... And some engines (not car engines, but inboard diesels
and I THINK outboards) are MOST efficient at or near WOT.

I assume WOT means "wide open throttle". This means I need to get an
outboard engine that will run at or near its rated max horsepower when
it can push the 18-ft boat at the cruising speed that I intend to
operate the boat at. (The speed that I intend to run the 18-ft boat is
like 10 to 15 land-miles per hour). This sounds good to me because this
means I can buy a low cost small engine and run it near top speed
instead of buying an expensive big engine and run it at low speed. This
works out great for my money. Thanks for the good news.

40-50hp sounds fine for his boat and his needs. I'd say go 4-stroke for
quietness, although you might want to check the specs of your boat to see
if there's an engine weight restriction.

Thanks for confirming that 4-stroke engine is quieter than 2-stroke.

I am not sure if I really need a 40-50hp engine because my intended
cruising speed is only 10 to 15 land-miles-per-hour. Actually, I don't
know because I really don't know how small an engine can push the 18-ft
boat at 10 to 15 miles-per-hour cruising speed.

And considering your other post about taking it on and off, you MIGHT get
by with a 25hp 2-stroke, depending on how slow you're willing to go.

Actually, I am thinking of 9.8hp because many lakes in New Jersey
(where I live) have restriction to limit the engine to no more than
10-hp. And a 9.8hp 4-stroke engine is around 90-lb; two persons with a
carrier should be able to hoist it. I am currently leaning toward
getting a 9.8hp as long as it can push the 18-ft boat with 4 adult
males on it at 10 to 15 land-miles per hour. What do you think?

Thanks for any info in advance.

Jay Chan