Prop help

[ramboot]

I am looking for help for the right prop for my 85 hp merc model 850, serial number on motor is 7082238, numbers worn but last couple numbers are 17? Not sure what is best to run as its only for fishing and getting from a to b , no water sports.

 

[jbuote]

Hey ramboot! :welcome:

Not a pro, and I have Johnson outboards, but I've found that in general, selecting the right prop is more a matter of testing out what you have and going from there, not so much engine make or type...

It's all a matter of boat type, what your normal load is etc..

The guru's will certainly get you to the right prop, but I'm willing to bet some of the first things they'll want to know (and I may have missed something) are:

1) What's max RPM operating range of your engine according to manufacturer spec? (some will already know that)
2) What RPM are you achieving currently at WOT?
3) What speed are you reaching at WOT currently
4) What is current size of prop on boat (Diameter and Pitch)

The idea is you want to be mid to high end of manufacturer WOT RPM range when full throttle and loaded as you normally would be..
If mainly fishing, how many people? Say normally loaded is 2 people, cooler, tackle, rods, reels etc..
Load up the boat that way when you test...

You'll need to have a tachometer for the engine if you don't have one already, and a GPS speed reading.

From that info, these folks will point you in the right direction and get you set up nicely!
Maybe a mod can move this to the prop section.. Certainly fits here, but better exposure maybe? :noidea:
Hope it helps, and welcome again!

 

[Chris1956]

Gee, what boat is it on? How long, how heavy and what type hull?

Motor is inline 4 right?

 

[ramboot]

It’s on a 16 ft, fibreglass closed bow v, goal max load 1490lbs. Measure from one blade across to other is approx 12” and I am not sure what pitch, looking at last two numbers is 17.. motor is 85hp merc and compression test on all four cylinders is 117-120. Top speed is approx. 30 and I don’t know what the rpm’s are wot. The max load I would have in it would be no more tha 500lbs, gear and two people. Motor serial no. 7082238 model850. Hope this all helps somehow

 

[racerone]

The last 2 numbers would indicate a 17" pitch.----If acceleration is slow go to 15"-----If it takes of like a rocket then 19" pitch.-------Step # 1 is to make sure motor is properly tuned and that the boat is not waterlogged.----How fast is " approximately 30 " is this measured by GPS / speedometer / or your hair waving in the wind ?

 

[ramboot]

The approx 30- 33 is what the speedometer reads, the motor runs real good AND I DONT HAVE HAIR TO WAVE IN THE WIND!!!
Boat waterlogged? How would I test for that with everything being dry.

 

[merc850]

Find out the year first, then the max rpm range (could be 4800-5500) then install a marine tach and see what it's turning. Then you can select a prop that allows motor to operate near the hi end at full throttle.