proper prop installation

The topic comes up frequently on the proper method of installing our props. We have even heard of people loosing brand new ones because they weren't installed properly. We are also seeing new CNC machined props come through with recommendations to lap the prop to the shaft. This I feel is due to the machined finish on the tapered bore rather than a reamed finish. Since I had my X55 hanging in the shop, I thought I'd take a few pictures and go through the steps.

Keep in mind that our props are held on by a close fitting taper. According to one of the prop manufacturers, 70% of the torque is said to be transmitted through this fit and the rest is the key/keyway. I personally feel it's 100% because the taper fit shouldn't slip at all. I've seen some pretty sloppy keys with 0 evidence that any force has been applied to them.

Prop removed:


Lightly clean the shaft with some emery paper or a Scotch brite pad: (If you don't have a new prop, clean the bore of it too)


Here's what you'll need. High spot blue (Prussian blue) and valve grinding compound:


Put a light coating of the blue on the shaft taper area:


Put the prop on the shaft and rotate back and forth in a small arc. Remove the prop and take a look at the blue:




You'll notice any high spots as lighter areas of the blue. This shaft, I made myself in the lathe about 20 years ago when I put a new bottom on the boat. The prop hasn't been off since then. (I know where all the rocks are on my lakes!!!) The prop is the original (64) to the boat. The taper fit isn't that bad considering it wasn't ever lapped in.

Spread some valve grinding compound on the shaft:


Put the prop on the shaft and with forward pressure on the prop, start turning it back and forth and rotating in 60 degree increments. When the compound seems to smooth out (no grit) pull the prop back and then forward again to get the compound re distributed. Rotate again. Repeat several times until there isn't much "grit" left to the compound.


Blue again and put the prop back on to check the lapping. High spots removed:


If not, repeat the lapping again.

Your shaft and the bore of the prop should have a real nice and even mat finish on it.


Clean all the compound and blue off with some solvent.

Fit the prop to the shaft without the key and mark the forward position of the hub with some tape:


The tape is a indicator that when you put the prop on with the key that it's forward against the shaft's taper. A common problem is that if the key is cocked or not in it's proper place, the prop will bottom out on the key rather than the taper.

If you are using a new key, it may need to be fitted. If too wide, drag it across a flat file:


Wrench:


Now, if you can't tighten the castle nut up to the point of getting the cotter pin into the hole, you can actually back off the nut to the hole. Tightening the nut has seated the prop on the taper and remember it's the taper that holds the prop on. It's the reason a puller is needed to break them loose. Anyone who is able to remove a prop with a hammer and a block of wood has a problem!!

BTW, every once in awhile, someone asks "why can't I grease or Never-Seize the taper so my prop will come off easier!! Well, you now know why and the correct answer is to tell them to get a decent puller!




John,
Did you notice only my right cuff shows up in the thread. I was real careful after you've given me such a hard time!!!

saffer wrote: Explain key installaton a bit more please

Key fitting:
New keys are usually on the large side by a couple thousand (.002") and key ways may be slightly under width. File the sides until the key fits in both the shaft and prop. You shouldn't have to hammer it it but it also shouldn't be falling out.

Key position:
If the key is placed too far forward on the shaft, it can get into the area of the shaft where the keyway gets shallower caused by the radius of the milling cutter. This makes the key ride high causing the keyway on the prop to bottom out on the key and not the bore taper. This is why you want to keep a eye on the tape on the shaft.

We ought to have a tech section for stuff like this- agreed

Is the key then suppossed to fit exactly under the prop and not stick out fore and aft?

Bruce,
The rule of thumb is 1 to 1&1/2 the diameter of the shaft between the strut and prop. It may vary due to the depth of the taper machined into different props.

Kimbrian,
The key can't stick out aft of the prop because that's where your prop nut goes. Forward it can be longer as it doesn't hurt anything as long as it doesn't ride up in the shallower keyway area. If you want, cut the key length to the same length as the prop hub.

whilst im here, as you can see the pic on my entry is not a 88 SN, i somehow got this guys boat on but I cant edit it off.
HAve asked under general discussion for help but no help as of yet.
Any ideas?

Great info! I'm about to change props on my boat and I had no idea about lapping and what not. Thanks! This site rocks!

JJ

Hey 8122pbrainard,

Great write up and thanks for taking the time to put that together.

I have a few more questions:

One side of my key has rounded edges, what is that for and which way should it go?

What kind of solvent should I use?

What do you think about the castle nut vs. the nylock nut?

I was talking to Skidim and they were recommending the Nylock nut. I am thinking I like the castle nut and cotter pin.

Clew In

much to my amazement, i have a 3/4-10, i called and called looking for one, and i had one in my tool box, where it came from i dont know.

eric lavine wrote: Pete, can we use a block of wood between the hull and the prop blades to tighten? or even a pipe wrench on the shaft so it wont spin?

Eric,
You've brought up one valid point regarding the block of wood. (guys - The other with pipe wrench is a joke) I've seen the block of wood used by many people. I've done it myself just for a little extra leverage but great care must be taken. I know you recommend against it. The Nybral prop is tough. The prop blades must be able to take the torque of the engine which typically is around 300 ft. lbs. or better divided by 3. With reduction trans's, this is multiplied by the ratio. What I'm saying is it would take quite a lot of force on the wrench (probably with a cheater on it) to distort a prop however, I don't recommend the block of wood simply because I don't want someone to have a problem. Everything has a breaking point. A proper fit on the taper doesn't really need that much force to seat it.


Give me a break - a pipe wrench!! I shutter when I see one of our maintenance guys with the channel locks!!

eric lavine wrote: can i run the nut on with an impact?

Sure - you know the factory does. They sure don't lap the props on. The impact is called "form the prop bore to the taper with force".

Not enough torque? Get one of these:

Hey 8122pbrainard,

I received my Acme 540 today and I will try to put it on soon.

Thanks again for the details they will come in handy and are much appreciated.

Thanks,

Clew In

Hey,

I agree with you and maybe even add an installation procedure heading. When someone puts up an informative procedure it could live there and not get lost in the threads.

Any way thanks again to all the experts giving us helpful advice.

Clew In

When lapping a nibral prop on a stainless shaft, doesn't the prop lose most of the material?

Couldn't that be a problem?

I'll be installing a new ACME in a few days. Good write up.

Not really worried about anything. I was just initially surprised to see you using the prop to lap it with.

It makes sense now that I think about it, your trying to make the two pieces the same shape. They're close already so it shouldn't matter much which one loses the material. And a small amount of material also as you point out.

Hey 8122pbrainard,

I put my Acme 540 on today and everything went great. I had a little trouble finding the high spot blue. No auto stores have it and only two Napa stores in the Atlanta area had it.

When I finished putting the castle nut on and installed the cotter pin the thing looks like a butterfly on the end of the shaft. I just cannot get it to wrap tight around the nut. Do you know any tricks to wrap it tight to the nut or does it even matter?

Clew In