propping my 20' alaskan

Hello all. I own a 20" Lund Alaskan with a 75hp Merc 2 stroke and last fall for duck hunting i purchased a 13 1/4" X 17" michigan SS Prop and which drove the hull perfect with the heavy loads I can put in it during hunting season..This past week I decided to go up another pitch to a 19" yet down 1/8" in diameter to a 13 1/8" and it still pulls it out of the hole no problem at all and although I do not have a tach, I know it's still not over or under revving...The hull is running 35mph with this new 19" prop but would like to try to get it going faster yet, but I'm just not sure whether to go up to a 13"X21" and hope for the best but had hoped there was someone ready this who might have gone through what I'm doing right now and might shed some light on it for me..

I do know, without a tach you don't know for sure, but I know the engine is not under revving presently...Thank you..

Re: propping my 20' alaskan

You really need to know the Rpm and gps speed. or you could be flirting with danger.
Also a duckhunter, I know at times i'm somewhat over loaded.
Buy a tiny tach, than you will know what's going on.

Re: propping my 20' alaskan

Gps showed just over 35mph and I have An old chainsaw digital tach I should c if it still works and try it.

Re: propping my 20' alaskan

You need wot rpm and speed to evaluate your present setup and select a prop if necessary and to see what the results are.
A Tinytach is about $47 and will work on any motor.

Re: propping my 20' alaskan

This is a dumb way to measure rpm but:: using speed, gear ratio, and prop size and guessing your slip at 10%
I get about 4900 rpm . Motor is rated for 4750 to 5250.Is your speed by gps?
It would appear a 21 would take you out of the rpm range.