4 adults @ 180 lb each, plus ~100 lb motor
, plus ~50 lb gas and gear
, plus ~130 lb boat at 10 ft LOA
. Say 1000 lb total. With 18 hp, you're at 56 lb/hp.
With those numbers, it should be able to stay on plane once up, but it'll have a hard time getting there.
If I'm looking at the right charts
, the Nissan
10-20 hp models are designed for a 9.2" diameter prop. A very rough calculation would suggest a 9" diameter 10.5" pitch 3-blade for top speed (approx 19 kt at 1000 lb all-up). If your current
one is 9x9 and can't get her up on plane, I might try dropping pitch to 8" and recognize that you'll have to shift some weight around to get her up at full load.