"Gary Warner" wrote in message
...

#1 - What exactly could happen without that.

If you have a gas guage, the sending unit needs a ground in order to
operate. Since the sending unit frame is "ground" it will connect the tank
to ground at the same time. In many cases, it is the tank that is grounded
and the sending unit gets its ground from the tank.

I assume it's that
a static electric charge could build up on the tank or on me...and
when I touch the gas cap a spark happens in the tank?

Static discharge is a concern, not so much from you and/or fill nozzel but
from the fill plate to the tank. There isn't much you can do to equalize
the charge between the fill nozzel and the boat. As you pointed out, a car
is isolated from ground by the rubber tires. What you can protect against
is a buildup of charge between the fill plate and the tank The fill plate
and tank are generally connected via a rubber hose. If there is no other
electrical connection, it is possible for the gasoline running down the hose
to create a static buildup which create an arc at some point. Running a
ground wire between the two assures that this won't happen.


#2 - Is the real purpose of the ground to the engine is because
the engine is grounded to the battery or is it because the engine
is connected to the drive shaft which is in the water (true ground).

It is a good idea to have a "ground" bonding system that connects everything
together. The engine is usually the common connection point as it offers a
lot of opportunities to make such a connection, plus it is the source of 12
volt power when the engine is running. The battery would be a good spot
except that having too many wires connecting to the battery gets to be a
mess and is a real hassel when you have to change the battery.

There are several reasons why you want everything tied together with an
intended ground bond system. The primary reason is that if you don't
intentionally provide a ground path, you may end up with an unintentional
ground path that could cause problems. This is especially true of any
thru-hull fitting where the unintended ground path could be through the
water and result in electrolysis.

Rod