Ranger 196 VS 200hp Johnson Venom.  with me (260 pounds), my buddy (300 pounds), and all the normal fishing stuff, I'm topping out around 5,100 rpm and 47 mph (according to the gps).

Is this normal, or am I missing something?  I have no idea about the prop, other than it's silver and has 4 blades...

RPM is way too low.   This could be a motor problem, linc and sinc adjustment problem or a prop problem.  The first thing I would do is pull the air silencer off the carbs, press the hotfoot all the way to the floor and make sure the butterflies in the carbs are pointing straight out and not to one side or the other.  If they are not pointing straight out, you need to make sure the cam roller has not started come appart, it should be approx 3/8" in diameter.  If the outer shell has come off, it will be much smaller than that.  If cam roller is good, you need to get the book and do a linc an sinc.

If that's all good, next step would be to do a compression check, after warming motor up for at least 10 minutes.  Depending on year model, it should have 90 to 105 pounds of compression.

 If that's good, then I would check prop out.

If it's an OMC/BRP prop, it will have the size cast into it around the inner edge where the prop washer and nut are.  I would think a 23" pitch prop would be close to what you need.

That motor needs to turn 5,800 - 6,000 rpm for your best power and speed.

Thank you!