Decided to do something other than a boat this time, but still with a nautical purpose,- a 6 wheel twin diff twin prop swimming truck. Been done before, but I thought I'd have a go at one of my own design. Drew up some side and end templates and made a start, Purchased all the mechanical bits, (diffs, front axle, prop shafts, props, ESC, wheels,) over a few months from uncle ALI and added them to stuff I had in my collection.

Using cheap 3mm packing ply, I made the floor base ( to suit the diff and front axle widths) and strengthened it with 10x10 ally angle. Next,-cut out the sides and front and back panels. The floor was scored at the front and back to allow it to be bent up to the sides for gluing. Holes were cut in the floor to allow the diff heads to protrude through.

I had to modify the front axle to suit the width of the diffs by adding in an ally angle center section. Also had a play with setting the diffs and drive-shafts up with universals. Drive motor is a 380 1500 RPM @ 12v geared reduction drive, which is the one I first used in my Jeep tow wagon for my Hartley, (replaced it with a 1000 RPM version for more torque) and uses a 3s LiPo and Quicrun 40A ESC.

Knees made for the floor ends and floor ends bent up and epoxied. Ally frame and running gear temporarily in place.

Making the inner guards (before attaching the sides) and making an alteration to the ally frame to make fitting the front axle easier. Offsetting the inner guards to let the sides in over the bottom floor. A large 5mm ply plate was glued to the bottom (pic 3&4) to mount the front axle on, rather than the inner frame. Raises the front slightly, taking the load off the first diff to make steering easier (both diffs fully driving would want to push it straight ahead, as they are not proper differentials, just fixed axles)

Found some plastic garden irrigation pipe saddles at the big hardware, which were exactly the right size for the diffs, so had a play fitting them at this stage, Also made a box for the diff heads to sit up in. While doing these jobs, I'm constantly working out what will go where next, and making sure everything will be accessible later (even certain screws. This is another of my 'chainsaw as you go' projects, which usually include many remodels and changes to make things work out.

The sides are now glued on using epoxy with a filler powder to make it a glue, seals really well and is a good gap filler. Also bored some rough holes for the prop shafts, and glued on some flow directors for the props.

Hull front and rear panels glued on, and hull glassed to the waterline. Remainder of resin applied to the exterior topsides above the waterline at the same time.

The hull was sealed with a timber preservative thin resin inside. Screen surround and roof then fitted and removable rear screen made. all now painted with a sealer, primer and topcoat. Shafts were epoxied in at this stage, complete with clamp on oilers.

[ Clamps were made earlier while waiting for F/G to dry,- turned and bored for a press fit on the shaft tubes, then slit, flats milled and holes drilled and tapped for 3mm machine screws for clamping.]

Holes were drilled in the shaft tubes and the nylon filler tube holders located over them and clamped on. Shafts and tubes had to be shortened as well (bearings tapped out, tubes cut, bearings replaced).

The large hole in the front of the diff cover box is for the driveshaft seal.

ADDITION -

Forgot to add these pics of the buoyancy test with full weight on board, after sealing and glassing the hull. Taped up all the bolt holes etc and chucked it in the big blue clam, floated well, no leaks except for a few drops around the tape.

Rudders were fitted (std stuff and easy to get at) extra blocks glued to their bases for strength. Rudders themselves were made alongside other jobs (painting etc) so they were ready to fit. Arms are soldered to aircraft wheel collars. All holes for the equipment were drilled before glassing, and just needed re-drilling through the glass and parts bolting in place.

Trial fitted the swim motors ( 28mm 2200kv out-runners) on their ply brackets to check for alignment, (and made a few adjustments,) and found suitable positions for the twin 20A TrackStar ESCs . (excellent little ESC with a fully adjustable programme.

Also fitted the wheel steering servo on the pre set nylon bolts (servo screws to drilled blocks which slide down the bolts and are secured with nylon nuts, - see bolts pic 3 previous post) The main drive motor was also trial bolted back in place. Props and shafts were re fitted to check for clearance. A bit of colour was added as well. Next to be fitted will be the running gear and wheel steering linkage.

Makes things a lot easier you trial fit each item in its' position as you go along, as it's easier to work on an open area. If everything is pre fitted, and access to every bolt/screw is planned for, it only takes a short time to bolt everything back on after painting or glassing. Much better than trying to drill holes later in confined spaces. Even if you have to drill a hole above a difficult to access screw (which may need removing later) and disguise the hole (I've used punched discs of masking tape,- painted over), it can make life so much easier later .

Fitted the diffs/driveshafts (both pre filled with marine outboard grease and sealed) with the plastic saddles and marine silicone to hold them still. The diffs have each been fitted with an air pressure tube in the tops for a future pressurising system (tube can be seen in pic 5). Ply posts for the water drive ESCs arming switches epoxied in place as well. A link bar was also made for the tops of the diffs to stop them rotating under drive torque.

I've included some belated pics here to show various attachments better. These were all tested in position before painting and sealing the hull, then all removed for the next stages of work.

Swim motors, main drive/gearbox, wheel steering servo with waterproof boot, driveshaft seal, rudder servo and now diff pressurising system fitted.

The pressure is provided from air from a tiny air pump sitting behind the left rear wheel arch , which is linked to a brass tube manifold in the diff cover box by silicone hose. The 2 diffs are linked to the manifold tube which has an 'open to atmosphere' tube extending into the box. This pressurises the box and provides a small amount of constant positive pressure to the diffs to keep water out, (they are completely packed with marine grease and all seams sealed, but that's no guarantee water won't get in, hence the pressurisation)

A Perspex cover was made to fit the box and screwed down with a silicone sealer. (as it turned out in the water test, no water enters the box, and the tops of the diffs don't even get wet (sort of diving bell effect). Excess pressure just bubbles out under the hull. A small waterproof plastic box was used to house the RX in the cab, and screwed through the bottom to a ply plate glued to the front hull knees, with a slot near the lip just big enough to push the servo plugs through. The truck won't be going out in the 'open sea', so it should be sufficient.

A few lights being added (spots don't work as the polyester resin I poured in killed them, don't think LEDs like MEKP.) Flashing top light being made.

Windows in and tinted (PETG with car tint applied.) Front screen removable for access. Also made up a thermostatically controlled fan for the motor, as I found it got a bit hot under heavy going on a long grass test in the back yard. It uses a 55C thermo switch which is strapped to the motor, and in the power circuit for the fan. Fan is 3v and run from a small voltage reg, with power coming from the main 3s drive battery. All mounted on a ply shelf which attaches over the motor with 2 Neo magnets. (shelf shown unpainted).

Almost ready here for the big actual 'sea trials'.