A radio-controlled boat is a boat controlled remotely with radio control equipment. Sport boats are the most common type of boat amongst hobbyists.
Scale boats are replicas of full-size boats. They can be small enough to fit into your hand, or large, trailer-transported models weighing hundreds of pounds. More often than not they are a miniaturized version of a prototype, built using plans and/or photos, although there are variants of the hobby that utilize freelance designs. Another offshoot of this hobby is radio-controlled submarines.
RC sailboats use the power of the wind acting on sails to propel the boat. Model sailboats are typically controlled via a multi-channel radio transmitter in the hands of the operator with a corresponding receiver in the boat. By changing the position of the two joysticks on the transmitter signals are sent over two separate channels on a single radio frequency (assigned to the individual boat/operator). On the boat, the radio receiver is connected to two battery powered electric motors or servos. Signals from the radio transmitter are interpreted by the radio receiver and translated into instructions to change the position of the servos. One servo controls the position of both main and jib sails together (allowing the sails to be trimmed), the other the position of the rudder (allowing the boat to be steered).
Power boats are typically electric or internal combustion, (ignition engine or glow plug R/C engine based) and some are steam powered (conventional type, and also flash steam). (At one time some boats used engines working on the compression ignition principle. These were not diesels in the true sense of the word but the modelling fraternity frequently referred to them as such. A few enthusiasts still operate such engines.) The power is commonly used to rotate a submerged propeller, aircraft propeller or jet which in turn provide the thrust to move the craft. Typically power boats have two controls, rudder, outboard motor or stern drive and throttle control. Powered scale boats will often have additional remote controlled functions to improve realism, e.g. sounding fog horns, rotating radar antennae etc. Some of the more sophisticated powered racing boats may also have additional remote controlled functions. These may include remote mixture control allowing the driver to optimise the fuel/air mixture during a race. Another function occasionally implemented for racing boats using a surface piercing propellor is remote control of depth or angle of thrust. There are three main types of power boat. RTR(ready-to-run), ARTR(almost-ready-to-run), and kit versions are available. All thoroughbred racing boats are made from kits and the builders add their own gear and radio.
Radio controlled racing boats are designed for maximum speed and maneuverability. Various syles of racing include circuits of different shapes laid out on the water with bouys. Some enthusiasts race in the sea controlling their craft from a pursuing boat known as a "chase boat". These courses will usually be a few miles long and the competition is judged against the clock to find the fastest in class. Within the various styles of racing there will be a number of classes depending upon engine size and type.
What's it worth? Take a look at this Rc Boat price guide: sold listings for a value indication.