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Re: prop clearances

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  • Lewis E. Gordon
    I went back and looked up Geer s The Nature Of Boats and did not find the 2 inch ROT. He mentioned that 10% prop diameter was a bare minimum and that 14% to
    Message 1 of 7 , Sep 27, 2007
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      I went back and looked up Geer's "The Nature Of Boats" and did not
      find the 2 inch ROT. He mentioned that 10% prop diameter was a bare
      minimum and that 14% to 18% was much better as a minimum. So Farmer's
      15% is right in the ballpark for modern times.

      Lewis

      --- In AtkinBoats@yahoogroups.com, "Ronald Fossum" <artemis@...> wrote:
      >
      > It's interesting the amount of variance that knowledgeable people in
      this field propose. 'Course the truth is that propeller design has so
      many variables that it's surprising that anyone gets close to
      agreement with another. Usually the designer - particularly if we're
      talking 50+ years ago, as is the case here - is smack on. Weston
      Farmer, a contemporary of Bill Atkin, and an authority on propellers
      in medium speed boats of the 30s & 40s (worked for ELCO prior to and
      during WWII), always used "15% of the diameter is the minimum
      clearance". Which is about what the Rescue Minor design has.
      >
      > Ron Fossum
      > ----- Original Message -----
      > From: chodges31711
      > To: AtkinBoats@yahoogroups.com
      > Sent: Tuesday, September 25, 2007 1:54 PM
      > Subject: [AtkinBoats] prop clearances
      >
      >
      > Does the minimum 2 inch propeller tip clearance that Dave Gerr talks
      > about apply to Atkin tunnels? If not, do they have a vibration
      problem?
      >
      > Rescue Minor and Shoals Runner plans show less than a 14 inch space
      > from shoe to hull for a 10 inch prop.
      > I have seen one book excerpt at Amazon that said clearance in a round
      > top tunnel should be less than 1 inch.
      >
      > Charles
      >
      >
      >
      >
      >
      > [Non-text portions of this message have been removed]
      >
    • chodges31711
      In Mr. Gerr s Propeller Handbook he states those percentages and then said but not less than 2 inches. That would imply that 10% is for props over 20 inches.
      Message 2 of 7 , Sep 28, 2007
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        In Mr. Gerr's Propeller Handbook he states those percentages and then
        said but not less than 2 inches. That would imply that 10% is for props
        over 20 inches.

        Charles

        --- In AtkinBoats@yahoogroups.com, "Lewis E. Gordon"
        <l_gordon_nica@...> wrote:
        >
        > I went back and looked up Geer's "The Nature Of Boats" and did not
        > find the 2 inch ROT. He mentioned that 10% prop diameter was a bare
        > minimum and that 14% to 18% was much better as a minimum. So Farmer's
        > 15% is right in the ballpark for modern times.
        >
        > Lewis
      • adharvey2
        In the article accompanying the plans for Sand Dollar, Wm. Atkins writes: There is ample clearance for a propeller diameter of 15 inches. I think it may be of
        Message 3 of 7 , Sep 28, 2007
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          In the article accompanying the plans for Sand Dollar, Wm. Atkins
          writes:
          "There is ample clearance for a propeller diameter of 15 inches. I
          think it may be of interest to explain that the tips of the blades can
          clear the bottom of the horn timber by very little and still perform
          efficiently. There is little, if any, advantage of two or three inches
          space between the tips of the blades and the hull. And there is real
          disadvantage in this practice - the angularity of the propeller is
          increased, a feature which should always be avoided. So far as
          damaging the propeller or damaging the bottom of the boat if drift is
          picked up I might ask what avail will be the 2 or 3 inch clearance be
          if it is invaded by a piece of, say 3 1/4 inch thick, drift wood? Any
          drift wood as light as 1/8 inches to 1/4 inches thick will do little
          damage, excepting to itself, and if you strike anything much heavier
          than this it will damage the propeller despite any amount of clearance
          provided. And becides a hull with a proper keel, shoe, and protected
          rudder is not likely to pick up floating or half submerged drifting
          things. For the same reason I can see no use in fitting a metal strike
          plate on the bottom of the hull over the propeller."

          Most of that relates more to protection than efficiency but I
          thought it best to keep it in context.

          Andrew Harvey



          --- In AtkinBoats@yahoogroups.com, "Ronald Fossum" <artemis@...> wrote:
          >
          > It's interesting the amount of variance that knowledgeable people in
          this field propose. 'Course the truth is that propeller design has so
          many variables that it's surprising that anyone gets close to
          agreement with another. Usually the designer - particularly if we're
          talking 50+ years ago, as is the case here - is smack on. Weston
          Farmer, a contemporary of Bill Atkin, and an authority on propellers
          in medium speed boats of the 30s & 40s (worked for ELCO prior to and
          during WWII), always used "15% of the diameter is the minimum
          clearance". Which is about what the Rescue Minor design has.
          >
          > Ron Fossum
          > ----- Original Message -----
          > From: chodges31711
          > To: AtkinBoats@yahoogroups.com
          > Sent: Tuesday, September 25, 2007 1:54 PM
          > Subject: [AtkinBoats] prop clearances
          >
          >
          > Does the minimum 2 inch propeller tip clearance that Dave Gerr talks
          > about apply to Atkin tunnels? If not, do they have a vibration
          problem?
          >
          > Rescue Minor and Shoals Runner plans show less than a 14 inch space
          > from shoe to hull for a 10 inch prop.
          > I have seen one book excerpt at Amazon that said clearance in a round
          > top tunnel should be less than 1 inch.
          >
          > Charles
          >
          >
          >
          >
          >
          > [Non-text portions of this message have been removed]
          >
        • chodges31711
          I think the problem with too little clearance is vibration (noise?) created as prop blades pass by the narrow spot. The vibration is less with more blades
          Message 4 of 7 , Oct 1, 2007
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            I think the problem with too little clearance is vibration (noise?)
            created as prop blades pass by the narrow spot. The vibration is less
            with more blades and/or more clearance.

            Charles

            --- In AtkinBoats@yahoogroups.com, "adharvey2" <adharvey@...> wrote:
            >
            > In the article accompanying the plans for Sand Dollar, Wm. Atkins
            > writes:
            > "There is ample clearance for a propeller diameter of 15 inches. I
            > think it may be of interest to explain that the tips of the blades can
            > clear the bottom of the horn timber by very little and still perform
            > efficiently. There is little, if any, advantage of two or three inches
            > space between the tips of the blades and the hull.
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