shaft dia for surfacing prop

Discussion in 'Surface Drives' started by stupidbaker57, Oct 11, 2011.

  1. stupidbaker57
    Joined: Jan 2011
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    Location: Lakeville Ma

    stupidbaker57 Junior Member

    I am building a surface drive for a small boat (9 foot runabout) that is powered with a 27hp motorcycle engine. The shaft needs to be 52 inches long. I would like to use a 3/4 stainless shaft. Would the length be too long for the dia., and would it have a tendency to "whip" in the center?
    The boat would weight in around 550 pounds with driver. and shaft speed would be around 7000 rpms.
    Any info will be welcomed.
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    Unless you are making your own propeller, 1" shafts and propellers are more readily available and usually cheaper.
  3. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    Bearing support for shaft should be no closer than 20 times shaft diameter.
    And no further apart than 40 times the shaft diameter. The twenty forty rule.
    A 3/4 " shaft should be good. You may have to compromise.
    1 person likes this.
  4. Frosty

    Frosty Previous Member

    The probs with a surface drive shaft is that it is always pushing from the bottom, shearing the shaft off just in front of the prop will occure if insufficient thickness is not used.

    Surface props also have large blade roots and unfortunately hubs for the same reason.

    However as the hub is not in the water it is not a prob. Surface drives are good if a few silly rules are adhered to that would not be normally be obvious on conventional drives.
  5. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    Thats rpm based isnt it Tom?

  6. stupidbaker57
    Joined: Jan 2011
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    Location: Lakeville Ma

    stupidbaker57 Junior Member

    Thanks, Tom. I have a 1" shaft that can be used on the runabout (cracker box) I'll spline it for and outboard prop. (I have a couple of 8 inchers)
    As for the hydro with the rear mounted engine position, I will be able to use a 3/4 shaft and machine a taper for the 6 1/2" prop that came today from Hill Marine.
    With a site like this, you get to save a lot of time in guesstamations.
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