 11ft Saturn inflatable boat. The transom is 15 inches in height. The site recommends a short shaft engine. I can get a good deal on a long shaft, but am worried that it may not work. Can I use a long shaft?
     
  2. baeckmo
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    baeckmo Hydrodynamics

    You have to change the transom mounting height. The anti-ventilation plate above the propeller should approximately be level with the lowest part of the hull. If you attach a long shaft engine without adjusting transom height, the "rig" will cause high drag, speed loss and an uncomfortable amount of spray. So, to make it short: Use the correct shaft length!
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Why is it such a "good deal"?

    Is there something wrong with it?

    Is it worth all the hastle?

    -Tom
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    Mount the long shaft 4 to 5 inches higher on the transom. This will mean you need to make an extension.

    This means you'll likely have to remove the plastic "clamp landing" plates. You have a plywood transom, which you'll screw and/or through bolt the extension to. It would be wise to have full contact with all of the transom with this new piece. Naturally, the new piece (extension) will have to be the same thickness as the original transom. There's a fair bit of shaping necessary for a goo fit, but it's not that difficult a project. Mount the new piece to the inside face of the transom, stopping just short of the tubes and let it rest directly on top of that horizontal piece on the bottom of the transom. Glue and screw for a solid job. Screw and sealant for a temporary job that will eventually need replacement.
     

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  5. baeckmo
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    baeckmo Hydrodynamics

    Note that there is a risk with the above scheme. Since the length of the lever from thrust point (=propeller cl) to top of bracket, there is an increased torque into the top of the transom. Short shaft transoms sometimes are juuuust about adequate for that length; they need reinforcements to cope with the increased moment.
     
  6. PAR
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    PAR Yacht Designer/Builder

    Yes, there is an increase in the leverage the engine has on the transom, but as I've described it, the new transom will be twice as thick, with the same HP and should easily handle the torque, good point though Baeckmo.
     
  7. CDK
    Joined: Aug 2007
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    CDK retired engineer

    I think the weak point is not the transom itself but the bonds to the air chambers.
    If there is a substantial amount of hp involved, there should be tie bars with a 45 degree or less angle to the bottom to take up the torque, especially if the transom is modified.
     
  8. WickedGood

    WickedGood Guest

    Go for it.

    on a little inflateable its not going to make any differanece.

    I put a long shaft Saluki injun on my 12 ft Alum rowboat the "Row-Dent" and it worked fine.

    It actually helps keep it from flipping over because of the ballast
     
  9. jimbo2010
    Joined: Sep 2010
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    jimbo2010 Junior Member

    I used a long shaft for years worked fine.

    It's not supposed to?

    But it works,
     
  10. troy2000
    Joined: Nov 2009
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    troy2000 Senior Member

    If it's truly a good deal on the long shaft motor, go for it. My opinion is that if you're just poking around on an inflatable, instead of entering races or commuting, you'll probably do just fine. I doubt it will affect performance drastically enough to worry you.

    If it does, you can stash the motor for future use, sell it for a profit, or do the transom conversion.
     

  11. Easy Rider
    Joined: Oct 2009
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    Easy Rider Senior Member

    But The Short Shaft Is Better.
     
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