Looks like i need a new prop also!

So my boat had a 80 35hp Johnson engine on it with a 10.1/4x14p prop on it and would do 28mph
Now boat has a 83 50hp merc. It came with the same size prop and will still only do 28mph as it should with the same prop size.
BUT im afraid to just go WOT for fear of ripping my transom off or the seats out of there mounts the acceleration is WAY to much!!! Hitting full speed is almost instant. Its insane!!!!!!!!
So I feel I need more prop and to get better top end speed.
Anyone have any recommendation?
Im thinking 12X16ish or something. I would like to get about 40mph

My boat is semi v 14foot with a total weight of about 900ish pounds fully loaded.

After looking at my original paper work that came with this engine Looks like I need a 10DX16P prop.
Chart says 34-40mph on 13-16Foot boat weighing 900-1400 pounds
Sound about right?

Sounds about right in theory. All you can do is try it.

To be exact you need to put a Tach on the engine... compare current RPM's to recommended operating range. Only then will you know exactly how much to increase the pitch from the current prop.

True enough and I'm working on that right now