Fitting a new prop
 

I bought a new prop for the boat. The instructions said to make sure
the taper of the shaft and that of the prop matched.
75 to 85 percent contact area was suggested as a minimum.
I dry fitted the prop to the shaft,
without the key, and rotated the prop a few times with the shaft fixed.
It was easy to see that I had good contact at the "big end"...maybe
30 % contact area.
My plan is to use valve grinding compound to lap in the taper for a
better fit. My question is: what grit should I use? My instinct says no
larger than 200, and no smaller than 400.
Do you folks lap in new props, and if so, how do you do it?
Thanks.
NormB

On Thu, 21 Oct 2004 10:22:04 -0700, engsol
wrote:
I bought a new prop for the boat. The instructions said to make sure
the taper of the shaft and that of the prop matched.
75 to 85 percent contact area was suggested as a minimum.
I dry fitted the prop to the shaft,
without the key, and rotated the prop a few times with the shaft fixed.
It was easy to see that I had good contact at the "big end"...maybe
30 % contact area.
My plan is to use valve grinding compound to lap in the taper for a
better fit. My question is: what grit should I use? My instinct says no
larger than 200, and no smaller than 400.
Do you folks lap in new props, and if so, how do you do it?
Thanks.
NormB
===============================

Sounds like the prop may have the wrong taper for the shaft. If the
taper is correct, little or no lapping should be required.

Rotating the prop is not the correct way to check contact. The correct way
would be to use Prusion Blue inside the prop hub and fit the prop to the
shaft in the correct 'keyed' postion but without the key. Draw it up
slightly, just to seat it, with the prop nut, remove and observe the high
spots transfered to the shaft.

If it looks like your getting less than the 75-85 % then scrap the high
spots and re-blue.

When you get it where you want it, check the key to ensure that it isn't too
thick for the depth of the keyway in the new prop. There should be a few
thousands clearance so that it doesn't effect the prop fit. The fit on
either side can and should be a zero clearance fit (no play at all in the
key way of the shaft or prop. If your prop has a different keyway width than
your shaft, then have a machine shop make a step in the key for you..

FWIW. I've been though this prop fitting many times with navy ships in dry
dock, but in 40+ years of boating in a dozen boat, I have never felt a need
to blue in a prop. And I have never had a prop problem that I could
attribute to the prop fit.

My experience and opinions, FWIW.

Steve
s/v Good Intentions