Fast electrics: The brushlss revolution

Radio Control Boat Modeler, Jun 2003 by Turner, Jay

For decades, fast electric boats have taken advantage of advances in RC car technology. Our radios, batteries and motors are generally identical to, or slightly modified versions of products designed for cars. This gives us a wide variety of products to choose from, and thanks to high sales volumes in the car industry, the prices of these products are now quite low. Though boaters often demand more from these products than car racers do, we have been able to get by pretty well.

Until now, the performance of fast electric boats has improved in direct proportion to advances in battery technology. We've also seen a continued development of better speed controls, but the lack of progress in brushed-motor design has caused performance to plateau. Brushed motors are really simple; mechanical brushes act as switches to carry the battery's current to the motor's wire coils and to turn the motor poles on and off. They function well in these roles, but the more power we require from the motor, the more those same simple brushes resist current and place mechanical drag on the motor armature. Together, these shortcomings reduce the brushed motor's efficiency, and given the fixed amount of power available from any one battery, a lower efficiency means less power going to the propeller.

Enter the brushless motor. Brushless-motor design has been used for decades in industrial and military applications, and today, it can be found in everyday items such as computer hard drives. Still, truly consumer-available brushless motors have only recently begun to play a role in the RC hobby. Model airplane enthusiasts were the first to reap the benefits of brushless technology, but the fast-electric boating community is quickly following.

ADVANTAGES

Instead of using brushes to serve as switches and carry the current through the motor, brushless motors use a special speed control to perform those functions. (You cannot run a brushless motor with a speed control designed for a brushed motor.) Recent advances in microprocessor and transistor technologies allow brushless controllers to operate much more efficiently, and they now exhibit considerably less electrical resistance than motors with brushes. Without the frictional drag of the brushes on the commutator, there is a net gain in efficiency that translates into more power.

Another advantage of brushless motors can be attributed to their design. In simple terms, a brushed motor has a wire-wrapped armature spinning inside two magnets held within a steel can. The brushes transmit the current to the spinning armature (arm), and their contact with the arm is a major source of power loss. In contrast, the brushless motor is built inside-out. The arm is a simple steel shaft (with the magnets attached) spinning in two ball bearings; all the wire is in the motor can. This means that there can be a permanent connection between the ESC and the motor windings; brushes aren't needed. This also means that it is possible to use more wire of a larger diameter in the brushless motor for less electrical resistance. Combined, these design features mean more efficient motors, especially at very high power levels. In addition, because there are no brushes to wear, brushless motors require hardly any maintenance and will run just as well after dozens of high-power races as they did when they were brand-new.

SETTING THE STANDARD

The first brushless motors to make inroads into the fast electric boating community were the Aveox motors. Basically identical to the motors/ESCs used in RC planes, they provided high power levels and required virtually no maintenance when used in 12-cell race boats.

At first, attempts to adapt the brushless RC airplane motors to accommodate the much higher amp draw required by RC boats caused some problems, but the necessary modifications were made and the Aveox-powered 12-cell boats showed a real power advantage on racecourses. This power advantage didn't go unnoticed by top-level racers, and every major fast-electric race saw an increasing number of brushless-powered racers in the winners' circle.

Other manufacturers of brushless RC airplane motors also took notice of the success of the Aveox racers. Hacker and Lehner, both in Germany, quickly began to produce boat-specific speed controls when racers began to use their products in various cell-- count classes. At the 2001 World's Straightaway (SAW) Record Trials in Los Angeles, the brushless-powered boats really showed their speed by setting hydro records that were close to 90mph. The best speeds that brushed-motor technology could achieve with similar setups were in the low 70s. In 2002, a brushless-powered 32-cell outrigger exceeded 100mph, and the limits of brushless performance are still unknown.

WHICH BRUSHLESS SYSTEM IS RIGHT FOR YOUR BOAT?

Don't read what the record-setters at the latest SAW trials used in their boats and put the same combo in yours. In most instances, you will overheat your ESC and perhaps permanently damage your battery cells. The market is changing as newer ESCs are introduced, so ask the sellers of the systems what they recommend for your specific application. I've seen the following two combinations run very well for top-level oval racing; these setups can be used for sport boating by using smaller props.