"Me" wrote in message
...

Maybe you can tell us, "How to make a 12Vdc Motor that has brushes and
can't cause sparks between those brushes and the commutator"? It can't
be done, Dufus.

It can, actually, but it often isn't economically feasible.

What can be done, is to seal the motor as best you can,

That can be very unwise. There are much better ways of addressing the
explosion hazard than sealing the motor case. Most motors need air for
cooling. High current motors are going to get very hot and the resulting
pressure increase would be very hard on seals. Maintaining an air tight
seal, especially around a rotating shaft, can be very difficult.

and then mount it above the expected location of the explosive gases,
with a suction hose down to the lower bilge area, and a VERY short
discharge line.

And how do you regulate the level of the explosive gases? Gas fumes are
heavier than air, and will tend to settle. They are not like water,
however. You can't rely on the heavier fumes settling to the bottom and
staying there. Think about that for a moment: If it really worked that
way, the entire atmosphere of the Earth would separate into layers based on
the relative weight of the gas. It doesn't happen. Air circulation causes
eddy currents and stirs things up.

You can't rely on the position of the bilge blower to keep it safe from
explosion. If for no other reason it would be possible for the entire
compartment to fill (top to bottom) with the necessary gas/air mix to create
an explosion.

The Coast Guard requirments state that all electrical components installed
in the engine compartment be ignition protected. That does NOT mean sealed.
The most common way of protecting a motor, generator or alternator is to put
a metal screen across the air vents. Gas fumes can enter the motor, they
can get to the brushes, and they can be ignited. They haven't been
compressed so the potential engergy is fairly low, and volume inside the
motor case if small so the resulting explosion is trivial. The gas fumes
can be ignited: the flame traves out on a wave front until it hits the metal
screen. The screen absorbs the heat from the wave front and quenches the
flame. As long as the screen doesn't get so hot that it ignites the fumes on
the outside the "explosion" stops there. No harm done.

You should have a similar screen across the air intake of the engine (often
referred to as a "flame arrester") so that a backfire through the intake
system can't ignite any fumes. There should be one on the distributor.

With the proper ignition protection it doesn't matter where the device is
mounted. Note that the starter is usually mounted very low on the
engine......

Rod McInnis