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Not another Skipjack build...

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  • Not another Skipjack build...

    Hello all. First post since 2011. Since running DIY free-running model subs in the backyard pool as a kid. I've always in the back of my mind been interested in doing an RC sub. Recently, I saw the Moebius 1/72 Skipjack kit on sale on Amazon for $79 so I placed it on order and here I am. I've perused most other SJ builds for ideas and inspiration. I'm not new to RC, I've been involved with the RC tank and warship combat sides of the hobby for years now. So I know my way around custom RC systems work. I see that water tight cylinders are the norm nowadays. Since I'm hoping to use as much stuff out of my warship combat parts bin (and I don't have any acrylic tube in stock), I'm thinking of going a different route with this build. In warship combat, we use 150psi CO2 to power the bb cannons and we have various successful methods to waterproof servos and electronics. Most RC combat captains today no longer use waterproof enclosures for the electronics but instead waterproof each component. Since I've got all this stuff on the shelf, I'm thinking, why not build a sub warship combat style? I'll forego the WTC and use the CO2 system to move the ballast.

    So for this build, I'm contemplating:
    • Free flooding hull, no WTC
    • All electronic assemblies individually water-proofed
    • "Hardened" off-the-shelf waterproof servos
    • Direct drive 755 motor, open to the water (motor has Oilite bushings, no internal fan)
    • CO2 powered ballast system
    • 4oz paintball CO2 bottle with commercial off-the-shelf 150psi pressure regulator
    • 24v power with custom ESC
    • Warship combat trigger board to operate the ballast solenoid valves
    • Vex 75mhz radio

    Can any of the experienced sub captains shoot any big enough holes in this idea to convince me to go a more traditional route?


    Steve Tyng
    Rockville MD
    Last edited by SteveT44; 07-17-2020, 11:31 AM.

  • #2
    I would be delighted to shoot you an idea or two. Starting with:

    DON'T DO IT!

    You're welcome.

    The Horrible
    "... well, that takes care of Jorgenson's theory!"


    • #3
      Originally posted by He Who Shall Not Be Named View Post
      I would be delighted to shoot you an idea or two. Starting with:

      DON'T DO IT!

      You're welcome.

      The Horrible
      AS STATED!!


      "Firemencan stand the heat"


      • #4
        Originally posted by SteveT44 View Post
        Can any of the experienced sub captains shoot any big enough holes in this idea to convince me to go a more traditional route?
        Hi Steve,

        I'll agree with Hagar...sorry, I mean Dave. But only for the electronics and motor, I'll explain why. You've given us a great thought experiment, understandably not the first one of it's subject though.


        There is a reason more people die from electrocution in fresh water than salt. The very thing thing that you think helps us, it's greater insulation properties than salt or pool water, hurts as as well. Unless you diving in distill water, it will have some conductivity. The human body is more conductive than the surrounding fresh water, so electricity seeking the shortest path to ground, lets say around an ungrounded light/AC outlet around a lakes swim dock, will use said human body to find ground, killing the occupant.

        Now unless you can ensure EVERY piece of conductive material is 100% insulated from the water, that includes ALL connector pins, where insulation terminates covering the wires. edges of the electronics PC Boards (I design boards, I know people cheat by extending Cu on inner layers to the edge exposing the inner Cu . These boards aren't water rated), the motor casing, the brushes in the motor, the windings, the solder connections to the windings. Can you? You have potential human bodies all over the boat! (Probably not the nicest analogy, but you get the point.)

        Are you going to completely pot the RX? Fill it with goo even at the connections? How will you swap servo leads easily when you want/need to change something. Are they IP68 rated?

        Think service and replacement, much less simple maintenance.

        Trust me, I'm not saying NO. I just want you to understand your application, or need to do this.

        Now what will happen, either because you missed something, or normal wear and tear, stray currents will enter the water. Seeking the most direct path to ground, will begin to corrode other metals that are less noble, semiconductors and what not..

        You'll have quite a bit of maintenance on your hand after each run. Squirting WD40 or what not into the motor...a mess.

        So let's say you solve the above engineering issue, or are rich enough to do a complete electronics swap yearly,. NOT to be facetious, just want to guarantee your number of surfacing's equal the number of dives over this models lifetime.

        Here's a photo of something I THOUGHT I waterproofed, see the corrosion flag (green discoloration) on the positive leads (anode)? What's going on UNDER there, where you CAN'T see or readily inspect? I plan to replace this unit this coming off season. Look again carefully at this photo, see what looks like dust? More on that in a moment.

        Click image for larger version  Name:	anode.jpg Views:	0 Size:	36.0 KB ID:	141784

        The point is, water is the most relentless $#*! I to Dave.


        Contaminants in the water. Put aside saltwater, the above would happen probably without energizing the model. Most if not ALL fresh water bodies of water have junk floating in it. Particulates such as plant debris, organisms, silt, even before the molecular level. I know, I have to wash my boat every time I use it.
        Remember what I said about the "dust"? Look at the following.....

        Click image for larger version  Name:	Silt1.jpg Views:	0 Size:	59.3 KB ID:	141785

        Click image for larger version  Name:	silt3.jpg Views:	0 Size:	58.9 KB ID:	141786

        Thats sand, silt etc. It gets into everything! You want that in your motor windings, it's brushes?

        Ok...say you go brushless like a lot of folks say can run in water, go for it.

        BUT DON'T RUN IN GROTON!! Or any other lake with suspended ferrous material. This will jam between the rotor and the magnets!

        Click image for larger version  Name:	Silt2.jpg Views:	0 Size:	111.0 KB ID:	141787

        That's a rare earth magnet, with SAND sticking to it!! Also you can see the rust that was suspended in the water!

        That will get into the exposed motor, collect on your magnets, and never get out. Granted Groton is one place that I've seen this, but who knows where else you want to run?


        I do agree with the servos, I'm starting to keep them in the wet. They're cheap and you can waterproof them yourself. Bob shows his method on his YouTube Channel. If 3 $2 blue servos from China last a season, why not? I pay more for Propel in one season. The trick is ALL those electrical connections, 9 now for three servos. I did them individually here using 2mm Gold Bullet Connectors w/ Moisture proof Heat shrink and 1/8" Silicon tubing.

        Click image for larger version  Name:	ServoLeads.jpg Views:	0 Size:	42.1 KB ID:	141788

        I believe Bob sells EXCELLENT 3 conductor waterproof connectors. So in the future I won't need three individual thru-hull electrical connectors for each.

        I also do this with 3.5mm Bullet connectors on my power leads now avoiding the aforementioned corrosion. Connections >10A.

        Click image for larger version  Name:	PowerLeads.jpg Views:	0 Size:	53.0 KB ID:	141789

        Some folk leave the metal exposed and just add grease. I just loath corrosion of any sort and want to ensure a virgin contact between connectors limiting resistance. But that's me.

        So my "go forward" is to maintain a dry compartment for motors and electronics, yet keep the servo's in the wet.


        To prevent what I just discussed, and the freedom to locate the servos where I want throughout the hull in an optimum position for each. I can ALSO greatly limit the size of the Dry compartment, and locate that where I want. That's MY application.

        My design goal is:
        3-Easy access for maintenance and replacement.

        I always suggest to make that boat BULLET PROOF, make it run in ANYTHING. GO anywhere.

        Click image for larger version  Name:	Untitled.jpg Views:	0 Size:	86.9 KB ID:	141790
        Did I say organisms??

        I haven't yet, because some extra precautions would be needed, and trim would be greatly affected, but SHARK should be able to run in salt water.

        What's yours?

        Look, I dig what Dave is saying, even the way he states it because he doesn't want you to waste time in the long run

        A true submariner who won't mince words. Probably a lot more efficient than I did, that's why we love him!

        At least that's what I believe he means lol!!

        But in the end, if you DO want to try it, please go ahead and prove us wrong. Trust me I love seeing that!!

        Just don't ever discount experience, aka wisdom.

        I get enough of that "NO" myself with people saying my boats aren't true RC subs because I like 2.4Ghz. But at least I tried something to add to this hobby.
        I like what I do very well thank you.

        My only point is: Think it through, and ask yourself in the end will it meet your requirements?

        Is it worth it??

        Have fun storming the castle shipmate!

        "Sub" Ed

        PS: I see you live in MD, you should join us at SubFest 2020 min GA. I'm in NY and I'm going.
        DiveTribe SUBFEST 2020 - Cohutta, GA - Sept 18th to 20th, 2020
        Last edited by QuarterMaster; 07-17-2020, 05:17 PM.
        v/r "Sub" Ed

        Silent Service "Cold War" Veteran (The good years!)
        NEVER underestimate the power of a Sailor who served aboard a submarine.


        • #5
          I would not pass on the opportunity to go to the SubFest in Georgia if you can do it. Even if you do not have a boat. There will be subs there you can run and you will learn a ton in one day.
          As far as trying something new, I am all for it, but unfortunately many start few finish in this hobby.
          If you can cut, drill, saw, hit things and swear a lot, you're well on the way to building a working model sub.


          • #6
            I'll third the attendance at the event in GA. Regardless of whether you have a boat, you'll learn a ton and build a network of like-minded individuals, and have a TON of fun doing it.



            • #7
              Originally posted by SteveT44
              Time for some hull bonding.

              It's common knowledge that the hull sections of this kit are anything but round when they come out of the box. I wanted to to address this before gluing the two bottom sections together. I temporally hot glued this 124mm diameter half frame into the aft section to get the half circle I wanted. I didn't permanently bond this frame in place because I haven't decided on the final frame design yet.

              When I placed the aft section flat on a flat surface, and then placed the front section over it. I figured out that pressing down on the front section draws the piece into shape. To hold it in shape, I hot glued bits of scrap onto the bench. Here we see the bits holding the bow section tight and flat to the bench and giving a nice snug fit to the aft section at the mid point.

              The aft section is taped to the bench to hold it in position for the glue-up.

              Testor's red is spread on the inside of the joint of the bow piece...

              ... and then the bow is lowered over the aft piece and snapped into the holding pieces on the bench. A clamp is used to draw the sides in tight and a lead weight to gravity assist the clamping from above. This is left to cure overnight.

              The next morning, a single bottom piece. At this point I decided to see how accurate the glue-up was. I strung a line from bow to stern ...

              ... and found out the mid point is almost 1/8" off center, bummer.

              Investigating further, I marked lines on the bench and determined that the stern is almost 1/4" from where it should be. The red lines on this pic are where the hull sits naturally if the lead weight wasn't there. In hindsight, I should have marked a center line on the bench before the glue-up and positioned the hull pieces to that before bonding. To correct this, there is enough give in the assembly to force it into alignment. I'll probably glue some more positioning bits on the bench and then use a heat gun to gently coax the hull straight. I'm also planning to bond a length of aluminum angle into the bottom which will keep things straight (assuming I bond it accurately).

              That's it for now.

              Good, sound kit-assembly on display here. And the solid gold part of any such WIP is identifying problems, determining their cause, and coming up with a fix. Good stuff. Suggestion: make heating the plastic to conform should be you last, desperate method and done only if you have another kit in the stash. I would saw, buttress, and re-glue. But, that's me. Your mileage may very.

              "... well, that takes care of Jorgenson's theory!"


              • #8
                Originally posted by SteveT44

                I agree, heat is the last option. Before cutting, I think I'll try bending 1/4" past center and let it sit for a day or so and see what I get.
                To no avail. It will spring back. Break out the razor-saw, pal, and get to chopping.

                "... well, that takes care of Jorgenson's theory!"