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Re: [AtkinBoats] prop clearances

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  • Ronald Fossum
    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
    Message 1 of 7 , Sep 26, 2007
      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]
    • 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 2 of 7 , Sep 27, 2007
        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 3 of 7 , Sep 28, 2007
          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 4 of 7 , Sep 28, 2007
            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 5 of 7 , Oct 1, 2007
              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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