Shaft size

Discussion in 'Inboards' started by discovery, Jul 11, 2014.

  1. discovery
    Joined: Aug 2013
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    discovery Junior Member

    Apart from the usual shaft size problems us men have, :rolleyes:, is it possible to go too big (seriously).
    My boat runs a 2" shaft behind a 1.5 to 1 gear, and I cant help but think of the extra drag it would have in the bearings and such over a 1 5/8" shaft which is recommended size for my power combo.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    It depends on your speed. Drag increases exponentially so if you are running under 25 knots it probably won't pay to do all the modifications.
  3. NavalSArtichoke
    Joined: Oct 2013
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    NavalSArtichoke Senior Member

    Shaft size depends on a lot of things: how much power or torque is being transmitted, how far apart the prop and the gear are, what things below the water might whack into your shaft (i.e., how long do you want to last, ..., the shaft I mean).

    You could probably run a shaft which is half the recommended diameter, but you also increase the chances of winding up with a broken shaft when you are out in the middle of nowhere.
  4. d1970
    Joined: Jun 2012
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    d1970 Junior Member

    Think of it as insurance,whatever the extra cost is vs a thinner one.

  5. Petros
    Joined: Oct 2007
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    Petros Senior Member

    A larger dia shaft can theoretically be thinner and lighter, but that would make it more suspect able to incidental damage (dents, corrosion, etc). You could still use small bearings and a large shaft, this is typically done on many cars and trucks, though it means that you will not have a one piece drive shaft. A larger dia drive shaft is also less susceptible to vibrations since it is stiffer (a stiff shaft is is always good). though that is also a function of free length and rpm.
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