The OTW "module" uses a pump to empty and fill the ballast tank.

Model subs, just like the real ones, have a watertight and pressure resistant compartment (pressure hull) where all the stuff that hates water (motor, radio,  servos, etc.) lives. Most of us call the pressure hull a "WTC" or WaterTight Compartment / cylinder. The parts of the model that don't have to be dry are free–flooding, with holes or slots to allow the water in and out. Of course, you  need to get inside the pressure hull for assembly and servicing. Plus, the  mechanical connections to operate the rudders and diving planes need to get out. These openings in the WTC are typically sealed with O–rings and gaskets.

Jim Butt - the Bob Villa of WTCs!
PHOTO: Tail feathers of Jim's Alfa sub and linkages. WTC is fashioned from Schedule 40 PVC waste pipe found at any hardware superstore i.e., Lowe's, Home Depot, etc. 3" ID (inside diameter) pipe is about 3.5" outside. Plenty of room for electronics if you're careful.  You can also buy clear Lexan tubing from McMaster Carr.

This is the body or pressure hull (that's where the electronics live) of Jim's WTC. It ain't pretty but it works! Jim is a positive guy, but also a pragmatist. Note his offer for a reward if the boat is found. He doesn't mention what the reward is, however.  :-)

Ballast tank (L) and forward section of WTC (R) showing mounting brackets. Ballast tank blow valve is aClippard  #ET2-12 standard 2 way normally closed 12 volt solenoid.

Six volt models are also available, either one goes for about 35 bucks. Get it from a local Clippard dealer - go to their website www.clippard.com to find out if there is a dealer close to you. Jim makes his own Propel storage tanks from copper pipe - PCV just isn't meant for that kind of pressure.

Business end of the WTC showing universal dogbone connector. (Blue squares are part of the ESC showing through from the inside.) Exposed electrical connections are coated with RTV (room temperature vulcanizing) silicone (it's not that kind of silicone - if it were, there'd be lots more guys interested in RC submarines!) Be careful to test the brand you buy on a piece of scrap. Some silicones  will melt flotation foam and craze (tiny spiderweb cracking) the polycarbonate tubing, stick to shag carpeting, your cat Fluffy. . .  and to paraphrase Martha Stewart, "That's NOT a good thing!" I use Permatex clear I buy at the auto parts store.

Electronics are mounted on tray attached to end cap. Studs slide through holes in end cap and held in place by nuts. Drive train may also be clearly seen.

Stainless steel studs protrude through gasket cut from sheet rubber. Gasket material is .062 (1/16") red Buna rubber gasket sheeting fromMcMaster Carr Supply. (Leftovers can be used for lining your underwear for the first time your sub doesn't come up from a dive in murky water!) Studs are made by tapping threads into holes drilled in PVC pipe, screwing in bolts and then removing the heads with a Dremel cutoff wheel. It helps if you have a nut on the studs so after the screw's head is cut off, you can back the nuts off to refresh the threads. You can make a diagram for spacing out the studs by drawing up a circle the same size as your tubing on a simple computer drawing program, then use the program to space out dots where the studs should go so they wull be even. Otherwise you're going to have to put the lid on the same way each time.
To get even pressure on the gasket when tightening the nuts and reduce the chance of a leak, don't do them in order. Rather, go back and forth (1, 4, 2, 5, 3, 6) across the top, in a way similar to tightening a cylinder head on an engine.

Forward WTC clear endcap. Note "L" brackets to mount WTC into free–flooding hull. Jim left the screw heads on this one, since it doesn't ever need to come off.