Apr, 2007 NOTE:

I the eight years since I wrote this article some things have changed. Hybrid vehicles were the first to use the 42-volt power standard and Mercedes and BMW will probably by first to use it in non-hybrids. MOSFET power transistors get better and less expensive every year so their cost has become less of an issue. And the Advanced Smartcontrol has been replaced by the SCO family of controls which are available with a 24 volt option.  

I'd like to start this page with a story about a model boater I encountered at a regatta a few weeks ago. He was using a typical direct drive set up with a 540 type 3 pole motor. His plastic fuse holder had turned into a molten lump of white plastic. Some of the more experienced modelers were explaining the consequences of putting 10 amps through a fuse holder rated for 3. The moral of the story is this: current is the enemy.

At this point I am going to define the relationship between current, voltage and power. A motor, as we all know, is simply a device which converts electrical power to mechanical power. Power, which is what we need to turn our propellers, is measured in watts or horsepower (with 1 horsepower = 745 watts). The mechanical power output of the motor is somewhat less than the electrical power in. This is because no motor is 100% efficient, some of the power is always lost as heat. Electrical power is defined as current (measured in amps) times voltage (measured in volts), this is formula number 1:

If we have 4 amps at 6 volts going into a motor, the input power is 4 x 6 = 24 watts, but if we have 4 amps at 24 volts the power is 4 x 24 = 96 watts, which is about 1/8 horsepower. The ability to deliver more power with less current is why the automobile industry is interested in 42 volts. It is much easier to build a high voltage/low current alternator than it is to build a high current/low voltage one. Also, the amount of copper wire in cars is increasing to the point where it is becoming a significant part of a car's weight. By tripling the voltage, the current is reduced to one third of what it was at 14 volts and you can use much smaller wires. Current is the enemy, thus the automobile industry is seeking to reduce it by 2/3rds.

So much for the automobile industry, what are the advantages of higher voltage for model ships/boats? The first advantage is you can avoid embarrassing incidents like the molten fuse holder in my first story. Another advantage is the one I mentioned on the "What's New" page; a wide variety of good, inexpensive motors to chose from. One more advantage is a cheaper E.S.C.. High current MOSFET power transistors cost more than low current ones.

There are some disadvantages. One is the need to carry more batteries (and the cost of buying them). The other is trying to find controls that run at higher voltages. This is probably a good time to mention that all of our new Advanvced Smartcontrols are available in the high voltage version which will operate at 24 volts.

To prove the concept really works I built the 40 inch landing craft model you see in the following photos. The basic material is 1/4 inch plywood stiffened with pieces of pine. the construction is essentially a box with a bow section attached. In the first photo only one side of the hull is attached and the various pieces of equipment, batteries, speed control, etc., are being test fitted. 
 

 

On the evening of Saturday, July, 24th, 1999, I took the landing craft to the pond for its' first test run. I was delayed by a brief thunderstorm and it was almost sunset when I finally got it into the water. The couplings I need to use the geared Pittman motor had not arrived so I ran with one of my Johnson 545 type motors and a 55 mm Graupner plastic prop. Everything seemed to be working but the hull did not have sufficient ballast and the prop was cavitating badly. Into the trunk of my car I went in search of something heavy. A set of socket wrenches in the cargo deck fixed the problem. After about 40 minutes of hard running it's starting to get dark and it was time to pull it out. The Johnson motor was warm but not hot. The speed control was cold but the voltage regulator was getting pretty hot. This makes sense because it has to get rid of all that excess voltage. I think I will rewire to run the regulator from just one battery. If I add any more servos the regulator may overheat and go into thermal shutdown. It would be real embarrassing if it happened in the middle of a regatta.

The photo below shows the Johnson test motor.

On August 8th I went to the Picton fun-run. The couplers for the geared Pittman motor had arrived so I installed that motor and the 4 inch Radestock prop. I was able to get the same speed as with the Johnson/Graupner set-up using only 12 volts; current was down to about 330 milliamps at 12 volts. A nearly 100% jump in efficiency! At 24 volts with the big prop, the landing craft seemed to almost leap out of the water, just what you need for those "Full Military Power" situations and only 660 milliamps of current. The next photo show the landing craft at Picton. With no paint or wheel house it got nicknamed the "floating casket" for obvious reasons.

Months later, in November, 1999, the completed model sits in the dock at the Hobby show. Note the Sherman tank on board.

Starting with the basics, the diagram below shows how to connect two 12 volt batteries to get 24 volts.

The next drawing shows how I use a double-pole/double-throw/center-off power switch to select either 24 or 12 volts for motor power while always getting 12 volts for the voltage regulator.