This may not be your best answer, but you do have several choices. Again, these are choices. You have a 50hp Evinrude - is it on a fast boat or a slower boat? Simply to fish, the short answer is you can put on any prop that will fit your lower unit. You don't say what your choices are in availability or if they are 3-blade, 4-blade or whatever. Switching from a 3-blade to a 4-blade will equal an inch of pitch change. Do you use a tachometer? If you do, good, that way you can keep the rpm's out of the red zone. You can keep it that simple. Or, you can complicate it. One inch change in pitch is about equal to three inches change in diameter, and maintain approx the same rpm range. (I'm trying to remember some formulas from 40 years ago when I was dabbling in Naval Architecture and Marine Engineering.) On our inboards we basically used the largest diameter that we had room for, and enough pitch to control the rpms - and we didn't run full throttle - ever. We also didn't listen to the 'experts' at the marina. We did what we needed to do to make the boats do what we wanted them to do - simple. On my outboard, it was supposed to have a 3-blade - I switched to a 4-blade and got better power to get the boat going under load and just as much speed when light and never did exceed the rated rpms. Most of my outboards were slower boats, like the mid-'50's Crestliners. They were rated for 70hp. I ran an 18hp Johnson for over 20years on my 15ft, sometimes with a 10hp or 7 1/2hp beside it in a pseudo twin arrangement. The engines were small enough to not create a synchronizing problem. Never did get it over 20knots. With a 40hp Evinrude, I did get it to 25knots, but it had an electric shift and I didn't like that for trolling. Forever a dead battery. I later traded for a newer generation 25hp Johnson - which I loved, and it would do 22knots. In all of this, this boat spent most of its life on the ocean and on big lakes and rarely got over 15knots with the loads of people and camping gear that we carried. The 4-blades that we weren't supposed to use were far better for our application than the stock 3-blades.
In your application, with the fishing as first use, I would get the closest to your present diameter (larger diameters get further away from the turbulence caused by the skeg on the lower unit, thus getting a better bite) that was available, but not larger than the 'factory issued' prop. Did your engine reach its rated rpms with the aforementioned prop size? If it turned too fast, than you could use more pitch. Conversely, if it turned too slow, you could use less pitch. This is all less rocket science than if you were running a high performance rig for water-skiing or racing, etc. About all I can offer - but should be enough to get you up and running.
Regards,
Sweetwater
