How far aft is good clearance required under prop?

Discussion in 'Props' started by Northeaster, May 4, 2016.

  1. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    (i.e. between prop and lower skeg / rudder support bar)

    I made a mistake in building my lower rudder support (placing it above the lower skeg bar rather than below it), and now only have about 0.5" of prop clearance..

    So, in looking at options, one would seem to be rebuild so it placed bar below the skeg support, which is possible.

    Or, if cut the rudder support bar shorter, so it was not directly below the prop, would this be acceptable? i.e what if I had 2 inches of clearance under the prop tips, but if that amount was reduced a few inches aft of the prop?

    Note, - It is a 15 inch prop.

    Any help would be appreciated.
     

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  2. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    3" away from the blade tips would be a minimum. I've seen closer, but it's not ideal. It looks like a simple fix, to cut and redo.
     
  3. Northeaster
    Joined: Sep 2011
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    Location: Eastern Canada

    Northeaster Senior Member

    Thanks for the reply!
    Do you mean that you would recommend just cutting shorter and placing still above skeg support bar, as it is now?

    Or. remove, add a longer bushing between hull and rudder blade and configure rudder support to go under the skeg bar?
     
  4. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The more clearance the better, so cut what you have back partly or wholly into the strut and put a new hunk of steel on to support the rudder heel fitting, straight back from the strut. I'd recommend a lap or scarf joint pon this new piece, to get enough weld surface area for the scabbed on new piece.
     
  5. SamSam
    Joined: Feb 2005
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    Location: Coastal Georgia

    SamSam Senior Member

    I thought the extra piece was so you could take it off and the drop the rudder out. How does the rudder come out if the skeg is welded into one piece?
     
  6. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    The hell fitting could be welded to a plate that's bolted to the support, for rudder removal.
     
  7. sdowney717
    Joined: Nov 2010
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    sdowney717 Senior Member

    I would cut the lower support back like your thinking and then try the boat and see if it is noisy. 2 inches you can probably live with that even though not ideal. It would be worse if it was a prop tip to hull issue.
     

  8. yodani
    Joined: Nov 2010
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    Location: Danube Delta

    yodani Senior Member

    Or instead of a solid bar you could use a U profile bar face down and overlap it with the skeg. The U bar is strong enough and will add only 5-6mm in height.

    The simplest fix would be to shorten the overlapping part near the second hole and eventually drill another in between the other two. I think this will work well.

    Cheers,

    Daniel
     
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