The Hull

I went through the scales, and figured I'd do a 1:12 model... due to the cube power effect, it will have about 1/2 the displacement of a Dean's 1:10 DUKW, but just seems more manageable. It's still 31" long by 8" wide, should be about 14 lbs to simulate a full load.

Dimensions are scaled off the Italeri 1:35 DUKW, a nice kit by the way. Here's a sample of a quick CAD layout. No, I don't offer CAD files, and these are too sparse for use by others. Making a shareable and usable CAD drawing is as much work as building the model.

The Framework

I cut most pieces out of 3/16" plywood I had laying around. Junky stuff, but stiff, and I'll seal it up with resin. Wouldn't want this in a real boat. Sides are 1/64" aircraft plywood, so it could be bent around the bow. Thin, but wait there's more.
Not shown in the pictures yet, but after I sealed everything up with resin, I decided to laminate a skin of 0.015" styrene on the sides. Gives a better finish, and a nice base for gluing on other styrene bits. Sides are real strong now too with this composite lay-up.


DUKW hull, top side is coming later.


Underside. I cheated squared off the round prop tunnel, and as the axles will be centered, so are the drive shaft tunnels... the original is all off-center and funny.

Skinned


Sheathed overall in styrene, with basswood ribs. Would have used styrene for the ribs too, but the price was adding up for all those sticks! Already painted the plywood cargo bed olive.


Close up of my stand-in driver, Bull the Crash Test Dummy. It will be interesting to find a 1/12 quartermaster figure...
Another benefit of the thin styrene sheathing, not visible in these shots: You can use a pointy tool from behind to punch rivets, of which there are many on a DUKW.


All modules and hatches off, showing the interior.. pretty good access to all corners.
A Pittman 31:1 gearmotor with cog belt drives the axles' propshaft, which goes fore and aft into the two tunnels... obscured here by the propeller's propshaft, driven by a Speed 400 and an MPI 3.33:1 planetary gearbox. The motor needed to be in the bow, as there wasn't enough clearance under the cargo floor. An intermediate shaft with Dumas dogbone couplings at each end transmits the power.
The Pittman is in a simple split wood clamp; one screw holds it together. As the Pittman has an off-center output, I can rotate it in the clamp to tension the belt. The 31:1 ratio will give a nice scale road speed, if I use 1:1 bevel gears in the axles.
See the "POWER" section for updates on the main drive.

Underside: 1-3/4" prop at right, with the rudder disappearing into the shadows. Axles' propshaft poking into the two tunnels in the middle.

Displacement

I predicted 14 lbs. displacement based on:
19,570 lbs dead weight + 5000 lbs (2.5 ton) cargo = 24,570 lbs. Since displacement scales with volume, we divide the prototype weight by the linear scale factor cubed: 24,750 / (123) = 14.3 lbs.

Floated the hull in the bath, and loaded it to the top of the wheel arches; total was 10 lbs. This is about the lightest I'd want it.
Then loaded it to just submerge the 2nd rib below the gunwales... 14.5 lbs. Ta-dah!  Wouldn't want to load her down any more than that. The bare hull and motors weigh 3.5 lbs, so I have quite a bit of headroom for adding batteries, axles, and the final details. Oh- a little more margin- the tires are hollow and seem watertight, and six of them can hold up another 1/2 lb.

7.5 lbs for the nearly complete model, and 2.5 lbs of lead shot get me to the minimum displacement.

I wondered for a while how to stash the ballast. I'm always turning this dukw upside down, so the ballast needed to stay put whilst inverted. Didn't want to screw lead plates to the floor, but I finally realized that I had some convenient compartments built into the hull- just behind the rear wheel arch, and just forward of the front axle. As luck would have it, they accepted the requisite amount of lead shot perfectly, all I had to do was put a lid on each spot.


21 oz. of lead shot in the bow compartment...

and with a lid on it.

 

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