ORIGINAL: n.h.schmidt
Hi That is a old MRP gearbox. They most often came in their Hydro boats and one of their Cats. With the orginal gears you got 1to 1 ratio but, with two motors. If Simon is right about the gear tooth count 30 pinion and 70 spur ,you will have 2.33 to 1 ratio . Thats a real high ratio for fast boats but can work. You will need hi reving motors to get it going .
His estimate of ratio is bang on.
It is a little highly geared but can do the job.
I guess that you are looking reduce this ratio.
one option would be to go to a good hobby shop and take a look at gear wheels that are used in RC cars, I know my lhs has a good supply of them.
Alternativly you could look for a replacement gearbox unit out of a Kyosho or Academy boat, I have both sorts in 3 of my boats and all are at 1.5 : 1 and can take one or two motors.
See how ya go !
Keep us posted.
Cheers.