Prop shaft straightening

[motthediesel]

In my continuing quest to get rid of the nasty driveline shakes in our A27, I am endeavoring to add a mid-shaft bearing to our 1-1/4" bronze prop shaft. I did not want to have to remove the shaft, but as it turned out, it was necessary.

I was able to remove the shaft coupling quite easily using an OMC outboard flywheel puller. What I had not anticipated though was they difficulty of pulling the shaft back far enough to slide the new bearing on from the front. Apparently corrosion and general gunk had built up on the shaft inside the stern tube, and when I used the prop threads to pull it back, it pulled the rubber inset out of the cutless bearing.

So, at that point I decided to bite the bullet and remove the shaft entirely. That involved dropping the rudder, which I wanted to avoid doing, (but turned out to be quite easy). The shaft was then pulled out, and taken into the shop. It was setup on two greased V-blocks, and a dial indicator was used to test it for true.



The indicator was attached to a large and heavy piece of 1/2" steel, and the shaft was rotated to see how concentric it was. At the fore end it tested perfect, in the center it was good, only off by .002". At the minor diameter of the prop taper though it was not so good, off by nearly .01". No sign of any damage to the prop, so I would suspect that perhaps at some point a line was wound up on the shaft maybe?

So -- how to straighten this thing? I thought first about using a jack, but then I did a little research and decided to try "peen straightening". With that technique, you first determine where the bend is, then find it's "inside" point. That point is then placed on a wood block and struck repeatedly with a hammer. The idea (as I understand it) is to release the compressive stress in that area, and to allow the tension forces in the other side of the shaft to pull it back to it's former shape.

This is a "beat, test, repeat" sort of thing, and I'll be damned -- it works! Here's a little video clip of my final test, under .002" at the minor end seems good to me.

https://youtu.be/uTFxKgmuc_M

The cardinal rule of diagnostics is of course to "change one thing at a time". But -- these things all require the boat to be out of the water, so to hell with the cardinal. To that end, I am also changing our prop. Not that I think it's unbalanced, or damaged in any way, but because I think it's too big. It's a three blade 19x12, and that huge diameter means only about 1-1/2" of tip clearance to the hull, WAY under recommended. Also, interestingly I think, too big for a 1-1/4" shaft -- according to Dave Gerr's "Propeller Book" anyway.

So I'm going to be trying a 16x16 three blade. Why? Because, by coincidence, I happen to already have one It could be that will be too much pitch, but the boat is definitely over-propped with the 19x12 as well -- we'll see what it does with this one before we spend any more money

Tom

So it's getting a 16x16 three blade.

[Bob Noodat]

I am just glad I was finally able to show you the light.........

[Dieselram94]

What is your plan to add a bearing onto the shaft?
A standard pillow bearing on a mount glassed to the hull?

[tego]

I think you'll like the 16x16. A 19" is way too big. The factory prop on the later 27"s was an 18x14 on the 1 1/2" SS shaft. The 1 1/4 bronze shaft is nowhere as strong as the later SS shaft. 16" is not too much pitch, since you're reducing the prop diameter by three inches. You'll likely be a little underpropped but who's in a hurry with our boats anyway? Ben

[motthediesel]

Here's the bearing:



Here's it mounted in the boat:



I just epoxied it in and filleted it with thickened resin, tomorrow I'll tab it in with fiberglass tape and more epoxy resin. As you can see, I cut an new hatch just aft of the fuel tank. That will also give better access to the bilge pump and switch.

Tom

[Nancy]

Great project! Well done!

[nebulatech]

I'm impressed! Thank you for sharing.

[Bob Noodat]

Very nice indeed. That is a heckuva long shaft! Excellent idea making access, I do not see how Albin can have left the centre of the shaft inaccessible or failed to add an intermediate bearing.

Are you taking the new prop to be checked for balance before floating your boat, so to speak?