Cooling a Rc Electric Boat
By Paul Pachmayer
How Too: Water Cooling Water
cooling your motor and speed control is an effective and easy way to
increase the performance of your boats electrical system. Cooling
coils for motors and water pickups are available from several
dealers if your not a "do it yourselfer" but for this how too
everything will be made from scratch. This particular installation
is done on a RRH Mutineer Mono and I'll be installing the water
pickup through hull.
What you need;
3 foot piece of 5/32" K&S aluminum tube. Brass 5/32"
K&S Tubing. Medium silicon fuel line. 5 minute epoxy Salt
First thing to do is locate where the water pickup
hole needs to be drilled. In general I use the same measurement on
all my monos that use through hull pickups, 3/4" forward of the
transom and 3/8" from the keel. Mark this location with a pencil.
Now you need to drill the hole in the transom. Use a
5/32nd inch bit, start by drilling straight into the hull.
On my monos I always put a piece of 3/32 or 1/8"
balsa doubler on the inside of the hull for extra support. If your
working on a plastic boat or a wood one that has only the sheeting
in this area, glue a piece of 1/8" plywood about 1 inch square over
the hole you just drilled. It's a bit easier to glue the piece in
after you drill the hole because you can see exactly where the
plywood needs to go. Now drill through the plywood doubler.
Once you've drilled through the hull you need taper
the hole inside and out. To do that you keep the drill running and
tilt the drill toward the front of the boat while making sure you
keep the drill bit parallel to the keel of the boat. The straighter
you keep the bit the easier it is to finish off.
Take your time doing this step so that you don't
scar the hull. I have put some nice gouges in the bottoms of hulls
by not paying attention to how close the drill chuck was to the
hull. Believe me, it happens quickly so stop when the chuck is about
a 1/16th or so away from the hull. The finished hole will look
something like this.
Now that the hole is drilled you need to take the
5/32" brass tube and bend it in a tight U shape. The easiest way to
do this (if you don't have a tubing bender) is to anneal the tube so
it bends nicely. Don't be to concerned about the tubing loosing some
of it's shape/roundness, you can correct a small kink by gently
squeezing it with pliers. The finished curved tube should look like
I bent that tube using my fingers only, just a
little at a time until I was satisfied with the shape.
Now take the U shaped tube and slide it into the
hole. Make any additional bends to the tube so it fits nicely in the
hole. The tube should exit the hull like this. Notice how the tubing
lays nice and flat against the bottom of the hull
While holding the tube in position on the outside
check the tubing inside the hull and make sure it isn't in the way
of any fastening hardware you'll need to get at.
Once your satisfied with the tubing's position make
a mark on the tubing just where the end of the hole is and trim on
the mark. Notice I marked where the hole ends on the hull with a
pencil, this makes marking the tube a bit easier.
Now that the tubing is trimmed glue it in position
from the inside and outside with thin CA. After the CA sets mix up
some 5 minute epoxy and spread it around the tube inside the hull.
Once you have the inside epoxied take a small amount of the epoxy
and spread it on the outside of the hull around the tube, this will
fill any small gaps or chips in the wood around the tube. Remember
to work quickly on the outside of the hull and flip the boat over as
soon as possible because the larger amount of epoxy on the inside
will be moving a bit.
Now while the epoxy sets you can make your cooling
coil for the motor. First take your 3 foot piece of 5/32" aluminum
tubing and crimp one end with a pair of pliers.
Now you need to fill the tube with salt, the easy
way to do this is with a small funnel. I found the small funnel I
use at the hardware store, it's made for filling oil lamps and fits
perfectly on the tubing.
Fill the funnel about 3/4 of the way up and tap the
tube with your finger nail. The salt will fill the tube pretty fast
using this method.
Once the tube is filled remove the funnel (over a
garbage can) and crimp the end closed the same way you did the other
end. With the tubing filled with salt and crimped on each end it
acts like a solid piece and won't deform while you wrap the tubing
to make the coil.
The best thing I have found to wrap the tubing
around is a D battery. The D size battery is about a 1/16" smaller
in diameter than a standard 05 car motor. This gives a very snug fit
and works perfect. If your making a coil for a 700 style motor just
remove the torque ring from the motor and use the motor as your
Start wrapping the tube around the battery, take
your time to insure a nice tight coil. when your finished wrapping
the tube it should look like this.
Take the tube off the battery and cut the crimped
ends off with a cutoff wheel on a Dremel or a razor saw, a fine
tooth hack saw blade should work also. Now the fun part, getting the
salt out of the coil. I just tap the tube while rotating it over a
garbage can, it takes longer to get the salt out than it does to
make the coil! Once all the salt is removed put your new coil on the
motor by "screwing" it on the can.
Now that the epoxy has had a chance to set you'll
need to finish off the pickup tube on the outside of the hull. If
your careful you can use a Dremel and cutoff wheel to remove most of
the excess tubing, if not just sand the tubing down with a sanding
block until its flush with the hulls bottom. Your end result should
look like this.
Now install the motor in the boat and make sure the
ends of the cooling coil are in a position that allows you to put
you silicon tubing on easily. Cut a piece of the fuel tubing and
slide it on the coil and pickup.
Put your speed control in position and cut another
piece of fuel tubing to fit between the coil and ESC. Then cut one
more piece to fit between the ESC and the hull exit. The water exit
is just a small piece of 5/32" brass tube placed in a convenient
location, on this install my exit is though the transom just above
The last thing I do is make a "thermostat" for the
cooling system. Just like in a car you don't want the water going
through the system so fast that it doesn't have a chance to remove
heat. What I do is take a small piece of the 5/32 brass tubing and a
small piece of 1/8" tubing and ca the smaller tube inside the larger
one. I cut the exit fuel tubing about half way down it's length and
install the little thermostat in. Restricting the water flow this
way gives the water a bit more time in the system to remove heat.
This seems to work pretty well in most applications.