Prop Replacement

[Rub-a-dub]

I have a new Merc 4S 75hp with a 17 pitch aluminum prop. I want to go stainless. I currently run 5000rpm full throttle.
By going stainless, should I keep the same pitch etc. ?

 

[Sea Rider]

What's the min-max wot rpm range factory stated for that engine ? 5 K wot revs seems way low. That engine should rev 5000-5500 or 5500-6000 rpm range. If running towards the lug side need to go to less pitch props to pull wot rpm towards middle or max range as currently loaded.

3 blade alum props works fantastic well, just need to prop engine right.

Happy Boating

 

[82rude]

If we take solas chart as gospel and your motor is 2015 or newer your dead in the middle of 45 to 5500 rpm.A 15p aluminium should put you close to max rpm and I would guess a 15p stainless would be close to what your rpms are now.Any chance of trying one out?http://www.rubexprops.com/boat-propellers/rpm-range/?make=Mercury

 

[82rude]

May I add boat type ,gear ratio etc also come into play.For an example I have a 2011 60 hp evinrude etec with the large gearcase.Boat is older starcraft ss160 .RPM range of 55 to 6000 .My solas 17p 4 blade will do 5900 rpm,my mich wheel 17 3 blade 6000 rpm (both al)my omc sst (stainless) was reworked to 16p and hits dead on 6000 rpm.All 3 props are within 1.5 mph of each other and are very tolerant of load changes without losing any performance or speed.2.67 gear ratio also.Yours is a 2.07 ratio and maybe less tolerant to changes in weight etc.

 

[WesNewell]

IIRC the recommended rpm for that engine is 5250-5750. Running 5000 is a little low. Going from alum to ss should bring it up a little, but there's a lot more to it than just prop pitch. If the engine is mounted too low that can also reduce rpm because of more drag and also causing considerable speed and fuel efficiency loss. If you don't want to mess with what you have now, then a 16p prop should bring the rpm close to in the recommended rpm range. Personally, I'd pull the intake air restrictor out to increase power of the engine and stay with the same pitch or maybe go up in pitch if the rpms max out.

 

[Rub-a-dub]

Thanks guys for all the info. I will see if my dealer will allow me to demo a smaller pitch prop and try to bring
the rpms closer to the max 5500

 

[Watermann]

A SS prop is a whole lot of expense for your small motor, normally motors with twice your power benefit from the expensive performance SS props. You're best served with a 3 blade AL prop.

Just to be sure are you properly trimming your motor out once up running on a plane?

 

[82rude]

Wes he has a 4 stroke the rpm,s are as I stated.https://www.mercurymarine.com/en/ca...m=us+&utm_source=Overlay_IP=ca#specifications

 

[82rude]

Watermann your most likely right about new ss props.Mine was used and my friend was the prop repairman/seller so I got a deal I couldn't refuse.Since he repitched to 16p for me I'm liking it a lot.Its nice to have a friend thats an expert in prop repair.

 

[b.gagnon]

You need to give way more info....
Boat type?
Engine mounting height?
Light or heavy load?
Are you trimming the motor up?

You are dead in the middle of the range so your setup seems good depending on the answers to the above questions!
If you go stainless you will have to lower the pitch by 1-2" depending on the design.

 

[QBhoy]

Just for info...despite what is posted above...a stainless prop will NOT generally bring the revs up over a aluminium prop. Quite the opposite.