I can totally endorse what Greg Faux has posted about the need to make up an exhaust by pass - this is absolutely essential for the smooth running of any outboard engine enclosed in a well, and also for the well being of the crew!
And Philip Linsell briefly alluded to "yacht props" - there are two key factors to maintaining drive, and hence speed, in rough waters - the first is to use a Longshaft outboard [which you already have] and the second is to fit the largest diameter, smallest pitch, 3 bladed propellor that you can buy for whatever make of engine you have. Expect it to have large area blades, and perhaps to be described as a sail boat or work boat prop If you have through-hub exhaust, you need a sail boat prop with deflectors that reverse the direction of the exhaust when you go astern, in order that the exhaust gases do not break up the water flow to the blades and destroy your astern thrust.
Many outboard propellors are interchangeable between brands, so don't be afraid to ask other brand dealers if yours doesn't seem to have what you need.
When my Seal 22 was in commission, I used to run a 7.5hp two-stroke longshaft fitted with a 9.25" x 6.5" three bladed prop, which easily drove the boat at maximum hull speed, and made it very manoeuvrable.
Geoff Sheddick
Parker 27/146 "Stroller'
Geoff Sheddick
Parker 27/146 "Stroller'