Prop Pitch vs RPM

Hi There,
I have a 1988 Mercury Classic Fifty, 45 HP motor. I have added some new decking to the boat(which had added @ 300lb additional weight) I currently have a 14 pitch prop, but would like to lower it to a 12, maybe an 11 pitch to help increase the low end torque and get me on plane faster. My question is: Will this significantly increase my RPMs? If so, will it be so much so as to cause damage to the engine with over revving? Unfortunately, I do not have a tacometer. So I am looking for opinions on this. Thanks for the help. If it helps, I have a 1978 Ranger 1601, pro fishing boat, that is rated for a max of 55hp.

Re: Prop Pitch vs RPM

A classic 50, 45hp sounds strange. It does appear there are no 50s in 88.
OK I have to say you shouldn't make any prop changes without a tach.
and it would be very good to have a speedo or gps.You can get a tinytach for about $40 and easy to insatall.I have a feeling your 14 is right on the edge of too much prop before the floor.Especially if you take a couple of buddies and some cool ones.It is generally agreed that lugging your motor with todays fuels can result in
premature repair problems.and poor performance.If you insist on a change without a tach at least give it a good test with the 14 and figure your rpm change is ABOUT
200 per inch.If I was guessing on a prop change (a suicide pick) I would guess toward the lower number.If you end up under propped you can still use it by being careful with the throttle.If overpropped you will be lugging it at anything higher than
a faster than high displacement speed.

Re: Prop Pitch vs RPM

In 1988 there were both a 45hp and a 50hp.

The 45hp was a 4 cylinder 43.8ci with WOT RPM range of 5000-5500. (I think this is the one you have)

The 50hp was a 3 cylinder 49.8ci with WOT RPM range of 5300-5800.

Going from 14p to 11p will drastically increase your rpms. You need to get a tach and select a prop that will allow you to run 5500RPM +/- 100RPM at WOT with an average load.

Re: Prop Pitch vs RPM

I owned one of those 4 cyl 45s back in the day that thing was the smoothest running little boat motor I ever owned, it ran like a 4 stroke, the only draw back was it had tiny little pistons and didnt make a lot of low end power but man was it smooth running