Generally, less than 20 percent of the rated max hp will significantly
degrade performance.

I assume you mean an outboard engine will reduce its performance
significantly if we operate it below 80% of its max hoursepower rating.
Do I understand this correctly?

In other words, I need to run an outboard engine at or near its peak
horsepower rating. This also means that I need to buy an outboard
engine whose _max_ horsepower rating matches the cruising speed of the
18-ft boat that I intend to operate at.

I probably want to cruise at a leisure speed of 10 to 15-miles-per-hour
(here the miles are not nautical miles). What kind of horsepower an
outboard engine should have in order to push a 18-ft boat with 4 adults
males on it at the cruising speed of 10 to 15 miles-per-hour? Any idea?
Thanks. I appreciate your help.

As for the difference in noise between a 40 and 50 hp engine, it's
insignificant. Besides current engines are much quieter than previous
two-strokes.

Good to know this. This means I should concentrate on getting the
proper horsepower to push the boat instead of being distracted by other
factors.

Jay Chan