I am currently upgrading and tweaking an old RC boat I have had for about 17 years now. Its a Graupner Hydrospeed Cobra. I started with a graupner speed 500 race motor and upgraded it to a 600. I upgraded the stock 42mm prop with 1.4 pitch to a 42 mm with 1.6 pitch and then upgraded the battery packs from 7.2v 6 cells to 9.6v 8 cells. I also implemented a water pickup and coil for liquid cooling to beat the heat. I run one battery pack at a time and the speed is quite good, but i still want more. the speed controller in the boat is an old Cherry Electronics microswitch which has connectors for two battery packs. the way i believe it was supposed to work was with two 4.8v packs in series. When the boat is at half throttle only one pack would be tapped into, but at full throttle it would open up on both packs. Reverse would only draw from one pack. As i have never been able to find these packs I have never run it in that configuration. So i have only been using one pack, making half throttle on the controller equal to full power. If i push the control stick to far, the power cuts out as there is no second pack to draw from. I would like to run more voltage to the engine so as to obtain more speed. I am pretty much out of room inside the boat for engine upgrades, so a bigger can is out of the question. i fiddled around with hooking up four AA 2100 mah NiMh rechargeable batteries in a holder i bought from radio shack. I hooked that to one of the battery connectors on the speed control and my 9.6v pack to the other. this resulted in a little puff of smoke and a melted battery holder. Hind sight is 20/20 and i can now see the error of my thinking. I have pondered hooking up two 9.6v packs, but i can only imagine that 19.2 volts would be far too much for the motor (best spec i can find for the speed 600 7.2v motor says max 12 volts). the speed control indicates it can handle 10.1 amps and 125 volts, so i am not so concerned about it. it might be old, but it works fabulously. the question (as i am sure you have all been waiting for) is: Does anyone know of any kind of battery isolation circuit or diode that i could put between the speed control and my 4 AA battery pack that would prevent the other pack from overloading this smaller "homemade" pack? If such a thing exists, i could then run the small pack for slow forward and revers, and both packs for high-speed forward. I have thought about taking apart my 9.6v packs and making my own 12 cell pack, but i'd rather not go that route yet, especially if a simpler solution exists. Thanks! ps - if anyone can think of other ways to increase speed please do not hesitate.