surface drive

Instead of sailing, say cruising, since you're talking about using the engine, not the sails.

And I THINK (I'm not sure!) that you change the pitch of the propeller, not its slope. As I recall, at some speeds, some pitches are more efficient.

Behaps an avid boatsman will come along and set us all straight.

Well there is an innovation for propellers on boats, and especially for yachts where the motor is stopped and the blades of the propeller fold down to reduce drag as it goes through the water.

So in this example, I suspect this might be the case.

If so.... I would then substitute  'If it is needed'...  for... 'If necessary'

and 'the vertical angle of the propeller's slope/incline can be adjusted for sailing at slower speeds....to..... 'the rake of the propeller blades can be adjusted for sailing at slower speeds'.