On Thu, 16 Aug 2007 09:59:09 -0700, Frogwatch
wrote:

I will defend my suggestion although it sounds impractical for the
Alberg. A coupler properly made, probably from SS about 1" ID larger
than the shaft with deep holes drilled and then milled to size will be
as well balanced as the original. A few barnacles on a prop would
produce a far greater imbalance.
I have been amazed at how my 3 bladed prop does not shake even though
it gets covered with barnacles. I hit a mast from a dismasted Laser
this past year and made a huge ding in one blade and it still turns
smoothly. This causes me to believe that it is alignment, not balance
that is most critical and a well made coupler would maintain that
alignment. Besides, you can always re-align the shaft (fairly easy on
my S2).


Here's a little engineering insight. If a drive shaft is lightened
by drilling out an axial hole whose diameter is one half the outer
diameter - the shaft's strength in torque is reduced by 5% and its
weight is reduced by 25% This is important in aircraft
construction.

So as applied to cutting and shutting a propellor shaft, if its
diameter is x, and it is cut then bridged with a coupler,
the coupler's diameter ought to be just over twice the shaft's
diameter, to bridge across the gap. As to passing the torque into
and out of the coupler - the ideal connection is splines, the more the
merrier, within reason. But this is unlikely to be an option.
Another option, much simpler to arrange, is two well-fitted bolt holes
at right angles to each other, at one shaft diameter back from the
cut, and the other at two diameters back if possible. This is likely
to be weaker than the original torque capability.
There is another option, which will be familiar to you from
examining an old style (English) bike pedal crank.
A flat is worked on the shaft, and the pedal crank has a hole
to pass the shaft. But it also has a hole at right angles to the shaft
through which a cotter bolt with a tapered flat passes to bear on the
main shaft's flat. The cotter, when bolted tight, eliminates slippng
between the two pieces, so a wear point is eliminated.
If an eccentric hole can be drilled in the coupler, so a cotter bolt
can bear on the shaft, it would stop relative motion.
But there are many other ways you could concieve of connecting a shaft
into and out of a coupler, I'm sure.

Brian Whatcott Altus OK