Sure you can sail a boat without a motor, but if you're going to have one, why "underpower" it. Sailing through Hells Gate with boat that displaces around 11000 lb with a 10 hp can be done, but what if the winds are in the wrong direction, you're pressed for time, trying to beat a storm, you mistime the tide/current, etc. and need to get in quick? A 25 Ft Catalina 5 HP motor is much better than a P26 with 5 HP motor as the Pearson displaces almost 20% more water weight. (4500 versus 5400 lbs).
The "general" rule of thumb is 2 HP for every 1000 lb displacement. It's just a guideline. The Pearson 34 came with 16 HP motor, but it displaces about 12,000 lbs. My boat is a Pearson 10M and it has a 27 HP motor and displaces 13000 lbs. I once had to get up a river, that has a natural 3 kt current, during max ebb flow of the tide. Total current in the throat between the jetties was 6 kts. At one point with the motor full, we were going 1 kt SOG. We couldn't sail up, because the wind was on the nose and the channel at that narrowest point is about 150 ft wide. Our boat is 33 ft.
We passed a small 22 ft sailboat, towing a dinghy
, full motor on (9HP) and full sails. It took him almost 3 h to go the 3 mile up river. When he arrived at the dock, he said sometimes he was going backwards and had to sail eddy to eddy.
The weight different between a 6 hp motor versus a 10 HP motor is nothing compared to the boat weight, plus stuff, plus crew, etc.
I recommend a 9 HP motor.