. You should be able to crack on in a Ribcraft 4.8 after all they are nearly as good as a Searider 4.7
so I think 20 knots is fairly conservative, unless that's near your best cruising speed.
BTW I would advise you to never go out without spare fuel, a method of getting it into your engine and 2 stroke oil if you have one.
Plan for the weather, suitable clothing, make sure yoo have a paper copy of passage plan obviously depending on the time of tides you can get a nice push once you get past the Needles if the tide is flowing