Setting Up a Fast Electric Gear Drive

by Jay Turner

Most 05 motors in direct drive fast electric boats run best with propellers similar to the Octura 1/8th inch bore series, from the x427 to P735 size. As the propeller size increases, the motor usually needs to have more turns of wire on the armature to avoid overheating the motor. It becomes quite a balancing act to match the motor, prop and cell count to come up with a fast, reliable combination for direct drive, especially if more than 6 cells are used.

One method to over come this problem is to use a gear drive. A single motor drive allows an 05 motor to operate in the rpm range where its maximum efficiency is found, but by using reduction gears the motor can spin a much larger prop than it could in direct drive. For example, a certain 14-turn car motor running direct drive on 8 cells can turn an x432 prop and still not overheat; using a slightly larger x435 prop results in a faster boat, but the pack dumps too soon, and the motor is scalding hot because it is forced to turn too slowly by the large prop.

Using a gear drive does two things to help this situation. First, it allows that 14-turn motor to run faster, drawing fewer amps while turning a larger prop like an x640. The larger prop is much higher in pitch and its larger diameter makes it more efficient at transferring the motor's power into forward thrust, so the boat goes faster. Second, it allows a lower turn motor to be used, and a lower turn motor will usually produce more horsepower. The result is often a faster, more reliable boat.

There are some downsides to a gear drive. It is more complex mechanically, and needs more maintenance than a direct drive does. There are some mechanical efficiency losses in the gears and bearings, but this is usually more than offset by the increase in thrust efficiency with the larger propeller. The gear drive takes up more space, and it costs more than a direct drive motor. But in many instances the alternative to a gear drive is to use a larger, more expensive motor like a cobalt magnet motor or a brushless motor.

If you have decided on a gear drive, which one do you choose, and how do you set it up? For a six to eight cell boat, a drive using a single motor is best ( either a dual-motor drive with only one motor, or a single motor drive ), and the best gear drives available are those from Hughey Boats. These are high quality, reliable and proven drives, and simply are the best for use in fast electric boats. Attempting to use one of the plastic entry-level gear drives will only cause frustration and require constant parts replacement if the model is expected to run at fast electric racing speeds.

Setting up a gear drive is not overly complex, and good results can be attained if we remember a few basic rules. First, there should be a minimum of friction in the drive system. The gear drive must be in proper alignment with the drive cable – if the alignment isn't perfect, the drive will not be efficient and you are giving up free performance. The gear drive bearings must be clean and free of rust. They should be sprayed with motor cleaner every time the drive is out of the boat, then re-oiled. Use a thin, high quality oil like Marvel Mystery Oil from the auto parts store to oil the drive bearings.

There MUST be some play between the gears – running them tight will suck up lots of power. Set the gear clearance by running a piece of paper through the gears while turning them. If the paper won't go through or is torn, move the gears a bit further apart and try again. Always use some lubricant on the gears; light grease is a good choice, and a very thin coat is plenty.

High quality hand wound motors will perform better on a gear drive, but decent machine wound modified motors will work okay too. Dress the motor with hard or silver serrated brushes and heavy springs, and use some timing advance as well. The exact amount of timing will vary according to the motor and its use, but 3/16th inch of advance is a good place to start. Oil the motor bearings before each day of running, and adding comm drops before each pack will help the motor run better, as well as improve brush life. Water cooling isn't essential, but is valuable, especially on a boat with a closed motor compartment.

After running three to six packs through the motor, pull it and check the brushes for wear. If they have worn down more than 3/32nd inch, it is a good idea to change them. Clean the commutator, and check it for grooves or burn marks. The comm should be turned periodically, depending on use. Sport boaters may be able to get by for ten to twelve packs between comm turning, but racers will see a serious performance drop if they wait more than three to six runs between trips to the lathe. [ Note that this motor maintenance program is no different than it would be for a direct drive motor, and in fact the direct drive motor will likely need brushes and lathe work more often to maintain peak performance. ]

Lets rig an 8-cell surface-drive monoplane for fast sport running and occasional racing use. We could start with a 13- to 14-turn motor, 1.8:1 gears in the drive, and an x440 prop. Run the model on open water at full speed until the batteries start to dump; record the time it took for the battery to dump, and determine if the amperage draw is too high. If you are using 1700 mAh batteries, and they take less than 2 minutes to dump, you should use a numerically higher gear ratio, like 2:1, or go to a smaller x438 prop. Either or both of these changes will increase the motor speed and reduce the amp draw. Not only will a sport boat run longer on a charge, but the motor will run cooler and last longer.

For a lightweight hydroplane and 8 cells, a more potent motor can be used, since the model will be easier to push to higher speeds. A 12- to 13-turn motor, 1.8:1 gears and an x640 prop can give a model like the DPI American Dream or a wood outrigger speeds in the upper 30s and lap times under 16 seconds. Remember that it takes more than just a lot of power to make a boat fast, and the strut depth and angle, boat weight, and overall trim can make a powerful boat either very quick, or awfully slow. Maximum performance will require some experimentation; don't be afraid to use different ratios or props than I mention in this article to keep your boat running efficiently.

Gear drives are also used with fewer than 8 cells, and with more than 8 cells. A six-cell boat can use a 10- or 11-turn motor, while a 12-cell boat wired in series can use two 10- or 11-turn motors. Gear ratios will depend on the prop, hull type and trim, but should fall between 1.7:1 and 2.1:1. Boats running either 6 cells, or 12 cells in series, should run high pitch props, since the power band of the motors are compressed by the gearing, and a high pitch prop will be needed to regain the differential between the motor's average lap rpm and top end rpm. Try props like the Octura x600 series, or the 1.6, 1640, 2137/3, V937/3, P747, or similar props.

Gear drives are an effective alternative to the more expensive exotic motors, and many modelers are more comfortable with the 05 motors anyway. While they can be a bit more work to set up than an exotic, they can give comparable performance.

This Article was written by Jay Turner Exclusively for Rum Runner Racing. Use of this article is prohibited without written permission from Rum Runner Racing.


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