Carl,
That instruction is only relative to a boat with inboard power.

IF your outboard has a electic start or a battery charging capability,
of course you will need appropriate wire for that. If it does not, you
do not. When my boat had no electric require at the engine, there were
no wires even going there. For the first two sesasons with the electric
start, there was an isolated system with a garden tractor batttery stuck
in the locker next to the fuel tank.

This boat has no external metal and so no ground bond at all and there
are 300+ in outboard version just that way.

Matt Colie

Carl wrote:
I have just bought a fibreglass sailboat that needs rewiring. Reading
Don Casey's book "Sailboat Electrics Simplified", he states that the
battery MUST be properly grounded to the engine block. That's fine if
the boat has an inboard engine - but does this mean that I have to run
a grounding cable through the boat to the stern and attach it to the
outboard engine? Or is there an alternative "earth" on a fibreglass
boat?