Marty,
If you haven't bought the trolling motor yet I'd rethink your choice from
the standpoint of naval engineering. Your boats natural hull speed (amount
of waterline in the water on displacement type hulls) will be a deciding
factor on boat performance. For this reason I'd save a bunch of money and
weight and go with the smallest full variable speed trolling motor you can
find.
What I'm trying to say is there is a point where more HP doesn't help if the
boat isn't on plane, in displacement hulls the square root of the effective
waterline is the maximum efficient speed. Any additional power after that is
a massive use the hp or in your case battery amperage with rapidly
diminishing returns. In canoe racing it's called "trying to climb the hull
speed hill." More muscle needed to optimize canoe speed with less to show
for it.