Prop Selection on 85hp merc

[las4444]

I need to purchase a new prop since the existing one is pretty beat up. The prop is on a '77 850 merc and is 13 1/4 with a 17 pitch. The motor pushes along a 20' smokercraft pontoon at 20mph @ 4500 rpm. I cannot get the motor beyond 4500 rpm. The motor is strong for it's age. The current prop gets the boat up and out to top speed just fine.Should I just purchase the same spec or change the pitch? My goal with the boat is to pull tubers and general run around the lake.

T

 

[Bondo]

Re: Prop Selection on 85hp merc

Ayuh,... You're in the Wrong Forum,... Xs 2...
You have an Outboard,... This is the I/O forum....
You have a Prop question, it goes in the Prop Forum...

I'll see if a Mod. can move it for ya...

 

[Bob_VT]

Re: Prop Selection on 85hp merc

It's really only necessary to ask the question once

 

[Bondo]

Re: Prop Selection on 85hp merc

I cannot get the motor beyond 4500 rpm. The motor is strong for it's age.

Ayuh,.. It don't sound that Strong...
I'd think you're gonna need a 15" or even 13" pitch to get the Rpms Up...

 

[las4444]

Re: Prop Selection on 85hp merc

First, please forgive me on what I consider strong. I'm new to boating and have little basis to compare to. The motor has 125psi on all cylinders, fresh carb rebuild, and fuel pump kit. It has great spark that will jump 1/4"+. I used an adjustable spark gap tester. The motor is not sluggish and does not struggle when pulling two kid tubers. Have not had a chance to pull adults.

With that said, I'm not sure if it is the motor or prop or both. Even if the motor is a bit weak I would still like to get the best prop pitch for my needs.

If the lessor pitch would give me higher RPM's, is that better for the motor and/or for speed?

Thanks for your advice.

 

[Moody Blue]

Re: Prop Selection on 85hp merc

That's a lot of boat for that motor/prop. Should be running about 5000-5200 rpm at WOT. Lugging the motor below that rpm is really not good for it.

As Bond-o suggested, you need a prop with somewhere between a 13 and 15" pitch. Do you have power trim on the motor? If not, is the motor properly trimmed out using the transom tilt/trim pin?

With the lower pitch, you'll have much better hole shot (good for tubing) and slightly reduced speed but your motor will be happier.

 

[hwsiii]

Re: Prop Selection on 85hp merc

las4444m, I need to know what your gear ratio is.



H

 

[trendsetter240]

Re: Prop Selection on 85hp merc

Just wanted to add that we need the model number too.

The 1977 850EL has a WOT range of 4800 - 5500RPM.

THe 1977 850XS has a WOT range of 4800 - 6000RPM.

 

[hwsiii]

Re: Prop Selection on 85hp merc

LOL, Trend you are SOOO right.



H

 

[las4444]

Re: Prop Selection on 85hp merc

Model is 850EL and the gear ratio is 2:3:1

 

[hwsiii]

Re: Prop Selection on 85hp merc

las4444, your prop slip right now is right at 37% and your RPM are 1,000 below what they should be.

Prop Slip

If you will buy a Solas Amita 4 blade aluminum prop in an 11" pitch your RPM will be right at 5,500 and you should gain a couple of miles per hour in speed.



H