World's smallest RC sub?

Radio Control Boat Modeler, Dec 2001 by Revel, Guy

You have, no doubt, seen the radio-- controlled submarines that make the rounds of the model shows and scale competitions; many of them have also appeared an the pages of this magazine. These subs are impressive feats of engineering; they dive by filling water ballasts and navigate under water using independent motors and propellers. But if you want all of that in a package small enough to be hidden in the palm of your hand, you'll have to travel to Munich, Germany, because that's where Robert Kuhlewein specializes in miniature RC boats. The miniature masterpiece you see here is a practical model, but it's best suited to navigate in an aquarium; running it in a larger body of water would risk its loss.

What Robert has done is absolutely incredible. Reducing the weight of a model is not much of a problem, but he also reduced its size so that its inside diameter is barely larger than the three 35mAh button Ni-Cd cells (the smallest available) that power it. It is a tribute to Robert's incredible engineering capability that, in addition to the battery pack, it accommodates a receiver, two proportional servos, two motors and a working water-ballast system that enables it to dive at any speed. Some of the electronics are modified production pieces; others had to be handmade. The receiver is a stripped-down version from Becker, which is a German manufacturer that specializes in miniature equipment for small model aircraft. The two miniature servos are based on the 2.4-gram WES-Technik units that are popular with indoor RC planes, but they have been rewired to save precious space. The speed controllers are homemade; two provide directional control via the transmitter's mixing function through the differential command of the motors.

The servos are used for diving and underwater maneuvers but are used in two ways: one moves the tiny aft dive plane that's between the two propellers; the other changes the sub's static buoyancy by pumping water into or out of the ballast tank. This is where Robert really displayed his creativity, as there is no "pump" in the usual sense of the term; instead, he adjusts the volume of the ballast tank, which is nothing more than a short length of rubber hose. In this ingenious design, the servo alters the length of the hose using a set of tiny gears, as the servo torque alone is not enough to compress the rubber.

All the equipment is epoxied onto a hand-cut board and pushed into an aluminum tube. An O-ring seals the joint between the front and rear sections. The submarine's exterior was carved out of balsa and carefully sealed and painted.

The completed submarine weighs a mere 58 grams (2.1 ounces), and it is 160mm (6 inches) long; its beam is tiny at 25mm (1 inch). While this is surely an extreme example of miniaturization, you can bet that it will be only a matter of time before someone makes an even smaller one.