are you trying to plane it out Pat

just want a little more top end speed so if i need it .   it will be there if i need it really dont go that fast unless one of them good ol iowa thunderstorms roll in on ya.  you know how good we check the weather  lol. fish rain or shine. they have came up late at night.
she does plane out ok has one of those fins on the lower unit.

Yep what everybody has already said.  Check your WOT (wide open throttle) rpms under a normal load...if your up around 5000 to 5500 your pretty well maxed out under most engines.  My Evinrude typicall runs 48/4900 wot with a 17 inch pitch prop.  I could go down to a 15 inch pitch prop pick up another 300 rpm which means  15X5200 = 78000 inches or 6500 ft in one minute (this is theory and doesnt work that way)
whereas 4800 rpm X 17 inches = 81600 inches or 6800 ft in one minute.....In both cases that would appear to be over 60 mph, but due to water resistance and prop slippage in the water it really equates out to about 35 mph. with the slight edge in speed and fuel economy going to the 17 inch prop.

what they said ... you gotta have a tach in order to get the proper pitch put on your boat and if it's not worn off of the back of your motor mount that little tag on there will tell you what your wot should be hitting while on plane ... I'd guess that your max rpm should be about 5500 but can't say that for sure ... my 115 optimax wot range is 5000 to 5750 and you're supposed to prop it out to where you're on the upper side of that range ... I've got mine running at 5600 with an 18" pitch and I could easily go down to a 17" which would give me more power and probably more speed also but don't listen to that stuff about stainless props hurting your lower unit cause they won't ... they're designed to slip when they get in a bind just like the aluminum props do and the stainless props will obviously last a lot longer than the aluminum props ... dings that could destroy an aluminum prop will just put a minor ding in a steel prop. those props aren't solid ... they've got hubs in them that are designed to slip if they hit something so you don't have to worry bout damaging your motor just because you switch to a stainless steel prop. if I were going to guess at what prop a 16 ft aluminum boat needed with a 50 merc I'd guess about 15x13.5 and if I wanted more power I'd drop down to a 14" or maybe even a 13 depending on how much weight I ran in the boat all the time ... if it's really heavy then I'd guess a 13 would be right but you're supposed to have a plate on there that tells you how much weight you can run in the boat ... you could probably run a 17 if you're a small feller ... I had a 16 ft old timey bass boat with a 55 horse triumph evenrude on it which had a 15" prop on it and went 28 mph and I went up to a 17" and it dropped my speed to 27 mph and I lost a lot of rpm ... I lost about 1mph with the higher pitch prop... I dropped back to a 13" and it gave me 29 mph and I could haul as much weight in the boat as I wanted.