Changing propellers is an expensive complicated process. You really need to check the performance of your existing propeller first.
My suggestions: -
1) If you don't already have a GPS
get one. Plotters are best but a plain GPS
that reads in knots or, even better, tenths of a knot
will work fine.
2) Next time you go out check your GPS speed and engine
RPM. Make notes in your log. Check your engine as you go and don't use more than 80% power unless you have to.
3) After you get a feel for how the boat is look for a quiet calm stretch of water
and do a couple of runs up and down it at various engine speeds. Average the speeds in each direction. Make note of the results.
4) Now calculate your boats theoretical maximum hull speed
. This is often taken as 1.4 times the square root of the waterline length in feet.
Now, did your boat get anywhere near its hull speed
? Half a knot
is probably not worth worrying about. More than that and you've got something to chase over the next few years...