What do you mean by slow speeds? Is this a planing boat or a displacement
boat? Generally it is considered bad for gasoline engines to idle them for
extended periods. If the carbs are set up properly though it should not
foul the plugs very much. You can speed up for a few minutes occasionally
to clean them out. I don't think you are really going to find idling around
in a planing boat to be very satisfactory. You might experiment with
running on one engine to see if you can achieve the low speed you want with
the endgine running above idle.

"Diego" wrote in message
ups.com...
I have a boat with twin 350 hp Crusader gas engines. I would like to
cruise this boat between Florida and the Bahamas. It will be optimal
for me to cross the Gulf Stream at high cruising speeds to get out of
harms way, then reduce to a very slow speed when moving around the
islands in order to conserve fuel and increase my range. I've heard
that this can cause fouling of spark plugs. Has anyone tried this and
can comment on doing something like this? Does the plug fouling
happen over a lengthy period of time (weeks) or rather quickly (hours)?