DC Motor Basics
Today, there is a wide selection of electric
motors for a wide range of hull types. For this brief, I will mostly talk about
scale model boats. Most of the information can be of help to other boat modeler's
Electric motors are used mainly for powering displacement
hulls where the boat speed achieved is relatively unimportant. The main aim in
motor and propeller selection with this type of hull is to operate the motor as
efficiently as possible, so that the demands on the battery are at a minimum. Direct
Current (DC) motors provide the modeler the ability to select a motor that will provide
scale speeds for the model boat it powers. DC motors are best used in Warships,
Tugs, Towboats, Cabin Cruisers, Cargo and other Ships. Deep V hulls use modified DC
motors that have lower turns on the armature. This makes them turn at a much higher
speed and gives the modeler the "rooster tail" of spray when the boat is
run. This technical page covers unmodified motors for "scale speed"
Current drain for DC motors is at a maximum
when the motor is held in a "stalled" condition, and a minimum when the motor is
free running with no load. Current drain increases almost linearly with decreasing
speed, until a near stall condition is reached. Here the current flowing through the
windings is high enough to cause sufficient heating to modify the resistance of the
windings. In some cases, too, the heating effect can be high enough to burn out the
windings, or cause other permanent damage to the motor. Some motors make a
"whining" sound, where the current is applied to the motor, but the armature
does not turn.
Typical DC motors can have a unloaded rpm of
5,000 or higher. For scale speeds, this is much too high. Scale speeds are
often over looked by model boat builders. Have you ever seen a tug try to pull their
hull up and out of the water to plane? I bet you have seen a model come close
though! Remember that you may have the ability to turn your propeller on your model
much faster than the full sized boat. Most engines on tugs for example, don't exceed
1800 rpm, while their propellers only turn at about 800 rpm as they do their work.
It's unrealistic to limit your tug to that rpm range, your models propeller works a bit
differently than the real one. We can, keep in mind this slow operation in speed,
and work our boats on the same principle. Warships can do about 35 knots, and if you
scaled down this speed to 1/96th scale, it's not that fast at all!
On the other side of this issue is the event
of "needing extra power"
in case of an emergency. I often only operate my boats no more
than about 2/3rd throttle, giving me the last 1/3rd for use only when I need it.
There may come a time when you may need to get out of the way of that jet ski, speed boat
or wave. Plus, running the motor at slower speeds makes the battery last
longer. Just remember that the closer you operate your boat to scale speeds, the
better it will be for the motor and the battery.
Gear drives work to reduce the speed of the
propeller and most of all, current load. Motor(s) with
"gear drives, make our models handle better. Since they are basically a
reduction drive and work like a transmission reducing the rpm's applied to the
propeller. This allows the model to move at a closer scale speed (thus being
closer to true counterpart - realistic speeds). Gear
drives can be "home made" or bought ready to assemble. Dumas for example,
has a line of metal gears and mounting plates. Modeler's who are into R/C cars may
have some spare nylon or plastic gears to use.
Metal gears do make quite a bit of noise when
in use. This noise can at times interfere with the R/C equipment causing the model
to be hard to control or steer. The servos can move in a "jumpy" or
"erratic" manner. This noise can affect electronic speed controls (ESC),
causing them to chatter or to run wild at certain speeds. I have used a very small
amount of white marine grease on the gears, and have had no problems. When run
"wet", I did not get the problems like when they were run dry. Care needs
to be taken in the routing of the servo leads, ESC leads and the receiver battery leads.
Try to keep them away from your metal gear drive. On the other hand, plastic gears
eliminate this problem all together. You can even intermix metal and plastic gears
with good results.
Gear drives also solve the problem of
synchronization when using twin counter rotating propellers. Gear drives can turn
both propeller shafts with one motor. The modeler can even link two motors together,
adding power to the model and still insuring the synchronization between the motors.
Even if the motor in your model has a
faster rpm than the application requires or needs, a gear drive will reduce the "Current
Load" on motor(s) too, including what it applies to the ESC also.