Prop for a 50 hp Mercury

Does anybody know what the diameter and pitch is supposed to be on an 1985- 50 horse Mercury 2 stroke? I put a few knicks in mine-a 10" prop and my spare is a 12" with less pitch. I read somewhere that if I use a bigger prop I should reduce the pitch so maybe the spare will be ok. I just want to know what the standard prop is for this motor. I just read an article by River Rat who told how to find the numbers on the old prop. It was a 10 3/8 X 13 and the replacement is a 12 X 10.5. Will this give me better gas mileage?

Re: Prop for a 50 hp Mercury

The dinged up one is 10 3/8" diameter and 13P the other is 12" diameter and 10.5P. The 10" is probably your standard prop. 12" pitch is not a standard pitch. The pitches go like 10 to 13 to 15 as the gearboxes get bigger from 40 hp up thru 90, and on to 150 and and beyond. If the 12 came with the engine/rig, it suggests that someone was wanting to do some "work" with the boat and needed a lot of thrust.....maybe like pushing a large toon.

Not knowing your boat I can't tell you what is best for your application. Light fast boat, the 10. Heavy boat, lots of folks an gear, water toys, the 12. You WILL notice the difference in performance. The 12 will pop your neck coming out of the hole but rpms will max out fast and your top end mph will be significantly reduced from what you are getting from your 10.

HTH,
Mark

Re: Prop for a 50 hp Mercury

I can't find any definitive info on these 80s 50 mercs. I do believe they have a numerically low gear ratio of
1.64 or 1.70 around 1985. Props are relatively small in diameter and pitch.
If you give us the info on your boat and activities Your wot rpm and gps speed we can tell you about your setup.
Just to clarify Prop size is all ways written Diameter X pitch Diameter is a secondary consideration
and follows the design., If you check prop sizes you will note that usually as pitch goes up diameter goes down as pitch goes down diameter goes up. If a prop is intended for your motor/boat combination the diameter will be right.
Generally a prop is referred to by its pitch. Diameter is only considered in certain very specific applications.
Your motor needs to be able to rev freely within its rated rpm range. 4800-5500. generally you prop close to the max rpm.

Re: Prop for a 50 hp Mercury

steelespike said:

I can't find any definitive info on these 80s 50 mercs. I do believe they have a numerically low gear ratio of
1.64 or 1.70 around 1985. Props are relatively small .
If you give us the info on your boat and activities Your wot rpm and gps speed we can tell you about your setup.
Just to clarify Prop size is all ways written Diameter X pitch Diameter is a secondary consideration
and follows the design., If you check prop sizes you will note that usually as pitch goes up diameter goes down as pitch goes down diameter goes up. If a prop is intended for your motor/boat combination the diameter will be right.
Generally a prop is referred to by its pitch. Diameter is only considered in certain very specific applications.

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As close as I can get is a 94 catalog and the 50 was listed as the 3 cyl looper @ 1.64. So I guess they ran what, a 10" standard wheel on those boxes without much pitch at high rpms which is what he dinged up????

The classic 4 cyl was rated at 40 and had a 2:1 box. I know the classic moved around in hp for a lot of years from 40 to 50 as I recall. His might be a classic. Didn't say.

Mark

Re: Prop for a 50 hp Mercury

I tried to reply to thank you guys for all of the good advice but my response must be floating around somewhere in cyberspace for the last two weeks. Anyway I decided to go with the aluminum spare that is the 12x10.5. I have a spare that is stainless steel and is the same size as the original but I have heard that if there is a chance that I might smack bottom again that aluminum is more forgiving and does less damage to the motor.

Re: Prop for a 50 hp Mercury

gpfishingdude said:

I tried to reply to thank you guys for all of the good advice but my response must be floating around somewhere in cyberspace for the last two weeks. Anyway I decided to go with the aluminum spare that is the 12x10.5. I have a spare that is stainless steel and is the same size as the original but I have heard that if there is a chance that I might smack bottom again that aluminum is more forgiving and does less damage to the motor.

Click to expand...

Ok, so how does the prop run out?

Agree on the SS concern and I too would tend to agree. However, I had a 125 hp engine years ago and I could not go out on a single fishing or skiing trip that I didn't bend a blade on the alum. That lasted about 6 months and OMC came out with their SST, stainless steel, teflon coated prop. I bought one and for the next 7 years (of owning the boat) I never had the prop off. You just never know what lurks below the surface, but with depth finders and all that we have today, personally that's all that I run. If in uncharted waters, I take it easy and learn the water and the hazards. That helps.

Mark