Prop change for '91 Mariner 75HP

[marty53]

current specs:
WOT rpm - 4600 (Merc reccommends 4850 to 5250)
Prop pitch - 21 12.75" diameter Quicksilver laser II aluminum
GPS measured speed - 35mph empty boat with just me and 2 6.6gal gas tanks
Boat - 16ft Invader 160 fiberglass Open Bow. Boat weight is about 850lbs
Hole Shot is pretty good right now
I haven't measure the height of the engine yet.

The engine runs like a top but it seems like I should be able to get a lot more top speed (40-45mph) and RPMS on this. Plus i dont want to damage it by running it under spec. How low should I go with the pitch? I was thinking 19 but maybe even lower.

Thanks in advance

 

[12vMan]

Re: Prop change for '91 Mariner 75HP

You should be able to go to a SS 14-1/4 X 19 if you're running at a lower elevation. SS won't flex out of the hole or at top end like alum.

I'm assuming you have a 125-140 I/O?

 

[marty53]

Re: Prop change for '91 Mariner 75HP

Thanks for the response-

No I don't have an I/O-- it says in my title its a 91' Mariner 75HP outboard. I dont really want to drop $300+ on a prop so I need to go with aluminum.

I was just wonder if dropping 2" of pitch would be enough to get me up around 5000 RPMS