Boy I hope you don't ever intend to go into rivers and that the lakes
have no current. A 9.8 with that load might not even hold its own in a
stiff current. I remember watching my brother in a 15ft aluminum boat
with an older 25 Hp on it sitting in the Little Kanawa river near
Parkersburg, WV. The river was high and flowing fast, but not yet at
flood stage. He was sitting still with the engine WOT. Could not make
any headway at all and barely got it back to shore where we could tie
it back up to the dock. At normal current it would do OK and get on
plane with him in it. I couldn't see it doing much with 4 full sized
men in it though.

Yes, you are probably right.

Some old post said that a 9.8hp would push a 12-ft boat only at 12 mph
with 2 adult males and other gears. I would guess that the same engine
will only push a 18-ft boat at 6 mph or less with 4 adult males and
other gears. This would be too slow even for me.

On the other hand, someone else in another post said that his 9.9hp new
engine with a new prop could push a 18-ft boat at 20 mph (probably with
only himself in the boat). I guess this would mean around 10 mph if
there are 4 adult males on the boat instead of just one.

Seem like the best course of action a

- Get a powerful trolling motor and use it in
electric-only lakes. Electric-only lakes tend
to be very small anyway.

- Get a 9.9hp outboard engine and use it in
10-hp-or-less-only lakes (there are many
lakes like this around here (New Jersey)).
And play around with it with various number
of passengers to see how slow it can get,
and whether the speed is acceptable. I would
think that this should be OK because those
lakes don't have any current to speak of.
At that time, I will get enough information
about the boat/engine combo, and I will be
able to decide whether I need a bigger
horsepower engine to run the boat in sal****er.

- If I discover that 9.9hp is only good enough
for freshwater lakes and is not appropriate
for sal****er use, I will get a larger engine
(like 40hp) and use the 9.9hp engine for
trolling.

You see, I need the 9.9hp engine to run the boat in 10-hp-or-less lakes
anyway. This means it will not be wasted.

Jay Chan