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gunbuyer · Posted: Wed Jun 01, 2011 6:23 am Post subject: Prop Problems

I have a omc 225 with a 14.5 x19 prop. It used to work perfectly. But then I slapped a new lower unit on the boat and now it runs 6400 RPM and 500 more RPM for my cruising speed... The boat previously ran 5500 RPM which is perfect in my book.

I was told if i put a bigger prop on it that it will bring the RPM down. Obviously the lower unit I got is a different gear ratio than the previous. My question is does anyone have an old SS prop 15 or 15.5 x19 or have a recommendation on what size and pitch prop I should get. I can fish tomorrow with my little prop and just keep it under 4500 but it still uses more fuel... Thanks for any and all advice.

tailchaser22 · Finger Mullet Joined: 08 Nov 2009 Posts: 15

You will probably want to start looking for a 22 pitch prop to start off with. That will probably get you closer to what you were running.

Mad Dog · Posted: Wed Jun 01, 2011 12:44 pm Post subject:

Gunbuyer, I would recommend you get "The Propeller Handbook" by Dave Gerr. It is pretty technical but I learned a lot about getting the right prop on my boat. Everything about props was a big mystery to me. I wanted to get as close as possible on my first purchase then consider how I would tweak the dimensions after logging some time on the water under various conditions. The book gives you several proven equations to either come up with a starting point or to understand what is going on with the prop you have.

Just a thought.

MD Wink

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DBE1982 · Flour Bluffian in training Joined: 15 Apr 2006 Posts: 376

I would be concerned with why the lower unit is a different ratio.. Are you running the same exact prop as before?? You are about 900 rpms off of where you need to be, good rule of thumb is 1 degree of pitch equals 200 RPM. You are looking at needing 23-24 pitch prop.

gunbuyer · Posted: Wed Jun 01, 2011 7:59 pm Post subject:

I took my prop to a prop shop off laredo today. He switched me for a 14.5 x 21 and 14.5 x 23... I am using the same prop as before and it is off by that much... There is definitely a gear ratio difference. He said he can double bend my prop if the 23 works or he can single bend it if the 21 works. Or i can just swap props + 50 or 100 bucks. The guy seemed knowledgable and fair. I have 3 props going offshore with me in the AM!!!!!!!! Thanks for the input!