I was running my Nikko sub in my mother's pool tonight and I went ahead and did the rear fin mod by cutting off the upswept fins with a coping saw. I then found out one of the reasons for these fins: They keep the prop underwater when running on the surface. I corrected for this by adding two extra small weights to the rear compartment of the sub (where the prop/driveshaft is located). This kept the prop just below the surface just about all the time. Surface speed was definitely increased. "Porpoising" while diving was reduced somewhat.

My next plans are to replace the prop with something larger and possibly modify the fins on the sail to be angled down. The idea would be to give the sub a tendency to dive as soon as these fin are submerged. I would like to make them adjustable to find the best angle.

A much more ambitious mod is to replace the radio system with a Traxxas TQ 3 channel. This would keep proportional speed and steering (yes I would need an ESC) and the third channel can be modified to work a single speed reversible switch/speed control. Range and response time should be improved.