Hi, Wayne, (and Anne) I agree with most of the ideas here, especially to
break the thing down into troubleshooting stages, but have just a couple
of points to query. I am by no means whatsoever saying that your ideas
are incorrect for sure, but when troubleshooting, it is best not to
eliminate possibilities without being totally sure that they can be
ruled out. I have only just paid attention to this thread, so I am
generalising a bit here maybe.
A vibration could be present even with perfect alignment.
There are several possibilities where this could be the case.
The first is the fact that, as already mentioned in this thread, there
is a large item (the hull) that is probably within range of the water
pressure created by the prop.
So, the torque will vary within each single revolution as the prop
blades 'line up' with the hull. This same situation would be amplified
if the prop has a blade that was out of line with other blades, or
bent/damaged in some way, or badly mounted to the shaft.
Another vibe source could be the possibility that the shaft is riding up
a worn bearing, increasing with speed, until it 'flops over' at the top.
So, the shaft stops cleanly rotating, and kind of whips around the inner
bearing like a planetary gear path might. This would be a large
vibration source with a heavy shaft such as a 2" one.
Further, the forces acting on the prop are not equal, as the prop is
mounted at an angle to the passing water as the boat moves. In short,
there are side forces that increase with boat speed. This is unlikely to
be a sole issue, but might amplify the actual root cause of any vibe.
The troubleshooting idea of running with no shaft, and no prop, are
great ways to isolate the issue if convenient.
A place I might start at though, would be to grab a dial indicator and
run it around the place where the driveline parts mate. I know that it
is not good to try to guess, and find a 'silver bullet' solution, but in
my opinion, the weak link in the problem here is the same as any problem
I get after recently working on my car. Chances are the fault is human.
The rig was running OK, we think, subject to ICE noise covering up an
errors. I would assume that the electric motor is vibe free, but like
you, would test it anyway if convenient, so it can be totally ruled out.
With the shaft being new, we have to assume for now that it is not bent.
For me, the weak spot in the way that boat engines in general are
fitted, is that in theory, we are often connecting two shafts (prop
shaft and motor shaft) end to end, and in most cases, maybe making the
assumption that the centre lines are in line.
If a flange is just 4" across the face, and it is say, 3 thousandths of
an inch running out, the diameter of a human hair more or less, then for
every foot of prop shaft, in rough figures, the shaft will be running
out nearly ten thousandths, or one hundreth of an inch. With an 8 foot
shaft, that runout gets to be approaching a tenth of an inch.
Actually, Anne, a convenient starting point might be to leave the rig
bolted together, and just see if the coupling is generally running true.
With the heavy shaft weight involved, and the good solid strength of the
shaft, flexing under motion or it's own weight is less likely than with
a smaller setup.
Another possibility, seeing as the shaft is new, is that the area where
the prop fits was incorrectly machined, and the prop is able to shift
around, or was shifted when clamped tight by the nut. If it is on a
taper (I am an ex-machinist, not a boatwright, and have never had my own
prop off, but I assume it is on a keyed taper?) then there is a
possibility that the prop is at an angle, and is just clamped in place
by the nut pushing the prop bore to the place, a 'high spot' where the
prop makes contact. If so, this might allow the prop to end up located
at an angle.
Finally, any situation where there there are more than 2 bearings
holding a shaft or hard-coupled shafts, is open to side forces/stresses
if not installed/assembled/checked very carefully. This thinking may (or
not) oppose the general way marine assemblies are conventionally done.
Don't know if this might help,
Good luck,
John
2c. Re: Prop Pitch
Posted by: "Wayne Steeves" wsteeves@neurotag.com wsteeves
Date: Mon Jan 2, 2012 8:30 pm ((PST))
Don't think so. The vibration would indicate alignment. It's not as easy
as you might think with the coupling tube that is used as opposed to the
plate couplers normally used (or whatever their called). Any play in the
shaft at the coupler end makes it more difficult.
If there is any play at all (1/16" is max before replacement) in the
cutlass bearing any slight misalignment will be magnified and cause the
shaft to flop around as RPM's and load increases. This could be the
problem if you did not replace the cutlass bearing when you replaced the
shaft. The old shaft being bent will have provided considerable uneven
wear to the bearing over time. I would run the motor disconnected from
the shaft and insure it is not the problem (doubtful), then attach shaft
without prop and run it. If no vibration problems then problem would
likely lie with prop itself. Also make sure your stuffing box has been
properly packed and did not wear the packing unevenly due to the prior
bent shaft.
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