Here's one I have not heard...How close should my prop be to my rudder?

Discussion in 'Inboards' started by missinginaction, Jun 4, 2014.

  1. missinginaction
    Joined: Aug 2007
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    Location: New York

    missinginaction Senior Member

    Or does it matter?

    Well, I figured I'd need a new shaft and I was right. My 41 year old OE shaft was just too worn for comfort. I figured I'd need a new coupling as well and the people at Walter Gear had one in stock for my old v-drive. So I sent the coupling off to a prop shop for a fit and face with my new shaft. The OE prop looks OK for now, I'll use it to get a benchmark once she's in the water in a few weeks.

    While I'm waiting for UPS to get the coupling to it's destination it occurred to me that now was the time to make any adjustments in the drive line. This boat's no speed demon. Just a single small block Ford (302), 27' Silverton sedan circa 1973. I'd like to optimize low speed handling and docking on this old girl that has been lovingly restored over the past few years.

    I have the ability to move the engine a few inches forward or aft. Going aft will allow the engine to sit a bit lower in the bay. Not a huge deal but since it's the heaviest single item in the boat it would seem that lower is better.

    The rudder is positioned slightly to the port side of the boat so that the shaft can be removed easily once the prop is pulled. I'm wondering if it would be helpful to position the prop close to the rudder to improve water flow over the rudder? I've never seen that question asked. If closer is indeed better, how close might be too close. I realize I'll need enough clearance to get the prop on and off without dropping the whole rudder out of the boat.
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    A prop too close to the rudder may cause some vibration. I think that if it works fine after 41 years it is better to leave well enough alone.
  3. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Your consideration is correct, the bare minimum clearance is the one which allow a comfortable extraction of the prop from the shaft.

    But there are other considerations too, which are related to the rudder and prop hydrodynamics and efficiency, the need to avoid cavitation over the rudder blade at high speeds and the need to avoid a degradation of vessel's maneuverability at low speeds.
    These considerations have led to the empirically-established rule of thumb which says that the clearance between the rudder's leading edge and the aft-most point of the prop's trailing edge should not be less than 10-15% of the prop diameter.

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