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Re: Gear reduction for Little Effort

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  • sals_dad
    ... Handbook by Dave Gerr. I think Duckworks carries it. ... I ll second Rob s recommendation. Atkin s drive systems were something special - even bizarre,
    Message 1 of 8 , Dec 21, 2006
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      > Nowhere is this all better explained than in The Propeller
      Handbook by Dave Gerr. I think Duckworks carries it.
      > --Rob
      >

      I'll second Rob's recommendation. Atkin's drive systems were
      something special - even bizarre, by today's standards. For "Noble
      Cab", Dave Gerr re-designed the drive train, and ended up specing a
      17x17 5 bladed prop, with something like 95% coverage ratio (wrong
      terminology, I know). That prop had to be custom fabricated in
      Australia - and cost more than any boat I have ever owned.

      For Rescue Minor, I consulted with several prop specialists. The
      unanimous response was "impossible", "will never work".

      After a long time with Gerr's book and a spreadsheet, I came up with
      solutions very similar to Robb White's. I have the spreadsheet
      somewhere, kinda rough, and no documentation, but it might save
      somebody a few hours working out Gerr's formulas

      Sal's Dad.
    • John Kohnen
      Bizarre? Most of Wm. and John Atkin s motorboats use direct drive, no reduction, for simplicity and lower cost, I d guess, and the low to medium speed engines
      Message 2 of 8 , Dec 22, 2006
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        Bizarre? Most of Wm. and John Atkin's motorboats use direct drive, no
        reduction, for simplicity and lower cost, I'd guess, and the low to medium
        speed engines available in William's day could turn a decent sized prop
        direct at a suitable speed without complaining. Nothing bizarre about any
        of that. <shrug> Nowadays even light diesels turn faster than many of the
        gas engines William favored, so a reduction gear might need be used. In
        some cases, if you're going to pungle up the dough for a reduction gear
        anyway, it might make sense to gear down enough to use a larger propeller.
        Generally though, if you can get a reduction gear that'll turn the prop
        shaft at about the speed the Atkin's intended that's a good start. Then
        consult Gerr's Propeller Handbook.

        On Thu, 21 Dec 2006 07:35:44 -0800, Sal's Dad wrote:

        >> Nowhere is this all better explained than in The Propeller
        > Handbook by Dave Gerr. I think Duckworks carries it.
        >> --Rob
        >>
        >
        > I'll second Rob's recommendation. Atkin's drive systems were
        > something special - even bizarre, by today's standards.
        > ...

        --
        John <jkohnen@...>
        Power always has to be kept in check; power exercised in secret,
        especially under the cloak of national security, is doubly
        dangerous. <William Proxmire>
      • John Kohnen
        For displacement hulls like those huge engines usually push a big, slow propeller is most efficient. But Little Effort is intended for planing speeds, so a
        Message 3 of 8 , Dec 22, 2006
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          For displacement hulls like those huge engines usually push a big, slow
          propeller is most efficient. But Little Effort is intended for planing
          speeds, so a smaller, faster propeller will work best. The propeller
          called for in the plans is 11" dia. by 12" pitch. If you can find, or
          make, a reduction gear that'll turn the shaft around 2,000 rpm that should
          be fine, but if you run a direct drive from the engine and prop it so the
          engine turns 2,000 rpm you won't get anywhere near the advertised 33 hp.

          If you can find a direct drive reverse gear you could use a cogged belt to
          the prop shaft. That'd make it easy to get just the amount of reduction
          you want.

          On Tue, 19 Dec 2006 19:27:20 -0800, Andrew wrote:

          > ...
          > I'm a complete neophyte when it comes to
          > inboards. The designed engine ran at 2000 rpm with no reduction, but
          > I'm told by my marine diesel mechanic friend in Seattle that 2000
          > shaft rpm is not the norm these days, but he's used to engines larger
          > than my car
          > ...
          > I'm sure prop technology has advanced some in 50
          > years. The kubota has an incredibly flat torque curve so it could be
          > happy at 1800 or 2800 rpm or whatever.
          > ...

          --
          John <jkohnen@...>
          Distrust any enterprise that requires new clothes. <Henry David
          Thoreau>
        • Ronald Fossum
          Thank you John for this and the previous posting. I m still moving ahead on a steam powered version of Rescue Minor and find that Bill s propeller selection
          Message 4 of 8 , Dec 22, 2006
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            Thank you John for this and the previous posting. I'm still moving ahead on
            a steam powered version of Rescue Minor and find that Bill's propeller
            selection (based on prop tables and formulas - including Dave Gerr's) is
            exactly right. My difference is the steam power part of the equation. Rob
            White's use of a different propeller (not very different really) was based
            on engine rpm (about 800 rpm faster) and the resultant hp at that rpm. But
            working forward to that from Bill's propeller, it comes in about right. I've
            had experience with "angled off center" props and I must say that, done
            correctly, they do make for improved handling. A radical example of what
            happens with an inline shaft and high horsepower and be observed when such a
            boat "guns the engine" when underway.

            Merry Christmas all.

            Ron Fossum
            ----- Original Message -----
            From: "John Kohnen" <jkohnen@...>
            To: <AtkinBoats@yahoogroups.com>
            Sent: Friday, December 22, 2006 12:47 AM
            Subject: Re: [AtkinBoats] Gear reduction for Little Effort


            > For displacement hulls like those huge engines usually push a big, slow
            > propeller is most efficient. But Little Effort is intended for planing
            > speeds, so a smaller, faster propeller will work best. The propeller
            > called for in the plans is 11" dia. by 12" pitch. If you can find, or
            > make, a reduction gear that'll turn the shaft around 2,000 rpm that should
            > be fine, but if you run a direct drive from the engine and prop it so the
            > engine turns 2,000 rpm you won't get anywhere near the advertised 33 hp.
            >
            > If you can find a direct drive reverse gear you could use a cogged belt to
            > the prop shaft. That'd make it easy to get just the amount of reduction
            > you want.
            >
            > On Tue, 19 Dec 2006 19:27:20 -0800, Andrew wrote:
            >
            >> ...
            >> I'm a complete neophyte when it comes to
            >> inboards. The designed engine ran at 2000 rpm with no reduction, but
            >> I'm told by my marine diesel mechanic friend in Seattle that 2000
            >> shaft rpm is not the norm these days, but he's used to engines larger
            >> than my car
            >> ...
            >> I'm sure prop technology has advanced some in 50
            >> years. The kubota has an incredibly flat torque curve so it could be
            >> happy at 1800 or 2800 rpm or whatever.
            >> ...
            >
            > --
            > John <jkohnen@...>
            > Distrust any enterprise that requires new clothes. <Henry David
            > Thoreau>
            >
            >
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