Prop compatability. New and just don't get it!

[bugaboo]

We bought our first old boat this year. It's a 17.5 foot, 75-76 Glastron with an old 85hp Mercury motor. I have no idea what year the motor is right now, old I assume as it says 850 instead of 85. It didn't come with any manuals on either the motor or the boat itself. I took the boat out fishing and broke the prop last weekend. I took the prop into a Marine store and they told me it is a 13x21. They wanted to charge me $190 for a new prop and hub. I found a Mercury 13 3/4 x 21 with a hub on Craigslist for $50. Do you think this will work? Will that 3/4 of an inch make a difference? Are props kind of a one size fits all type of deal as long as you stay within the make and pitch?

 

[cribber]

Re: Prop compatability. New and just don't get it!

As long is it clears the anode, has the same spline count, and shaft length you should be okay. You might give up a little WOT RPM as a result.

 

[bugaboo]

Re: Prop compatability. New and just don't get it!

Sorry, I am completely boat illiterate, as such, I have no idea what an andode or the spline count is. Will you elaborate a little for me?

 

[bugaboo]

Re: Prop compatability. New and just don't get it!

This is the ad for the new prop....

Mercury 3 blade prop. 48-78122A40L, flo-torqII hub assembly included.

The part number off the one I took off my motor is 48-77348-21

Does this help at all?

 

[Texasmark]

Re: Prop compatability. New and just don't get it!

The "Anode" is beneath the anti-vent plate (the horizontal plate just above the prop) made of zinc, but is used for trimming out the engine...besides being a sacrificial element to help in preventing lower unit corrosion...pitting is common....it is commonly referred to as the "trim tab". It's function is to reduce/eliminate steering torque caused by propeller rotational torque. To adjust, loosen and move the rear in the direction the boat wants to go when you take your hand off the wheel at the speed you normally run with the engine tilt in the position you normally run. There is a plastic plug just above it. Remove the plug and look down into the hole. There is a SS hex socket screw down there that is used to adjust it.

If the blades of the prop strike it when you install your new prop, either get a thin one, or take a file and file off the front of the tab until the prop clears it by 1/4 inch.

Splines are the "teeth" on the propshaft....the shaft the prop slides over which mate to the teeth on the inside of the prop hub. A Merc prop of the proper diameter (13+ inches for that engine) will fit Merc prop shafts....I think the spline count is 13 for Merc.

In mounting the prop, grease the shaft, ensure that there is a washer on the shaft that the prop can be pressed up against, the prop, next the locking nut...piece of stamped SS material with a hex stamped to accept the prop nut and tabs on the circumference to lock (3 at a time) into the rear of the prop, then the prop nut which is torqued to 55 ft-lbs.

In installing the prop nut, put a wooden 2x4 between the blade and the anti-vent plate so that you have something to turn against. Put the locking disc on with the hex detent towards the nut so that the nut can go down and into the detent and install the nut. As the nut nears the bottom, you have to hold the locking washer up against the nut so that the nut can go down and into it. 55 ft-lbs is not knuckle breaking tight. It is good and tight, but not strenuous. Once you get the nut tightened, continue to rotate it until 3 of the 6 tangs on the locking washer line up with 3 of the 6 slots in the rear of the prop. Then with a screwdriver and hammer, tap the tangs in place.

Mark

 

[Frank Acampora]

Re: Prop compatability. New and just don't get it!

The unpainted aluminum looking trim tab on the back of the engine just behind the prop is the anode. Most Mercury engines in that size had 15 splines or grooves on the prop shaft. HOWEVER: mercury made two different size gearcases and the props that fit the larger diameter gearcase will not fit the smaller ones so you must be careful. I believe the 13 3/4 prop may be from an I/O and not an outboard--it may not fit.

Perhaps someone who knows the numbers will jump in.

 

[bugaboo]

Re: Prop compatability. New and just don't get it!

Thanks for the replies. I figured it out that it does indeed have 15 splines. I will have to figure out more about the motor. I read somewhere else on this forum that I may be able to find more information by the serial number I will give that a try. I called a prop shop today. The guy who I spoke with asked me about the size of the boat and the engine hp. He immediately said that I needed a smaller pitch. He suggests a 13x17. That seems like quite a difference from the 21 pitch I took off. Any thoughts on this?

 

[bugaboo]

Re: Prop compatability. New and just don't get it!

Okay, I got it! The motor is a 1976 Mercury 850 Thunderbolt...4 cyl.

 

[Texasmark]

Re: Prop compatability. New and just don't get it!

Couple of variables here. 21p is a lot of prop for that engine size and boat. First question is who put it on, how long was it on there, what did they do with the boat when it was used, and what kind of engine rpm's did they run.

On the 17P, that would give you plenty of all around performance such that you could load your boat with family and put 1 or 2 on water toys while the rest of the family enjoyed the ride.

And....the prop guy just may have been in the business for some time and knew what he was doing.

Good luck,

Mark

 

[bugaboo]

Re: Prop compatability. New and just don't get it!

Thanks Mark. I think we will try and go with the 17 pitch and see what happens. We bought the boat from a friend who rarely took it out on the water and when he did it was to fish. He bought it at some dealer in the area a few years before and never made any adjustments to it. So who knows why the previous owner before him put on the 21 pitch. We have mainly taken the boat out to fish and get the family out swimming on the lake. We would love to throw the kids on some tubes on the back, but have been pretty iffy on whether or not we think the old boat has enough power for that. She gets up and goes sometimes, but other times when we try to give her all she has she sounds like she was thrown into neutral and doesn't want to go at all. So we just cruise at lower speeds to keep her happy. She seems to be a bit fussy and we go with what ever she is willing to give us that day. Thanks for the help!