This section deals with the installation of the 12v 750ml dive tank.

I am using two 7.2v NiMh battery packs wired in series for 14.4v total. I am not using a battery eliminator circuit, but am rather using the stock 4-cell NiCad for the receiver. The boat runs about 45 min on a charge and is very fast in the water. The Johnson 540 motors get pretty hot by the end of the run and I will put in one of those water cooling systems if I'm forced to replace these should they burn out. I will probably wrap the motors in thin metal tubing. One of their ends will connect to the free flooding area and the others will empty out around the props. This will use the Venturi effect of the prop wash to draw water around the motors.

The 750ml dive tank is big enough, in my opinion. The weight of water that the tank displaces is equal to the amount of weight you can float above the waterline. Proper trim at full flood should have just a sliver of your conning tower out of the water. Unless you're sitting in a glassy pool, I don't think you'd know the boat was sitting low in the water. Here is how mine is trimmed. The lead weights you see in the pictures were used during the trimming process. They were finally located in the bottom of the hull with velcro so they can be repositioned easily.

Here is how it is trimmed at full flood.

 

The tank is set at an angle, because the rod is too long and in the dead middle would hit the screw joiner. I am using some brass tube which will be slightly greased to protect the surrounding electronics and wires from the motion of the rod. This is essential to "packing it all in" later, which is how I get 6 amp hours on board.
The brass tube was 15/32. (just because it was lying around) I pulled the switches and marked the square plate which I later had to trim to be even with the axis of the entire tank. I also had to shear off a corner of the square plate later to allow the battery pack on that side to fit in.
Here it is just before I soldered it. The plate is drilled and bolted down. The tube has a little magic marker tic mark on it, because I had to pull it back from the brass gear a bit or it would interfere with the position sensor microswitch. There are two high tech thermal resistant spacers (tin foil) on the bolt so it's all centered, and I have a thermal resistant clamp holding it all together. (aka my crescent wrench.) Not shown but of great importance was the thick blanket of probably six layers of tin foil that shielded the motor from the blowtorch. Truth be told, a wise person would tack it with ca, then put it in a vice for soldering and just let the ca burn away during the process. I wasn't that wise, but the tank survived. I was careful to lick it with the minimum of heat and then quickly cool the plate before the heat flowed to the plastic of the tank.
I had to trim the black spacers down for the thickness of the brass plate. Then somehow the switch was moved slightly up, so the flood stop plunger caught on the switch instead of sliding up it. A little piece of brass tubing glued into the center of it solved the problem. BTW I was tempted to photoshop out that solder drip, but hey, who would I be kidding? (If I had a modeling handle it would probably be Hack 'N Slash.)
Here's the other side showing how the tube had to stop short of the purge cutoff switch.
I didn't do very well estimating where the hole would go for the tank fill, so now I will have to Dremel out an ugly oblong hole for the tube. Oh well, I wasn't going to win any beauty contests anyway.
My tray is missing quite a bit from stock, but it only has to be strong enough to draw the boat halves together hard enough to slightly compress the main sealing ring. I doubled up some of the plastic to give it more tensile strength. The tank motor on the port side forced me to put a little block of lead on the starboard side, up front.
Here it is inverted.
Here is a closeup of the battery. This is why I had to clip a corner off the plate I added to the tank.

I will post more as I have time to work on the project.

Thanks, --Jason Overhulser