Prop questions

MossdaleMonster · Post by **MossdaleMonster** » Tue Jul 29, 2025 8:42 pm

I have a 1983 40hp two stroke mariner. I bent all the blades on my prop from a rock pile. The prop dimensions are 10-1/4 x 14. I was wondering if I went to a 13 or 12 pitch prop if I would run into issues? Or should I just stick with a 14? I don’t really know how much more speed I can get out of it just figured every mph would help lol.

mark poulson · Post by **mark poulson** » Wed Jul 30, 2025 9:14 am

I wouldn't risk changing the prop setup on a 40 year old motor.

Whoopbass · Post by **Whoopbass** » Wed Jul 30, 2025 10:35 am

You would probably get a little better performance out of a 12 or 13 pitch prop but you won't get more speed unless the 14 pitch prop is not reaching max rpm. I've had many 25-50 hp motors over the years and I was always disappointed in motors in the 40-50 hp range because I thought they should be way faster then a 25 hp motor but that's not the case. They're usually only around 5-7 mph faster.

Kelly Ripa · Post by **Kelly Ripa** » Wed Jul 30, 2025 3:48 pm

I'm with you on that Mark. Bad enough wiping out the old prop with all the aches and pains it could cause but I would think the user and the motor were in a groove after that much time with the old prop. I would think you would have more RPM but is that a good thing on an antique?

MossdaleMonster · Post by **MossdaleMonster** » Thu Jul 31, 2025 4:44 am

Kelly Ripa wrote: ↑Wed Jul 30, 2025 3:48 pm I'm with you on that Mark. Bad enough wiping out the old prop with all the aches and pains it could cause but I would think the user and the motor were in a groove after that much time with the old prop. I would think you would have more RPM but is that a good thing on an antique?

You mean going to a 13 or 12 pitch prop would cause the rpm’s to run higher correct? I checked online as well and got the same feed back that you guys are mentioning. I just initially thought the rpm’s would be the same no matter what. I was thinking the launch speed would be slower but might gain a mph possibly. Think im gonna stick with a 14 especially after saying “we’re in a groove” lol. Ive been running that boat for 7+ years and only once have had to clean both carbs. IF IT AINT BROKE DONT FIX IT lol. Preciate the help y’all!

monte300 · Post by **monte300** » Thu Jul 31, 2025 6:07 am

Yes, lowering the pitch will increase rpm’s and lower top end speed. Lowering the pitch should also help your hole shot

WRB · Post by **WRB** » Fri Aug 01, 2025 5:55 am

Your Mariner is a Mercury made for export market. Check the Mercury prop recommended for your 40 Hp era engine. Used engine who knows what the stock prop should be? Definitely use a stock pitch prop.
I would also perform a compression check on the engine to determine if it’s OK since it is a second owner.
Tom