On Tue, 11 Sep 2012 04:04:57 -0500, Richard Casady
wrote:

On Sun, 09 Sep 2012 05:09:49 -0500, Rick Morel
wrote:

On Sun, 09 Sep 2012 08:18:17 +0700, Bruce
wrote:

If you were to use two universal joints and a short intermediate
shaft, as you suggest, then you would require a thrust bearing
somewhere on the propeller half of the shaft installation. Not that
this hasn't been done, it has, and is done. BUT, it adds to the cost
and space required for the engine/shaft installation and isn't really
necessary in most smaller boat installations. The rigid shaft and
coupling has worked successfully for years and years.

Look up aqua-drive (I believe it is called).

Ah-ha, I didn't think of thrust! LOL! Shows what happens when one
doesn't think something all the way through.

I was basically struck by the idea that a single U-joint wouldn't
really be very different from no U-joint. It would correct for angular
error only. That's why auto driveshafts have two, to correct for
angular and positional changes as the much "looser" mounted engine and
sprung rear end dance about.

You're right, Bruce, the rigid shaft and coupling is very KISS and
effective, and has proven itself. I've never had a problem over a
period of many years and many miles.

We had a jet boat with a drive shaft with a U Joint. A turbocraft with
a 109 hp Graymarine engine.

And no propeller :-)