I troll for bass with my 90 hp at 2 1/2 to 3 mph and use crank baits that run from 5 to 15 feet deep. I used to troll for trout but only had a small boat and an electric motor then. We did alright when we made the lure go down 5 or 6 feet. It was on Ohop and Clear. This was 15 years ago but fishing hasn't changed that much.
It mostly depends on how agressive the fish are I think.
Then again opinions are like elbows everyone has a couple.
Bill