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prop clearances

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  • chodges31711
    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
    Message 1 of 7 , Sep 25, 2007
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      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
    • Kenneth Grome
      Robb White never mentioned any vibration in his Rescue Minor, and from what I have read he built the bottom -- or at least the tunnel section -- exactly to the
      Message 2 of 7 , Sep 25, 2007
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        Robb White never mentioned any vibration in his Rescue Minor, and from
        what I have read he built the bottom -- or at least the tunnel
        section -- exactly to the plans. This does not prove there was no
        vibration of course, but I suspect that Robb would have mentioned it if
        there were.

        Someone else built a Rescue Minor recently, and it was supposedly built
        to plans. A friend of his posted some pictures of the boat online, but
        the owner does not seem to be a person who spends his time online, so
        we may never hear from him about this issue.

        I don't think anyone has built Shoals Runner in a very long time (if
        ever) so I doubt that there is any info on this design either.

        Note that these hulls are not rounded in the top of the inverted vee.
        They actually have a plywood joint there that creates a very shallow
        vee, so maybe Gerr's rule of two inches does not apply here.

        Sincerely,
        Ken Grome
        Bagacay Boatworks
        www.bagacayboatworks.com





        > 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
      • 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 3 of 7 , Sep 26, 2007
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          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 4 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 5 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 6 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 7 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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