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Re: engines; jet drives

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  • johnfader
    Don: syncronicity? Just yesterday I was thinking about the jet drive for use in my planned Sneakeasy. I was thinking about a complete drive from a jet-ski
    Message 1 of 13 , Jan 3, 2004
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      Don:

      syncronicity? Just yesterday I was thinking about the jet drive for use in my planned Sneakeasy. I was thinking about a complete drive from a jet-ski machine. These are far more powerful than I need for this boat, as it will be primarily a river cruiser used ad displacement speeds. I have been planning to use an outboard in the 15-20 hp range.

      I have a cousin with a Berkely jet and 455 CID Olds engine that I could have... guess that might be a bit much for a Sneakeasy :-)

      As for your application, maybe you shouldn't worry about air bubbles, eh? The drives work; no need worrying about whether there is some theoretical better arrangement. I see folks charging about the lakes all the time before these things, without a worry line on thier brows :-)

      Seriously, how much power do you need? From what I've read about outboard jets, they are really inefficient... as I remember, they are about as effective as a screw-driven outbaard one-third the size. Max, correct me please if I'm misremembering. Of course, with that goes 3X the fuel consumption for a given thrust, Compared to a prop.

      Anyone have any idea whether jets work well at low speeds? I'd guess their efficiency increases, but don't know for sure.

      Cheers/The Fader

      Don Sez:

      Happy Hollidays,
      I wonder if anyone in this group has experience with outboard
      jet drives. I have talked to designers who say that it is hard to
      make an inboard/jet drive work smoothly because alot of air
      (bubbles) are trapped under the hull and consequently fed to the jet
      pump with a poor result in propulsion. I wonder if outboard jets
      would work better since they can be adjusted for depth independent
      of the hull.
      On the Upper Delaware River where I live I see Carolina skiffs
      going though 6" deep water at 20-30 MPH with 100/+- HP engines on
      their transoms. Imagine how nice this might work on some of the PCB
      sharpie boats.
    • Jeff Blunck
      According to Mercury, their outboard jet drives are 25% - 30% less efficient. The advantages are strictly for very shallow running where debris can impact on
      Message 2 of 13 , Jan 3, 2004
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        According to Mercury, their outboard jet drives are 25% - 30% less
        efficient. The advantages are strictly for very shallow running where
        debris can impact on propellers and ruin the standard outboard.

        I did a fair amount of research on inboard jet drives for my Wyoming and all
        the manufacturers that I visited with expressed concern with an inboard jet
        drive on a flat bottom boat.

        Essentially flatties have to many air bubbles in the water following the
        underside of the hull. This has a great advantage in reduced power needed
        to get them moving and planing up to the point where the turbulence counters
        the power and they hit a wall where much more power is needed to go faster.
        This is why flatties where popular in the era of lower HP engines of the
        20's and 30's. Now all planing boats are designed with a vee hull which is
        usually constant from amidships to the stern.

        On an inboard jet drive, these air bubbles impacting on the turbine blades
        cause some loss of power and cavitations which can be deadly to an impeller.
        They like to see some vee in the hull design where the water pressure is
        equal across the bottom and sides, especially when the boat gets up on
        plane.

        They also expressed that the efficiency of jet drives are not realized until
        the boat exceeds planing speeds so if I was to travel at hull speeds most of
        the time, my fuel consumption would be excessive compared to a propeller.

        The Berkley jet people where very helpful and against their desire to sell
        me a drive, convinced me otherwise.


        Jeff
      • Don Tyson
        John, The outboard Jet I was on was very efficient once the owner got it adjusted well enough to keep the intake full of water and not air. This was a original
        Message 3 of 13 , Jan 3, 2004
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          John,
          The outboard Jet I was on was very efficient once the owner got it adjusted well enough to keep the intake full of water and not air. This was a original flat bottom Carolina Skiff 19'. I heard that the impellers don't last long if their fed alot of air. If entrained air under flat bottom boats (i.e.Sneakeasy) is a serious problem then I was wonder if the pickup unit on an outboard jet could be lowered several inches further than normal to get away from or , in other words, below this area of entrained air? Said another way: I wonder if lowering the intake would alow the jet to be fed pure water?
          Yamaha used to make a 25hp outboard which when converted to a jet was rated as an 18hp, perfect for a Sneakeasy.
          I would like to see a jet powered Wyo and since the Sneakeasy is supposed to be scaled down from that why not experment on the smaller cheaper boat?
          The other problem I have been wondering about is braking power. I understand that reverse is poor on jets. Any thoughts anyone? Jeff Blunck, What are you oing to use?
          Don

          johnfader <johnfader@...> wrote:
          Don:

          syncronicity? Just yesterday I was thinking about the jet drive for use in my planned Sneakeasy. I was thinking about a complete drive from a jet-ski machine. These are far more powerful than I need for this boat, as it will be primarily a river cruiser used ad displacement speeds. I have been planning to use an outboard in the 15-20 hp range.

          I have a cousin with a Berkely jet and 455 CID Olds engine that I could have... guess that might be a bit much for a Sneakeasy :-)

          As for your application, maybe you shouldn't worry about air bubbles, eh? The drives work; no need worrying about whether there is some theoretical better arrangement. I see folks charging about the lakes all the time before these things, without a worry line on thier brows :-)

          Seriously, how much power do you need? From what I've read about outboard jets, they are really inefficient... as I remember, they are about as effective as a screw-driven outbaard one-third the size. Max, correct me please if I'm misremembering. Of course, with that goes 3X the fuel consumption for a given thrust, Compared to a prop.

          Anyone have any idea whether jets work well at low speeds? I'd guess their efficiency increases, but don't know for sure.

          Cheers/The Fader

          Don Sez:

          Happy Hollidays,
          I wonder if anyone in this group has experience with outboard
          jet drives. I have talked to designers who say that it is hard to
          make an inboard/jet drive work smoothly because alot of air
          (bubbles) are trapped under the hull and consequently fed to the jet
          pump with a poor result in propulsion. I wonder if outboard jets
          would work better since they can be adjusted for depth independent
          of the hull.
          On the Upper Delaware River where I live I see Carolina skiffs
          going though 6" deep water at 20-30 MPH with 100/+- HP engines on
          their transoms. Imagine how nice this might work on some of the PCB
          sharpie boats.



          Bolger rules!!!
          - no cursing, flaming, trolling, spamming, or flogging dead horses
          - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
          - Pls add your comments at the TOP, SIGN your posts, and snip away
          - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax: (978) 282-1349
          - Unsubscribe: bolger-unsubscribe@yahoogroups.com
          - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com


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          [Non-text portions of this message have been removed]
        • Hugo Tyson
          Jet drives are best at speeds above 25 Knots where they are more efficient than propellers. Speeds under 25 Kts the re really inefficient as the motor is still
          Message 4 of 13 , Jan 3, 2004
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            Jet drives are best at speeds above 25 Knots where they are more efficient than propellers. Speeds under 25 Kts the're really inefficient as the motor is still working relatively hard to drive them. The propulsion is directed by the "bucket" on the nozzle, so the engine needs no gearbox in most cases. You adjust the thrust, therefore speed and direction with a lever.You can go from full speed ahead to full astern with just the pull of the lever. Motors are not stressed in these jet applications as they run at a constant load driving the impeller. One must realise that the major strain on a boat's hull is on the bottom, where the jet drive is mounted, not the transom in the case of OB motors or the engine bearers,gearbox in a inboard . I've heard at lower speeds they can be a bit tricky to steer.

            As to Outboard jet drives they aren't as bad as you make out, (being only a third of the hp.) Apparently a 115HP Outboard Jet drive is equivilent to a 90 hp outboard. So you do lose a bit of hp, but not 2/3rds!!.but gain lots of advantages as long as you're aware of the jet drive's limitations also. Remember most 115HP outboards are the same size and weight as the 90HP models.Only the rev. range,prop.dimensions and fuel injection/carburettion differ between most brands of 75,90 & 115 HP outboards..

            Of course the smaller the HP the greater the difference in output between jet and prop. I'm not sure what the smallest HP jet drive OB is but I don't think I've seen anything under 40hp which would be approximately equivilent to a 25-30hp outboard with prop. drive.

            Hugo Tyson, Tasmania Australia.

            johnfader <johnfader@...> wrote:
            Don:

            syncronicity? Just yesterday I was thinking about the jet drive for use in my planned Sneakeasy.


            From what I've read about outboard jets, they are really inefficient... as I remember, they are about as effective as a screw-driven outbaard one-third the size. Max, correct me please if I'm misremembering.
            Anyone have any idea whether jets work well at low speeds? I'd guess their efficiency increases, but don't know for sure.

            Cheers/The Fader

            Don Sez:

            Happy Hollidays,
            I wonder if anyone in this group has experience with outboard
            jet drives. I have talked to designers who say that it is hard to
            make an inboard/jet drive work smoothly because alot of air
            (bubbles) are trapped under the hull and consequently fed to the jet
            pump with a poor result in propulsion. I wonder if outboard jets
            would work better since they can be adjusted for depth independent
            of the hull.
            On the Upper Delaware River where I live I see Carolina skiffs
            going though 6" deep water at 20-30 MPH with 100/+- HP engines on
            their transoms. Imagine how nice this might work on some of the PCB
            sharpie boats.




            Bolger rules!!!
            - no cursing, flaming, trolling, spamming, or flogging dead horses
            - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
            - Pls add your comments at the TOP, SIGN your posts, and snip away
            - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax: (978) 282-1349
            - Unsubscribe: bolger-unsubscribe@yahoogroups.com
            - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com


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          • Jeff Blunck
            ... rated as an 18hp, perfect for a Sneakeasy. ... to be scaled down from that why not experment on the smaller cheaper boat? ... understand that reverse is
            Message 5 of 13 , Jan 3, 2004
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              > Yamaha used to make a 25hp outboard which when converted to a jet was
              rated as an 18hp, perfect for a Sneakeasy.
              > I would like to see a jet powered Wyo and since the Sneakeasy is supposed
              to be scaled down from that why not experment on the smaller cheaper boat?
              > The other problem I have been wondering about is braking power. I
              understand that reverse is poor on jets. Any thoughts anyone? Jeff Blunck,
              What are you oing to use?
              > Don

              Dropping the transom a bit on a Sneakeasy for the Jet would probably be
              ideal and work just fine. Seeing a Sneakeasy scooting along on a morning
              dew would be something to watch!

              I'm current built out to fit a 3.0L Mercruiser Stern Drive on my Wyoming.
              Bolger calls for a max of 150 HP on the Wyo for safety sake so the 130 HP
              Sterndrive should be ideal. I expect to cruise around 10 knts on 1/3
              throttle with a WOT range of about 22 knts. Who wants to bounce a Wyo and
              rattle the dishes going faster? At least in crusing mode with all the gear.
              :>)

              Jeff
            • jhkohnen@boat-links.com
              For a low-speed boat you might consider a tunnel stern, which is essentially a high-volume, low-velocity jet drive. In ascending order of construction
              Message 6 of 13 , Jan 3, 2004
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                For a low-speed boat you might consider a tunnel stern, which is essentially
                a high-volume, low-velocity jet drive. In ascending order of construction
                complexity and propulsion efficiency, here are some examples from Chester
                Nedwidek and the Atkin catalog:

                http://www.boat-links.com/images/Skeeter.gif

                http://www.boat-links.com/Atkinco/Utilities/Twinkle.html

                http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html

                On Sat, 3 Jan 2004 09:31:28 -0700, Jeff wrote:
                > ...
                > They also expressed that the efficiency of jet drives are not realized
                until
                > the boat exceeds planing speeds so if I was to travel at hull speeds most
                of
                > the time, my fuel consumption would be excessive compared to a propeller.
                > ...

                --
                John <jkohnen@...>
                http://www.boat-links.com/
                Distrust any enterprise that requires new clothes.
                <Henry David Thoreau>
              • Jeff Blunck
                Actually I have adapted the Wyo to something similar though it s untested yet. I put a 28 inch wide by 5 deep tunnel running 42 inches long at the stern.
                Message 7 of 13 , Jan 4, 2004
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                  Actually I have adapted the Wyo to something similar though it's untested
                  yet. I put a 28" inch wide by 5" deep tunnel running 42 inches long at the
                  stern. You can just see it on some of my pictures showing me move the stern
                  section with a trailer.

                  The key here is that the tunnel is still under the designed water line so
                  there should be no problems with reversing or cooling. It was not installed
                  to get into shallows with a stern drive but it does reduce the overall
                  operating depth by 5 inches. Theory here is to have a tunnel allowing me to
                  raise the Sterndrive high enough that it's mostly out of the water for
                  corrosion problems plus it can be removed with out shipping water into the
                  engine compartment so repairs can be made without pulling the 50 foot boat.

                  This tunnel will not be very effective at the top end speeds of the Wyo and
                  in fact may cause some cavitations, that's still to be determined.

                  Jeff

                  http://www.4dw.net/cosailor/


                  ----- Original Message -----
                  From: <jhkohnen@...>
                  To: <bolger@yahoogroups.com>
                  Sent: Saturday, January 03, 2004 11:14 PM
                  Subject: Re: [bolger] Re: engines; jet drives


                  > For a low-speed boat you might consider a tunnel stern, which is
                  essentially
                  > a high-volume, low-velocity jet drive. In ascending order of construction
                  > complexity and propulsion efficiency, here are some examples from Chester
                  > Nedwidek and the Atkin catalog:
                  >
                  > http://www.boat-links.com/images/Skeeter.gif
                  >
                  > http://www.boat-links.com/Atkinco/Utilities/Twinkle.html
                  >
                  > http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html
                  >
                  > On Sat, 3 Jan 2004 09:31:28 -0700, Jeff wrote:
                  > > ...
                  > > They also expressed that the efficiency of jet drives are not realized
                  > until
                  > > the boat exceeds planing speeds so if I was to travel at hull speeds
                  most
                  > of
                  > > the time, my fuel consumption would be excessive compared to a
                  propeller.
                  > > ...
                  >
                  > --
                  > John <jkohnen@...>
                  > http://www.boat-links.com/
                  > Distrust any enterprise that requires new clothes.
                  > <Henry David Thoreau>
                  >
                  >
                  >
                  >
                  > Bolger rules!!!
                  > - no cursing, flaming, trolling, spamming, or flogging dead horses
                  > - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
                  > - Pls add your comments at the TOP, SIGN your posts, and snip away
                  > - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax:
                  (978) 282-1349
                  > - Unsubscribe: bolger-unsubscribe@yahoogroups.com
                  > - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com
                  >
                  > Yahoo! Groups Links
                  >
                  > To visit your group on the web, go to:
                  > http://groups.yahoo.com/group/bolger/
                  >
                  > To unsubscribe from this group, send an email to:
                  > bolger-unsubscribe@yahoogroups.com
                  >
                  > Your use of Yahoo! Groups is subject to:
                  > http://docs.yahoo.com/info/terms/
                  >
                  >
                • Don Tyson
                  Jeff, I could be wrong but I think Dave Gerr ( Well known east coast designer/author) did some designs with this concept which is essentially the same as the
                  Message 8 of 13 , Jan 4, 2004
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                    Jeff, I could be wrong but I think Dave Gerr ( Well known east coast designer/author) did some designs with this concept which is essentially the same as the historic Seabright Skiff. The seabright skiff is uniqe in that it was launched from unprotected sand beaches directly into the rough Atlantic off the NJ coast. Having the prop up in like that alowed the hulls to be dragged up the beach at the end of the day. As long as the two bladed prop was horizontal dragging the hulls didn't cause any damage. I havent seen anyone enploy a stern drive to this bottom but it sounds inexpensive as there are many used 3.0 units out there for a song.
                    Keep me posted.
                    Don Tyson

                    Jeff Blunck <boatbuilding@...> wrote:
                    Actually I have adapted the Wyo to something similar though it's untested
                    yet. I put a 28" inch wide by 5" deep tunnel running 42 inches long at the
                    stern. You can just see it on some of my pictures showing me move the stern
                    section with a trailer.

                    The key here is that the tunnel is still under the designed water line so
                    there should be no problems with reversing or cooling. It was not installed
                    to get into shallows with a stern drive but it does reduce the overall
                    operating depth by 5 inches. Theory here is to have a tunnel allowing me to
                    raise the Sterndrive high enough that it's mostly out of the water for
                    corrosion problems plus it can be removed with out shipping water into the
                    engine compartment so repairs can be made without pulling the 50 foot boat.

                    This tunnel will not be very effective at the top end speeds of the Wyo and
                    in fact may cause some cavitations, that's still to be determined.

                    Jeff

                    http://www.4dw.net/cosailor/


                    ----- Original Message -----
                    From:
                    To:
                    Sent: Saturday, January 03, 2004 11:14 PM
                    Subject: Re: [bolger] Re: engines; jet drives


                    > For a low-speed boat you might consider a tunnel stern, which is
                    essentially
                    > a high-volume, low-velocity jet drive. In ascending order of construction
                    > complexity and propulsion efficiency, here are some examples from Chester
                    > Nedwidek and the Atkin catalog:
                    >
                    > http://www.boat-links.com/images/Skeeter.gif
                    >
                    > http://www.boat-links.com/Atkinco/Utilities/Twinkle.html
                    >
                    > http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html
                    >
                    > On Sat, 3 Jan 2004 09:31:28 -0700, Jeff wrote:
                    > > ...
                    > > They also expressed that the efficiency of jet drives are not realized
                    > until
                    > > the boat exceeds planing speeds so if I was to travel at hull speeds
                    most
                    > of
                    > > the time, my fuel consumption would be excessive compared to a
                    propeller.
                    > > ...
                    >
                    > --
                    > John
                    > http://www.boat-links.com/
                    > Distrust any enterprise that requires new clothes.
                    >
                    >
                    >
                    >
                    >
                    > Bolger rules!!!
                    > - no cursing, flaming, trolling, spamming, or flogging dead horses
                    > - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
                    > - Pls add your comments at the TOP, SIGN your posts, and snip away
                    > - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax:
                    (978) 282-1349
                    > - Unsubscribe: bolger-unsubscribe@yahoogroups.com
                    > - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com
                    >
                    > Yahoo! Groups Links
                    >
                    > To visit your group on the web, go to:
                    > http://groups.yahoo.com/group/bolger/
                    >
                    > To unsubscribe from this group, send an email to:
                    > bolger-unsubscribe@yahoogroups.com
                    >
                    > Your use of Yahoo! Groups is subject to:
                    > http://docs.yahoo.com/info/terms/
                    >
                    >


                    Bolger rules!!!
                    - no cursing, flaming, trolling, spamming, or flogging dead horses
                    - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
                    - Pls add your comments at the TOP, SIGN your posts, and snip away
                    - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax: (978) 282-1349
                    - Unsubscribe: bolger-unsubscribe@yahoogroups.com
                    - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com


                    Yahoo! Groups Links

                    To visit your group on the web, go to:
                    http://groups.yahoo.com/group/bolger/

                    To unsubscribe from this group, send an email to:
                    bolger-unsubscribe@yahoogroups.com

                    Your use of Yahoo! Groups is subject to:
                    http://docs.yahoo.com/info/terms/



                    [Non-text portions of this message have been removed]
                  • Jeff Blunck
                    The prop I intend to use is a 15 prop and with the cavitation plate about an inch below the top of the tunnel at rest, I would estimate that only the top
                    Message 9 of 13 , Jan 4, 2004
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                      The prop I intend to use is a 15" prop and with the cavitation plate about
                      an inch below the top of the tunnel at rest, I would estimate that only the
                      top three inches of the prop will actually be running in the tunnel itself.
                      Without a drive to test fit, I can only estimate that even in the full up
                      position, the bottom of the fin may still drag in the mud but the prop
                      should be clear. Either way I was only after the ability to have the drive
                      lift clear of the water and to remove the drive unit without pulling the
                      boat. I don't plan on motoring the Wyo in real shallows except to nose up
                      to a beach.

                      Jeff

                      ----- Original Message -----
                      From: "Don Tyson" <tysond99@...>
                      To: <bolger@yahoogroups.com>
                      Sent: Sunday, January 04, 2004 1:51 PM
                      Subject: Re: [bolger] Re: engines; jet drives


                      > Jeff, I could be wrong but I think Dave Gerr ( Well known east coast
                      designer/author) did some designs with this concept which is essentially the
                      same as the historic Seabright Skiff. The seabright skiff is uniqe in that
                      it was launched from unprotected sand beaches directly into the rough
                      Atlantic off the NJ coast. Having the prop up in like that alowed the hulls
                      to be dragged up the beach at the end of the day. As long as the two bladed
                      prop was horizontal dragging the hulls didn't cause any damage. I havent
                      seen anyone enploy a stern drive to this bottom but it sounds inexpensive as
                      there are many used 3.0 units out there for a song.
                      > Keep me posted.
                      > Don Tyson
                      >
                      > Jeff Blunck <boatbuilding@...> wrote:
                      > Actually I have adapted the Wyo to something similar though it's untested
                      > yet. I put a 28" inch wide by 5" deep tunnel running 42 inches long at the
                      > stern. You can just see it on some of my pictures showing me move the
                      stern
                      > section with a trailer.
                      >
                      > The key here is that the tunnel is still under the designed water line so
                      > there should be no problems with reversing or cooling. It was not
                      installed
                      > to get into shallows with a stern drive but it does reduce the overall
                      > operating depth by 5 inches. Theory here is to have a tunnel allowing me
                      to
                      > raise the Sterndrive high enough that it's mostly out of the water for
                      > corrosion problems plus it can be removed with out shipping water into the
                      > engine compartment so repairs can be made without pulling the 50 foot
                      boat.
                      >
                      > This tunnel will not be very effective at the top end speeds of the Wyo
                      and
                      > in fact may cause some cavitations, that's still to be determined.
                      >
                      > Jeff
                      >
                      > http://www.4dw.net/cosailor/
                      >
                      >
                      > ----- Original Message -----
                      > From:
                      > To:
                      > Sent: Saturday, January 03, 2004 11:14 PM
                      > Subject: Re: [bolger] Re: engines; jet drives
                      >
                      >
                      > > For a low-speed boat you might consider a tunnel stern, which is
                      > essentially
                      > > a high-volume, low-velocity jet drive. In ascending order of
                      construction
                      > > complexity and propulsion efficiency, here are some examples from
                      Chester
                      > > Nedwidek and the Atkin catalog:
                      > >
                      > > http://www.boat-links.com/images/Skeeter.gif
                      > >
                      > > http://www.boat-links.com/Atkinco/Utilities/Twinkle.html
                      > >
                      > > http://www.boat-links.com/Atkinco/Utilities/RescueMinor.html
                      > >
                      > > On Sat, 3 Jan 2004 09:31:28 -0700, Jeff wrote:
                      > > > ...
                      > > > They also expressed that the efficiency of jet drives are not realized
                      > > until
                      > > > the boat exceeds planing speeds so if I was to travel at hull speeds
                      > most
                      > > of
                      > > > the time, my fuel consumption would be excessive compared to a
                      > propeller.
                      > > > ...
                      > >
                      > > --
                      > > John
                      > > http://www.boat-links.com/
                      > > Distrust any enterprise that requires new clothes.
                      > >
                      > >
                      > >
                      > >
                      > >
                      > > Bolger rules!!!
                      > > - no cursing, flaming, trolling, spamming, or flogging dead horses
                      > > - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
                      > > - Pls add your comments at the TOP, SIGN your posts, and snip away
                      > > - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930,
                      Fax:
                      > (978) 282-1349
                      > > - Unsubscribe: bolger-unsubscribe@yahoogroups.com
                      > > - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com
                      > >
                      > > Yahoo! Groups Links
                      > >
                      > > To visit your group on the web, go to:
                      > > http://groups.yahoo.com/group/bolger/
                      > >
                      > > To unsubscribe from this group, send an email to:
                      > > bolger-unsubscribe@yahoogroups.com
                      > >
                      > > Your use of Yahoo! Groups is subject to:
                      > > http://docs.yahoo.com/info/terms/
                      > >
                      > >
                      >
                      >
                      > Bolger rules!!!
                      > - no cursing, flaming, trolling, spamming, or flogging dead horses
                      > - stay on topic, stay on thread, punctuate, no 'Ed, thanks, Fred' posts
                      > - Pls add your comments at the TOP, SIGN your posts, and snip away
                      > - Plans: Mr. Philip C. Bolger, P.O. Box 1209, Gloucester, MA, 01930, Fax:
                      (978) 282-1349
                      > - Unsubscribe: bolger-unsubscribe@yahoogroups.com
                      > - Open discussion: bolger_coffee_lounge-subscribe@yahoogroups.com
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                    • jhkohnen@boat-links.com
                      It was William Atkin who first modified the Seabright skiff with reverse deadrise aft, forming a tunnel. He also mated the Seabright skiff box deadwood with a
                      Message 10 of 13 , Jan 5, 2004
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                        It was William Atkin who first modified the Seabright skiff with reverse
                        deadrise aft, forming a tunnel. He also mated the Seabright skiff box
                        deadwood with a V-bottom, both with tunnel sterns and without. Most of us
                        think of Billy Atkin as a designer of Good Boats, but not as much of an
                        innovator, but his V-bottom and tunnel stern Seabright skiffs were truly
                        innovative. His gift to the boat design world, as he put it. Dave Gerr has
                        done some tunnel-stern boats, but I don't recall if he was inspired by the
                        Atkin designs. Here are some Atkin tunnel-stern Seabrights:

                        http://www.boat-links.com/Atkinco/Utilities/Everhope.html

                        http://www.boat-links.com/Atkinco/Utilities/ShoalsRunner.html

                        http://www.boat-links.com/Atkinco/Cruisers/NanukIII.html

                        And a non-tunnel-stern V-bottom Seabright skiff:

                        http://www.boat-links.com/Atkinco/Cruisers/NanukIII.html

                        On Sun, 4 Jan 2004 12:51:27 -0800 (PST), Don Tyson wrote:
                        > Jeff, I could be wrong but I think Dave Gerr ( Well known east coast
                        designer/author) did some designs with this concept which is essentially the
                        same as the historic Seabright Skiff. The seabright skiff is uniqe in that
                        it was launched from unprotected sand beaches directly into the rough
                        Atlantic off the NJ coast. Having the prop up in like that alowed the hulls
                        to be dragged up the beach at the end of the day. As long as the two bladed
                        prop was horizontal dragging the hulls didn't cause any damage.
                        > ...

                        --
                        John <jkohnen@...>
                        http://www.boat-links.com/
                        In this world of sin and sorrow there is always something to be thankful
                        for; as for me, I rejoice that I am not a Republican. <H. L. Mencken>
                      • hal
                        ... The efficiency of a jet drive is related to the diameter of the nozzle, among other things. A high speed jet will have a -small- diameter nozzle to
                        Message 11 of 13 , Jan 5, 2004
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                          On Jan 3, 2004, at 4:49 PM, Hugo Tyson wrote:

                          > Jet drives are best at speeds above 25 Knots where they are more
                          > efficient than propellers. Speeds under 25 Kts the're really
                          > inefficient as the motor is still working relatively hard to drive
                          > them.

                          The efficiency of a jet drive is related to the diameter
                          of the nozzle, among other things. A high speed jet
                          will have a -small- diameter nozzle to increase the
                          velocity of the stream, and will be inefficient at low
                          speeds because not much water will be flowing. A low
                          speed jet will have a -large- diameter nozzle so a lot
                          of water will flow at low speeds, but it won't go fast.

                          Most commercial jet makers allow you to choose nozzle
                          diameter based partly on the weight of the boat and
                          desired speed. And of course the power of the engine.

                          hal
                        • hal
                          ... I believe the inefficiencies of outboard jets are due to the type of pump used. Outboards us a centrifugal pump, where inboard jets such as berkeley,
                          Message 12 of 13 , Jan 5, 2004
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                            On Jan 3, 2004, at 4:35 PM, someone wrote:
                            >
                            > Seriously, how much power do you need? From what I've read about
                            > outboard jets, they are really inefficient... as I remember, they are
                            > about as effective as a screw-driven outbaard one-third the size. Max,
                            > correct me please if I'm misremembering. Of course, with that goes 3X
                            > the fuel consumption for a given thrust, Compared to a prop.

                            I believe the inefficiencies of outboard jets are due
                            to the type of pump used. Outboards us a centrifugal
                            pump, where inboard jets such as berkeley, Panther, and
                            jetski/wave runners use axial flow pumps.

                            hal
                          • Sal's Dad
                            I know of two Atkins tunnel drives built recently, both documented extensively in MAIB: Robb White s Rescue Minor, which IIRC is about 20 x5 , with a small
                            Message 13 of 13 , Jan 5, 2004
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                              I know of two Atkins tunnel drives built recently, both documented
                              extensively in MAIB: Robb White's Rescue Minor, which IIRC is about 20'x5',
                              with a small deisel, and a 34' deisel cruiser built by Alex Hadden - took
                              more than 2 years, in a professional shop with 2-3 guys working. Both are
                              reported to be remarkable . But don't try to design one yourself, or to
                              modify a Sneakeasy to this shape.

                              The JetJon site is a bunch of guys who are actively experimenting with old
                              jetski drives, stuck into various hull shapes. I suspect that the Sneakeasy
                              would be perfect for this, perhaps with some slight modifications, or using
                              the "step sharpie" variation to limit ventilation (NOT 'cavitation'). Draft
                              might increase a couple inches, but still, a boat that would plane at 20mph
                              in less than 6".... For low speed, consider a pair of trolling motors,
                              foreward and aft, as "thrusters"

                              One of the big jet mfrs used to make a small unit, for up to 20hp, iirc. It
                              was inexpensive, for toys and projects. I seem to remember they sold it
                              with no warranty.

                              To get a couple inches less draft in my Diablo, I cut a notch, or "tunnel"
                              2-3" up into the transom. The transom was 20", now it's effectively 17",
                              which works fine with a short shaft 25. Doing it again, I'd cut much more
                              out, and raise the transom...

                              Best regards
                              Curtis
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