GEAR DRIVES and FAST ELECTRIC BOATS By Jay Turner
Fast electric boats are fast because they carry a lot of chemical energy stored in their batteries, and that energy can be readily converted into mechanical energy by the boat's motor to produce horsepower. The more efficient the motor is in converting the battery's energy, the faster the boat can be. This is rather elemental theory, and it is very simplified. One part left out of the theory is the conversion of the motor's rotational energy to forward thrust.  The propeller is the connecting device between the motor's power and the water, and it changes the motor's rotational energy into the boat's forward motion through thrust. Discounting the propeller's design, the larger the propeller, the more efficient it will in pushing the boat forward. The problem in fast electric boats is balancing the propeller's size with the motor's ability to spin it.
For example, take the standard Mabuchi 540-type electric motor found in most R/C boat kits. This motor is very inexpensive, yet is moderately efficient in converting the battery energy. Put that 540 into a Kyosho Viper, and on six cells with the kit's 33 millimeter-diameter propeller the boat tops out at about 10 to 12 mph.  Now lets put a 15-turn modified motor in the Viper, with the same prop and battery. Suddenly the Viper picks up speed dramatically, and probably hits 16 mph; the motor gets real hot, and run time is under two minutes, but we want more speed. Substituting an eight-cell pack for the six cell one, we put the Viper into the water and are impressed with the very quick acceleration and the new top speed of around 20 mph. Unfortunately, the run time is cut short because the motor got so hot it melted the lead wires off the endbell!
What happened? We forgot that the slower an electric motor runs, the more amperage it draws from the battery. If we add more cells, the motor spins faster but it will draw even more power from the battery. Eventually we end up running the motor too slow for best efficiency, and it overheats. The result is either melted wires, a burned out speed control, or a burned up motor. How can we make the Viper faster without overheating the electrics? One common way to overcome the overheating is to use a smaller propeller. This loads the motor less, allowing it to run at an rpm which produces less heating. If we substituted an Octura x427 propeller for the 33 millimeter kit prop, the motor runs happily and run times are longer, but the boat is not as fast - why? Now the problem is we do not have enough propeller diameter to efficiently push the model. A propeller produces a "thrust cone" of fast moving water as it spins, and this cone of water accelerates the boat. But the cone can be too small in diameter - imagine trying to move a 12-foot fishing boat with a two-horsepower motor spinning a 2" diameter prop. The boat would hardly even get moving. Now put an eight-inch diameter prop on a two horsepower motor and the boat will hit six mph or more. The motors produced the same amount of power, but the smaller prop didn't push enough water to efficiently move the boat. The same idea applies to models too. The thrust cone of the 27 millimeter prop is too small for the rather large, heavy Viper, so it does a rather poor job of moving the boat. We can't use a larger prop without loading the motor too much; how are we to get the Viper really moving? There are two ways to do this. First, we can install a larger-framed motor which produces more torque and fewer rpm. Examples are the AstroFlight and Plettenberg motors, which run well on eight cells and can swing a larger prop. These are popular with many boaters, but they are very much more costly than even a premium modified car motor. Still, they will work very well, and several local club members have used these motor to goo effect in the Viper hull on 8 cells, getting top speeds of around 26+ mph and awesome acceleration with props of 40 to 45 millimeters diameter.  The other approach is to use a gear drive - yes, we are finally going to talk about gears! What a gear drive allows the boater to do is to use the smaller modified car motors to turn a larger prop without overheating the motor. Advantages over the single large-format motor direct drive are several, generally including less cost, less weight, and more tune-ability. This latter can be very important for the advanced racer, because he can optimize his setup for the particular boat and racing conditions. While the single motor direct drive allows only one tuning area ( changing the prop ), the gear drive lets the modeler change the gear ratio too, allowing him to get closer to the optimum setup for his model.
There are disadvantages to gear drives too. The gear drive is more complicated, with more parts and requiring more maintenance. And just the large degree of tune-ability can be a disadvantage for some modelers, since it can be confusing juggling gear ratios and prop sizes. My own experience with gear drives demonstrates their advantages, particularly in competition. At national-level competition, my best finishes were with gear driven boats, including one National Championship. With some experience, getting awesome performance from gears is both possible and probable. For simplicity, we will speak only about single motor gear drives here, although the most powerful setups are with dual motors running on twelve to sixteen cells. A gear drive reduces the propeller speed while allowing the motor to spin up to an efficient rpm. With the motor rpm high, the amperage load is reduced, and temperature drops and the motor lasts longer. With the prop speed lower, we can install a larger diameter prop to get the large, efficient thrust cone we need to propel the boat fast. Now, how do we set up a gear drive on our Viper? While I've used several manufacturer's gear drive assemblies, I've had the most luck with those produced by Ed Hughey of Hughey Boats. I won't turn this article into a sales brochure, but suffice it to say that I believe Ed's products are the best available. The quality is high, the assortment of gears is extensive, and the support he gives his customers is outstanding. I'll use Ed's assemblies when describing set ups from now on. If we want to get the Viper moving, we need a prop diameter of at least 38 millimeters, so we will chose a prop of this size. Since the Viper is a submerged drive, we will be limited to non-lifting props, and this generally means Octura X-series propellers. To start, let's pick the x438 propeller, with its diameter of 38 mm (1.50") and pitch of 2.1". We can see how this works, and once we have our Viper running pretty well, we can experiment with larger props with higher pitch.  What motor to use? While any motor will work, if we want high speed we need at least a machine wound modified 05 car motor. These are available for under $35 at various sources, and with care they can last a long time. For the ultimate in power, as is needed for national-level racing, the Trinity D2 and D3 motors, or other brands based on the Trinity Epic cans, are the best available. They produce more power, are more efficient, have better magnets and will last longer. How much more speed will they produce versus the cheaper machine-wound motors? Depends of course, but on some boats it can be three to five mph or more. For the rest of this article we'll refer to the less expensive machine-wound mod motors.
For an eight-cell boat, I'd pick a 15- to 16-turn motor for sport use, and a 12- to 13-turn motor for racing. How the motor is set up is vital to gear drive performance, and there are several tips which will get even your direct-drive car motor-powered model running faster. First, replace the brushes with either hard serrated or silver serrated types. These will produce more power, especially the silver ones; silver is a bit 'harder' on the commutator and produces more wear. Use heavy springs like the Trinity 4033 "heavy" spring to increase power output and reduce arcing and comm damage. Advance the timing to increase efficiency, power and to reduce comm damage. The exact amount of advance depends on the motor, battery and application, but between 1/8th and 3/16ths inch is in the ballpark. Make certain that it is advanced, not retarded!  Now, we have to choose the gear ratio, and this is the most difficult parameter to choose. The best method is to ask other boaters who run gears what they have used, but you can make some educated guesses. For a submerged drive boat running one motor and eight cells with a small diameter prop, we can begin with a 2.17:1 gear ratio. Hughey supplies a gear ratio chart with his assemblies, and we can get to the 2.17:1 ratio by using a18-tooth pinion gear on the motor shaft, and a 39-tooth gear on the assembly's output shaft.
As with a direct drive set up, the gear drive must be perfectly aligned with the drive shaft, and there must be no binding anywhere to rob power. With everything set up and ready to go, we can head to the pond with some freshly-charged packs and begin getting our Viper's performance maximized. With everything checked out on the bank, we set the Viper in the water and hit the throttle, being certain to have a stopwatch to measure the total running time. Run the boat at full throttle in very large ovals to keep the speed up, and after one minute bring the boat back in to check the motor and batteries. If the motor or battery is really hot, we will need to either change gears to a higher numerical ratio, or find out what is binding on us. Chances are, though, that everything will be fine, so we go out again and keep making the large oval course until the battery dumps. Once we notice a big speed drop off, stop the stopwatch and bring the model back in. The total time the model ran both times must be added together; if the time is less than two minutes with a 1700 battery we will need to either use a higher gear ratio or go to a less powerful motor. A smaller prop would work too, but those smaller than the x438 don't fit on the 3/16th inch drive shaft. The same approach would be used if we had a run time of three minutes, but wanted to run for four minutes. Going to a 2.44:1 ratio would speed up the motor, reduce the boat speed and amp draw, and result in longer run times.  If the total run time was well over two minutes, then we can try to make the Viper even faster, after we wait for the motor to cool down! First we can try putting on a larger pinion gear, going from an 18-tooth to a 19 or 20 tooth. This will slow the motor down, increase the amp draw, make more power, and usually result in an increase in boat speed. Then we time the boat again, this time usually just running it until the battery dumps and bringing it back in. Once we reach the run time we want, we are about where we want to be with the gear ratio. Note that it is ESSENTIAL to keep records of what you tried and how it worked. Believe me, it will save a great amount of time and reduce duplication of effort!
If we aren't satisfied with the Viper's performance - say we want faster acceleration from a stop - we can leave the gear ratio at the original 2.17:1 and use a larger propeller. The next larger is the Octura x440, and we continue to test the run time of the model, increasing the prop size until we reach the run time we target. The next step to increase the Viper's speed would be to install a surface drive - a subject for another article. But the same procedure recounted above would be used for any geared fast electric boat, either surface or submerged drive, mono or hydro. The differences would be that the hydroplanes would use larger props and usually numerically lower gear ratios, as would the surface drive monos.  An example would be the Graupner Key West catamaran I raced at the 1997 NAMBA Nats. In the eight cell class, I tied for first place points against the 'faster' three-point hydros with a 12-turn D2 motor, a 2.05:1 ratio, and an X640 propeller. When I later ran it on six cells, I used a 10-turn motor, 2.29:1 gears and the same X640 prop. In this case the X640 was the best prop for this hull regardless of cell class, being large enough to deliver a good sized thrust cone, but not so large it caused torque and handling problems.
Different hulls and cell classes do need different props - the extremely fast Dave Frank 12-cell sport hydro used an X645 prop with its dual-motor gear drive, while my national champion 10-cell Graupner Taifun ran best with the Octura 1.6 prop. Some very fast 12-cell monos use either x440 or x642 props. Again, the best approach is to find the prop which works best with the hull, then dial in the gear drive to optimize the boat's speed and handling. This degree of tuning flexibility is not usually present when using a direct drive motor. With that set up, you usually have to compromise with prop selection since the motor has a fixed rpm range in which it performs best. Sometimes you get lucky that the best prop for the hull fits the motor's power curve; sometimes you don't. Checking the gear drive's performance as described above by measuring run time is the easiest method, but to be more exact we need to use a marked oval course. Then we make changes to prop or gear ratio until we have the fastest laps we can get and still complete the race without dumping. For racing, this is the preferred method, although it takes at least four buoys and a helper or three. Gear drive maintenance is not too extensive. After a day of running, take the drive out if it got wet and dry it off, removing the motor and oiling it's bearings, taking out the brushes and using a comm stick on the comm and brush ends. Oil the drive unit's bearings, reassemble the motor and gears, then put a very light coat of waterproof grease on the gear teeth. This reduces wear and friction, but too much grease will make a mess inside the hull. As with any high performance drive setup, the motor will need to be rebuilt periodically with new springs, brushes and a turned commutator. Performance will gradually degrade, and the boater may not even be aware of the lessened performance until he times his boat on an oval course, or races it against other boats he used to beat. Gears will last a long time if not abused, as will the gear drive bearings. While gear drives aren't for everyone, they do allow for the maximum performance possible with the 05 car motors, and they can rival or exceed the performance of the high-end direct drive motors for less cost. While the best source of gearing information is from other boaters or Hughey Boats, the boater does need to make the commitment of some tuning time to get the top performance. This is just as important with direct drive boats, and the gear drive boater will be better prepared than the competition if he does his homework. If you aren't going fast enough now, try it with gears!
This article originally appeared on International Waters website and is published here with permission. | 
