The key thing is where and how you support the shaft. If it is a
single strut bearing system, you would like to have it end up centered
in the tube the flexible box hose attaches to in order not to have any
preset on the hose and maximize the possible movement in all
directions. One way to do that would be put wedges or a bushing in
the stern tube. You would then have three feet of shaft sticking out
with a coupling on the end and I would say, yes, you should weight it.

Simpler would just be to support it at the coupling and adjust for
equal spacing in the stern tube. The shaft will sag some small amount
but this will not be significant to the flexible stuffing box in the
way that the coupling face angle error would be with the overhanging
shaft bending down.

Speaking of the angles, do you have a dial gauge? It looks from the
pictures like the faces were cleaned up a lot. Unless the
transmission coupling was checked in the shop, you should set up a
dial gauge and turn it by hand to be sure it is truly square. The
coupling style you have makes this a pain to do as you have to be
careful not to knock the whole thing out of its base setting as it
skips over the cut outs. You want to also check the shaft the same
way. This is hard in the boat because the bearings make it want to
"screw" for and aft as you turn it, even if the helper manages not to
push for and aft on the prop.

The whole idea is to get the geometry such that, if the shaft was
weightless and just sticking off the end of the engine, the center at
the prop nut end would not be going around in a little circle as the
shaft turned. As the engine moves around on its mounts there will be
larger misalignments and flexing in the whole system. However, if the
shaft is attached at even a slight angle, the engine will be trying to
wave the whole thing around in that circle. Since this happens on
every revolution it is rhythmic and sets up impulses that something
will resonate to and you have vibration.

How many shaft diameters do you have between the coupling and the
shaft bearing? Might be better to look for any red flags now than
after it is all together.

--

Roger Long



"Skip Gundlach" skipgundlach at gmail dotcom wrote in message
...
Hi, Roger, and thanks for the response, left below.

The shaft in the M46 is pretty long. I've not measured it, but I'm
guessing 7-8 feet minimum.

The stuffing box is on a hose, which is flexible; there's a tube
from the stuffing box, through the hull, and out to the cutlass.
The last few inches of the cutlass are cut away ~45* to half
coverage, making a water vent, and the tube is kerf-width slit in a
couple of places on the bottom, between the keel and the Pstrut for
about 20* or so for more water lubrication.

So, I'm reasonably sure there's not an intermediate bearing; there
had been one, not far before the stuffing box, but it was removed,
presumably when the SS shaft (stiffer, I'd assume) replaced the
Bronze original.

However, there's lots of length between the stuffing box and the
tranny - 3' or more, I'd say. Does that impact your advice (for
which I'm grateful)?

Thanks.

--
L8R

Skip

Morgan 461 #2
SV Flying Pig KI4MPC
http://tinyurl.com/384p2 The vessel as Tehamana, as we bought her

"Believe me, my young friend, there is *nothing*-absolutely
nothing-half so much worth doing as simply messing,
messing-about-in-boats; messing about in boats-or *with* boats.
In or out of 'em, it doesn't matter. Nothing seems really to
matter, that's the charm of it.
Whether you get away, or whether you don't; whether you arrive at
your destination or whether you reach somewhere else, or whether you
never get anywhere at all, you're always busy, and you never do
anything in particular; and when you've done it there's always
something else to do, and you can do it if you like, but you'd much
better not."

"Roger Long" wrote in message
...
If you just have a single shaft bearing, just ahead of the prop,
you don't need to weigh the shaft. Just suspend it so it is
centered as closely as possible in the stern tube. Be sure to hang
and fix it by the coupling or close to it. Some stuffing boxes
also incorporate a bearing so check carefully. If there is a
bearing in the stuffing box, it's a bearing. If your stuffing box
isn't flexible, there probably is a bearing in there somewhere.

If you have two bearings, measure the length and diameter from the
forward shaft end to the bearing and calculate the volume and
weight (.28 lbs / cubic inch is close enough). Divide by 2. Then
calculate the volume and weight of the coupling. Add the two
weights. Hang the coupling so a pull scale inserted in the wire
reads this amount. You will then not be lining up to a shaft that
is drooping under it's own weight.

On further reflection: shafts in most sailboats are probably so
short that this isn't a big issue. On power boats, there will
often be several feet of shaft between the bearing and the gear.
Still, it wouldn't hurt to hang and weight it. We're talking about
very small tolerances here.

--

Roger Long