As seen above, the basic concept is a pair of ladders made from prop shaft tubing. The center cross members are very accurately matched in length and are carefully clamped in place before the soldering so that they remain parallel. If the gears bind you can heat the joints up and put a bit of pressure on the tubes. This puts just a bit of space in the joint and works very nicely as an adjustment. Using four gears gives counter rotation to the props. In this case I did not try for reduction though that would be pretty simple to do. I chose to drive the gears from behind to clear the space forward of the shafts for the rear cannon.
The main ladder was completed before it was mounted in the hull. I would recommend the whole thing be done before mounting as it's a pain to do in place accurately. I am using 30 tooth gears from the cars as they are easily available. There was a lot of thought about doing all the shafts on a single plane. The more I think about that idea the better I like it.
[01/13/98] - Warning!
This is a prototype design. It has been on the water a few times and
looks pretty impressive but sounds like an egg beater. A crusier hull was
driven by a 7.2v battery at about a 8 sec 50 foot course. After failing to
find a motor that matches the ship, I have removed the gears from the
ship. The usual motors used in the hobby are not quite strong enough and
the car motors are too strong. I still believe the concept is valid and
will probably look very hard at it again when I start building smaller
ships again.