Mercury Mariner 50 4-cyl 2-stroke can I run a 17 pitch prop

JoeFin

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Aug 13, 2018
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Looking to replace the beat up aluminum prop on a 16' Bass Dominator boat with a 1985 Mercury Mariner 50 Hp, 4 cylinder, 2 stroke outboard.



This is my first outboard - so be kind. I had V-drives and Direct-Drives (Ski-Boats) but gave up boating for almost 17 yrs and finally let this kid trade me out of an old tow truck I had sitting in my drive way. Boat was a hand-me-down boat from his Grandfather who helped tow it over for him, and the kid is a Heavy Equipment Mechanic so he fixed most every thing on the boat before trading it to me.

Most every thing ...

The prop on it is aluminum and is pretty beat up and has been shaved down some probably in an attempt to rebalance it after chewing it up good. It has cavitation at mid range RPMs probably upper end too but I don't seem to notice it when its flat out on plane.

Would like to replace it with Stainless Steel and mostly see 10.25 x 15 props being advertised. From what I know about outboards they have crazy steep pitch to make up for the shaft in the middle. PowerTech is advertising a 11.25 x 17 stainless steel prop but would like to hear from folks running a similar set up
 

WesNewell

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Jan 3, 2018
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While that boat only weighs ~511# I think a 17P will be way too much pitch for a 50hp engine. I've got a 60hp on my 1000# boat and I had to drop back to a 14P

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jimmbo

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May 24, 2004
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A 1980s 4 cylinder 50hp would have a WOT rpm range of 5000 - 5500. On a 16ft boat I would recommend an 11 or 12 inch pitch prop. on a very very light boat you might get away with a 13
 

WesNewell

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Assuming it's not going to be used to tow skiers or tubers and it's also ~1.83 GR a 15p prop would be the right pitch for a 2 person load. It would help to know current prop, load, and wot rpm.
 

Sea Rider

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Can't play with props without the help of a tach. If current prop is working probably not top though, tach it and report max wot revs achieved as currently loaded on clam flat water cond. Can go from there maximizing a less pitch prop to pull revs up if wot revs are under middle to min revs factory stated.

Happy Boating
 

jimmbo

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What pitch was the prop that you are replacing? I’m pretty sure your engines gear ratio is 2:1. You did say it was a 4 cylinder 50 hp
 

JoeFin

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Aug 13, 2018
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Thanks for all the replies guys I really appreciate the help

Jimmbo I looked over the existing prop and I think I have to take it off to see the numbers. I'll look into that and get back to you.

Sea Rider - when I redo the carpet this winter I would be adding a tach. It does sound healthy when its WFO.

I'll pull the prop tonight and see it there is any information on the mating surface.
 

WesNewell

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If it's a 2.33 GR, I think you might want to go with a 17P prop. The mercury prop selector suggest an 18p with a 50hp 2.33gr 2000 engine. And from what I can find, that's the gr of the 85 4 cylinder 50hp mariner. But I'm not sure. Look at the owners manual if you have it.
 

jimmbo

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If it's a 2.33 GR, I think you might want to go with a 17P prop. The mercury prop selector suggest an 18p with a 50hp 2.33gr 2000 engine. And from what I can find, that's the gr of the 85 4 cylinder 50hp mariner. But I'm not sure. Look at the owners manual if you have it.

The OP does not have a 2000 engine. Nor does the engine he/she has, use the 2.3 gear ratio. That ratio was used in a much larger gearcase, on higher HP engines, such as the 4 cylinder 65hp, the 4 cylinder 75, 80, and 85hp engines, and it was offered as a High Altitude option on the mid 1980s inline 90 and 115hp. It was never offered on the 44.4 cubic inch 4 cylinder. I doubt it would even bolt on
 

WesNewell

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I found conflicting reports of GR. One said 2.33, and another said 1.83 (22:12). That could be why I suggested he check the manual For his 1985 model. If it's a 1.83 gr, then he'd want a 15P prop, 2.33 gr and he'd want a 17p prop. I didn't think that was too hard to understand. Found another that says gr is 2.0. If that's the case then he might want a 16p prop.
 
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jimmbo

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If it is a 4 cylinder 50 hp it will have a 2:1 gearbox, if it is a 3 cylinder 50 hp, it will have a 1.83:1. The OP said it was a 4 cylinder, it will have a 2:1. BTW a 15" prop is too much on a 16ft boat.
 

WesNewell

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It's not too much for a boat that only weighs 511# with a 2.0 gr. It was too much for my 1000# boat with a 1.83 gr 60hp. but barely too much. Ended up running a 14P on mine after remounting the engine higher. if he wants a 12p I got a almost new spitfire with hub kit he can have for $50, but I think that would be a mistake for such a light boat with that engine. It maxed out rpm on my 1000# boat.
 

WesNewell

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Yes, I know this isn't his engine, but they are the same hp and with even a lower gr.
  • Usage: Overall good performance/0 ft
  • Engine Manufacturer:
    Mercury
  • Engine Type: Outboard
  • Engine Year: 2000
  • Number of Engines: One
  • Engine Family: 2-Stroke
  • Engine: 50 Classic
  • Drive / Gear Case: 40-50 HP 3cyl 2-S, 50-60 HP 4cyl 4-S, 1.83 Ratio
[h=3]Details[/h]
  • Calculated Pitch: 17.08
  • Calculated Weight with engine(s): 1,278.40 lbs
  • Calculated Engine RPM: 5,250
  • Calculated Boat Speed: 41.90 mph
 

Faztbullet

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Mar 2, 2008
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Those spec's mean nothing as the OP motor is not prop rated..it about 45HP. The spec's you posted are for a 3 cylinder and a 4 stroke...different gear ratios as OP is 2:1 and always has been no other ratio has been offered.
 
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