If full throttle is 5000 rpm, then 2/3 throttle would be about 3300 rpm, which
is about my cruising rpm.

Yes, I understand your math. But when I saw the mechanic (who
conducted the sea trial) set the throttle at around 2/3, I saw the rpm
was at around 4,600 rpm (and that was what the mechanic suggested me to
run the motor at). And when he put the throttle all the way, I only
saw the rpm increased to 5,000 rpm (the tachometer can display more
than 5,000 rpm). I don't know why the motor rpm didn't increase
proportional to the throttle setting. This is probably one of those
things that I don't know about motor / rpm / prop pitch ...etc.

Jay Chan