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The RPM Foxtrot by David Merriman

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  • The RPM Foxtrot by David Merriman

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    Last week I spent my time developing a purpose built Sub-driver (SD, some of you call them WTCs) for the Rick Palumbo 1/72 FOXTROT kit. This thing will be marketed through Caswell Inc. Rick's kit represents what I consider one of the most beautiful boats of the cold-war. The model is just the right size to cram in a full bridge and periscope-antenna array and deck items. A detail freaks dream boat!. Even so, this beast will not be so big as to be a storage, display, and transportation burden.

    Two things drove the design of this SD: the anticipated weight and displacement of the model's above waterline structure. And the need to keep the SD as short as possible in order to leave room for the inclusion of a six tube nest forward, and a four tube nest aft. I was able to achieve those requirements through use of a 2.5" Lexan cylinder -- big enough to accommodate a lot of gear in a short package, but presenting a cylinder small enough in diameter to keep the entire unit in the wet with the boat bobbing along in surfaced trim.

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    Atop is a completely outfitted, operational 2.5 FOXTROT SD. Below is a broken down FOXTROT SD, showing off the bulkheads, mounting hardware, seals, and fittings that goes into the manufacture of this Sub-driver.

    As is my practice, the SD is divided internally into three spaces: The forward dry space, relatively short, houses the mission switch and high-capacity Lithium-polymer battery; the middle space, forming the ballast tank, containing the emergency 'come-home' propellant gas bottle, conduit tube (that passes the power cable from the forward dry space to the after dry space), blow valve, and vent valve; and, finally, the after dry space where the propulsion and control devices are.

    A unique feature of the FOXTROT SD is the use of three 180 size, 7-volt, direct-drive motors, all mounted to the motor bulkhead. The FOXTROT, like some of her sisters, featured three main shafts! The starboard propellers is right-hand pitch (clockwise rotation, looking forward), the center propeller (behind the single rudder) is right-hand pitch. And the port propeller is left-hand pitch.

    Go figure...

    ... Russians!

    Other than the three-motors, the FOXTROT SD is pretty much a standard layout.

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    Neither Rick, nor I have put an evaluation FOXTROT hull in the water for trials, so right now it's a guess as to how effective differential operation of the port and starboard screws will be in turning the boat. My fear is that without differential use of the propellers, the FOXTROT model will turn as poorly as the German Type-21 that the FOXTROT was based on! Further, if the two outboard shafts are toed in, like the Type-21, then the yawing force will be opposite to that which is normal, or there won't be any appreciable yawing moment at all.

    So, to find out how effective differential throttle use will be on the boat's turning rate, I produced an evaluation motor bulkhead that has all three motors in parallel (starboard motor turning clockwise, center motor turning clockwise, and the port motor turning counterclockwise. These motors will be driven through one common ESC. This is the baseline arrangement. I get this one to test.

    A second evaluation bulkhead has the starboard and center motors wired together in parallel, and the port motor wired alone. Two ESCs will permit differential use of the starboard (along with the center shaft) and port shafts. We'll see if differential use of the propellers will effect useful yawing forces. Rick gets that one to test.

    The running gear is simple: big motors, directly coupled to the propellers through universal joints. The Germans taught me this about matching DC motors to propellers: "Keep the can size of the motor equal to or larger than the diameter of the propeller, and you can direct-drive things without fear of over-heating".

    Nobody does this game better than those Germans. Nobody!

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    The three 180 sized motors took up so much area on the motor bulkhead that I was forced to employ threaded studs as stand-offs to support the aluminum device securing platforms and bulkheads. Here you see an early version of the bulkheads and platforms within a cylinder, and a motor bulkhead with the three studs installed just forward of that cylinder.

    Note that I glue three micro-sized servos side-by-side. These will later be secured with a single brass strap to the device platform. Took about two days of trial and error to get an arrangement of bulkheads, and platform to fit all the devices in a tight and coherent arrangement.

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    And here is a motor bulkhead with all devices installed, and operational. Thank goodness for the Sombra Labs 8-channel receiver! It's not only small, but so selective and noise immune that I can put it in to what has to be the most terrible of RF environments: Pushed up close to three screaming propulsion motors (see the spark suppression capacitors on the ass-end of the cans?), and in very close proximity to servos, and other devices that squirt out RF noises of their own. Yet, when I turn on the transmitter and receiver, everything works with out so much as a solitary glitch!

    The guy who designed and markets that receiver should get a Nobel Prize!

    In this shot the motor bulkhead is upside down. Aft is the Caswell-Merriman ***/Snort ballast sub-system -- used to empty the ballast tank, Kevin McLeod's fantastic little ADF (angle-keeper and fail-safe) -- the new version is smaller and so adaptive that you can mount it in any orientation within the SD because it is programmed to sense the gravity line, regardless of the device's orientation within the model submarine.

    (Kevin also designed and markets the Lipo-guard, low voltage detector, needed to protect the Lithium-polymer battery from damage as it approaches the critical low-voltage state).

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    Next to the ADF is the bullet-proof little MTronics' ESC. Though small, it has the capacity to keep current going to the nearly stalled motors without breaking into a sweat.

    At the after end of the after bulkhead you see the green circuit-board of Kevin McLeod's little *** controller.

    The white wire making up to the forward face of the motor bulkhead is the receiver antenna. The lug it mounts to extends through the bulkhead to the wet face of the motor bulkhead where the external antenna makes up to it.


    The battery cable connects to the battery and switch after it passes through the conduit tube within the ballast tank. The cable makes up, in parallel, to the ESC, Lipo-Guard, and *** motor controller. The ESC has a BEC that feeds the receiver bus, that powers up all the devices connected to the receiver.

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    Looking down on the top of the motor bulkhead, the red thing is Kevin's Lipo-guard battery protection circuit, and under it is the Sombra Labs 8-channel, synthetic crystal receiver -- the brains of the SD! Just behind the motors, and strapped to the aluminum platform, are the three mini servos that operate the stern planes, bow planes, and rudder.

    Pretty tight packaging, huh?

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    The device mounting is pretty sophisticated! Three motor bulkheads here and the aluminum bulkheads, platform, and brass strap that secures the devices in place tightly, but with good access for adjustment, repair, and or replacement. There is a method to the madness!

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    Use of production templates, holding fixtures, and jigs insures speed of manufacture and uniformity of part fit.

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    A snorkel induction tube atop the FOXTROT SD will run up into the after fairing atop the FOXTROT's sail. Induction air will be pulled from atop the sail as it broaching the surface. That air, pushed into the ballast tank by the ***, empties it, pushing the boat to surfaced trim. The on-board bottle, with its charge of liquefied gas, is only used in emergency -- when, for whatever reason, the boat can not broach the sail. The emergency blow ballast sub-system is either actuated remotely, through the transmitter, or locally, through Kevin's missing pulse detector -- an element of the ADF device.

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    As tightly as I packaged things in the FOXTROT SD's after dry space, I still left a two inch space between the devices and the after ballast bulkhead. This space for 'future growth'. Who knows?

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    David D Merriman III
    Last edited by Outrider; 02-27-2011, 08:34 AM. Reason: Punctuation & clarity.
    Stop messing about - just get a Sub-driver!

  • #2
    More work on The Foxtrot

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    Mr Merriman cuts the hull at the waterline.

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    Note how the Sub-driver sites below the waterline.
    Attached Files
    Stop messing about - just get a Sub-driver!

    Comment


    • #3
      The jig for the dremel and the straps and wedges that hold the hull in place seem to produce a very nice cut...

      But I gotta ask... Why don't you just manufacture the hulls to split where they should for practical R/C ops? The plastic model industry isn't interested in Z-cuts and the like, but they're pretty much essential for R/C use. For a boat like this Foxtrot, which seems intended for R/C use, why not manufacture in the cut, even if it has to be a four-piece hull instead of a two-piece?

      Begin evasive maneuvers, disco-balls on, pop smokes, chaff, and flares!
      Last edited by Outrider; 03-23-2011, 06:18 AM.

      Comment


      • #4
        LOL! No need to hide, Jim. Your observation about providing that split, at the waterline, in the production parts has already been addressed.

        All that work I did was to turn a pre-production set of hull parts Rick Palumbo provided into a proper r/c model submarine -- those parts did not at all reflect what the production kit parts will look like. In fact, as you suggested, Jim, the FOXTROT hull parts will arrive to the customer split at the waterline -- no need for the customer to commit such atrocious acts of savagery with a cut-off wheel as I demonstrated in my recent postings.

        Great minds think alike, Jim.


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        David,
        Last edited by He Who Shall Not Be Named; 03-22-2011, 11:14 PM.
        Resident Luddite

        Comment


        • #5
          Regarding the 3 motors driving individual props, wouldn't it be simpler to just use 2 motor and power just the outboard props and leave the center prop freewheeling? Or just use a single motor and just power the center prop although a single prop might have difficulty moving the sub.
          Make it simple, make strong, make it work!

          Comment


          • #6
            Because we dont like to do anything simple and easy! Where's the fun in that?

            Its not a big deal all 3 motors working, and its the right way to do it.

            Comment


            • #7
              Any idea on cost for the boat and the WTC? I love the Rissian boats and the fact that most of them have no seperate rudders to run linkage to.
              Last edited by Tugfan; 03-23-2011, 12:27 PM.

              Comment


              • #8
                Originally posted by redboat219 View Post
                Regarding the 3 motors driving individual props, wouldn't it be simpler to just use 2 motor and power just the outboard props and leave the center prop freewheeling? Or just use a single motor and just power the center prop although a single prop might have difficulty moving the sub.
                I was going to say its a free world do what you like, but then i realised we were talking about a communist icon. So well err, you know err do what you like, within acepted boundries..
                Next time someone points out it takes 42 muscles to frown, point out it will only take 4 muscles to b1tch slap them if they tell you how mnay muscles you need to smile:pop

                Comment


                • #9
                  Would be great if this could be convert to a Golf class missile boat. Imagine firing some small rockets from the sail while at the pond.
                  Make it simple, make strong, make it work!

                  Comment


                  • #10
                    Originally posted by redboat219 View Post
                    Regarding the 3 motors driving individual props, wouldn't it be simpler to just use 2 motor and power just the outboard props and leave the center prop freewheeling? Or just use a single motor and just power the center prop although a single prop might have difficulty moving the sub.
                    Finish your damned KILO!

                    David,
                    Resident Luddite

                    Comment


                    • #11
                      Hey Dave,
                      Can the 2.0 subdriver work in your Skipjack? I really don't want to buy another one for the Skipjack, (if it ever comes back out) :(.

                      Comment


                      • #12
                        Originally posted by Tugfan View Post
                        Hey Dave,
                        Can the 2.0 subdriver work in your Skipjack? I really don't want to buy another one for the Skipjack, (if it ever comes back out) :(.
                        No! ...

                        ... you cheap-ass, SOB!

                        Spring for the 2.5 SD and do it right.

                        You wouldn't put one of those toy wind-up motors in your little r/c tank, would ya?! Hugh! .... Hugh!?

                        David,
                        Resident Luddite

                        Comment


                        • #13
                          I beg forgiveness. What could I be thinking?

                          Comment


                          • #14
                            Hurumph!....
                            Resident Luddite

                            Comment


                            • #15
                              Originally posted by Merriman View Post
                              No! ...

                              ... you cheap-ass, SOB!

                              Spring for the 2.5 SD and do it right.

                              You wouldn't put one of those toy wind-up motors in your little r/c tank, would ya?! Hugh! .... Hugh!?

                              David,
                              Oh! Gawd! Who let HIM out?

                              ROSE!!!!!! How did he get out? Did you give him his tablets today? Tempt him back into the cage with some raw liver, and put the tranquilizers in his water bowl!

                              Thanks Rose!
                              Stop messing about - just get a Sub-driver!

                              Comment

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