A good rule of thumb is put a motor that is at least 70% of the maximum rate HP for the boat - if you go below that, you are more likely to have performance problems (getting up on plane reasonably etc.). I assume you know about making sure you match the shaft length, controls etc., if replacing one motor with another.
I wouldn't put anything under a 50hp on a fiberglass 15-16ft hull (I think 70-75hp is just right) but it could work fine as long as you don't care if its slow to plane and not very fast. you might need the correct prop to run in an "overloaded" condition so to speak as far as engine being under powered for the boat
Back in the day, everyone aspired to own a 35. The Evinrude twin cylinder 40 was about the biggest engine you could get. Now, we have much larger engines so if you are looking to repower then go with at least another 65. If you already have the 40 go with it. It won't be a world beater but it will get you there and back.
I'd say that it also depends on where you fish. If you are running 20 miles upriver and have 2 hours to get there (and back), maybe not. If your lake is 1/2 mile across and 2 miles long, perfect!
I have a 17ft boat thats rated for 125 max HP. I have a 60 merc and for what it is the thing flies with less than 50% rated HP. No problems planing and goes over 40mph with a light load. 38-39 fully loaded. I bigger motor would be nice but not needed, especially on the lakes/rivers I fish.
Read the boats HP ratings, they can be found online and perhaps on your boat plate that most newer boats, and some older one's will have. Once you understand what the max HP is then you can decide what to run on it. The above 70% guide is a good one, anything less and you will most likely not be too happy and neither will who ever you eventually end up selling it to. The 65 it has now most likely is about right but do read up and learn what your boat is rated for.Got any pictures of your boat?
