Model Forum / Radio Controlled / Water Models / February 2005

Sounds like a cool design problem.

I think I'd explore non-seal designs. For example, you can arrange
for the end of the stuffing tube to be well above the water line. You can
still place the motors low by using a gear or toothed belt drive. You can
then use shielded, high-quality, ball bearings, or maybe precision
bronze or teflon on the underwater end. (Maybe a stack of teflon
washers for a thrust bearing.) Keeping the end of the tube above water
for the rudder is even easier.

A magnetic drive using high-strength rare-earth magnets on opposite
sides of a thin plastic wall is another idea, although it's fussier, kind of
bulky, and limits the amount of torque you can transmit. (The built in slip
factor might be a plus - it could save the motor and/or prop from
damage if debris is encountered.)

If you seal the battery in with the motor an explosion at some point is
not impossible. (Many battery types will vent hydrogen at some point.
You can, however, install a catalyst to deal with reasonable amounts.)

Assuming the motor and battery are in separate compartments, you
may need to cool the motor if it's sealed in. You could feed water
through a scoop and into coils of copper tubing wrapped around
the motor - I'd use two circuits for redundancy, one port and one
starboard, with one coil from each around each motor housing.
(Make sure the seals can stand the extra pressure when the air
inside heats up.)

If the hull isn't effectively penetrated below the water line, and most
compartments are sealed (you could use a flapper vent on the
battery compartment to keep water out, but let any gas generated
be vented) you only have to deal with whatever's still got a hole
and might collect sloshed water. If the prop shafts open into an
otherwise sealed compartment, they'll be very resistent to
overflowing, so that's good.

Well, two months isn't too bad. If it's possible, keep metal out
of contact with the water (one reason I think teflon bearings on
the prop end of the stuffing tubes may be a good idea). If you
do have metal in contact, avoid mixing metals, especially in
close proximity since that can generate a current and increase
corrosion. You could do like the "big boys" do and provide a
sacrificial anode of zinc, letting it corrode and protect other
metals. If the hull is to be made of fiberglass, you could just
use fiberglass tubes - they'd be easy to seal into the hull too.

What kind of power supply are you planning to use?
Solar recharging? This is really intriguing sounding
project. -Wm