|
When
it comes to yacht maintenance, one of the least understood and most
overlooked systems that require maintenance, engine and shaft alignment
probably heads the list. Here's a short list of the problems associated
with incorrect alignment and poor or worn out engine mounts:
- Rapid bearing wear.
- Causes stuffing boxes to wear out and leak, not infrequently
sinking the boat.
- Bent or broken shafts
- Drive system vibration that can damage transmissions, engine
mounts and the boat hull itself.
- Transmission failure caused by increased stress on the rear
output shaft bearings and gears.
- Loosening of struts, causing leaking and possible sinking.
- Oscillating propeller shaft causing stuffing box clamps to
loosen and work free, usually sinking the boat.
- Wear or worn out engine mounts cause drive shaft misalignment
to stern drive, causing universal joints to oscillate and wear
out.
These are but a few of the damages that can be
caused by basic engine/shaft misalignment. Now comes a discussion
of why, unlike in an automotive applications, boat engine drive
systems do not indefinetly remain in alignment and need to be periodically
checked and realigned. People tend to think of boats and yachts
as being rigid structures, but they're not. The fact is that boats
bend and twist in all directions, albeit not to any degree that's
visually detectable. Expecially fiberglass boats, because they are
a thermosetting plastic, can change their shape over time. This
can happen either as a result of improper hauling and shoring, or
it can happen simply as a factor of age and the effects of gravity.
This type of mount is fine for gas engines
but not for heavy diesels. This mount, on a brand new 45 footer,
is badly distorted. Note how it is cocked toward the engine and
forward. In this case, the mounts allowed the engine to move so
much that the shaft coupling came apart.
However, the primary reason why drive systems do
not stay in line indefinitely is due to wear and tear on all the
components that make up the system. Stress, vibration, friction,
slamming and pounding are all factors that contribute the the changing
size and shape of all physical objects. Foremost among these are
the engine mounts. Remember that it is the engine mounts that carry
nearly all of the thrust load of the propeller and transfers it
to the hull. Obviously, then, the engine mounts should be well-designed
and strong. Unfortunately, more often than not the builder has selected
the most inexpensive mounts that he can buy, mounts that are not
going to do a very good job of keeping the system in alignment.
In fact, the most commonly used engine mounts for
gas and small diesels is a mount that does a very poor job of maintaining
alignment. Shown in the photo above, this mount not only permits
the engine to rock forward and back, but up and down and laterally
also. It literally allows the engine to move like a hobby horse.
A basic rule in engine alignment system is that
that larger the vessel, the longer and larger diameter the shaft,
the more critical alignment is. That's because any misalignment
becomes magnified over the length of the shaft; the longer the shaft,
the greater the potential error. A shaft that's centered at the
strut bearing can easily be off by several inches with only a change
in a degree or two of angle at the engine. This factor is what accounts
for why shaft misalignment can cause so much damage on any shafting
system of considerable length.
This is also why small boats can get away with
a great deal of misaligment, and why they can get away with using
cheap, inferiror mounts. A shaft that is only ten feet long can
tolerate ten times the amount of misalignment of a shaft that is
twenty feet long. But by the time you get up to a fourty-footer
with a 15' shaft, the amount of movement and misalignment in the
system that it can tolerate without causing damage declines dramatically.
At this point, the thickness and mass of both the
shaft, propellor and the drive load all begins to play an increasingly
important role. The reason is simply because small diameter shafts
are much more elastic than large diameter shafts. All boat shafting
systems are self-centering to some degree, but the larger the diameter
shaft, the less self-centering it becomes. The shaft does not bend
but transfers all that misdirected energy (vibration) to other components
such as struts, bearings, transmission and the engine mounts.
How to Check Your System
- The first thing to do is to check the engine mounts. If they
are the vertical stud type set in a rubber base, these are the
kind that are prone to rocking back and fourth. Check the stud
to make sure that its centered in the base with the vessel at
rest. If its leaning in any direction, the mount is stressed
and the system is out of alignment (see photo #2).
- Conduct a back down test. One engine at a time, start from
neutral, put the engine in gear and accelerate hard, up to no
more than 1/3 throttle. Repeat this process in reverse, all
the while watching the mounts for movement. If the engine and
mount are moving more than 1/8" in any direction, your
mounts are not doing their job of holding the engine in place.
- Observe the shafts while running at two speeds, idle and cruise.
Observation of shaft runout will only prove the negative; out
of line shafts can appear to run true even though out of line.
On the other hand, a badly wobbling shaft means something's
wrong. Runout up 1/8" is acceptable at idle speeds but
not at cruise. If shaft wobble is visible over 1200 RPM, suspect
a problem. Again, that's becasue rotating shafts tend toward
self-centering. At high speed enven bent shafts can straighten
out and show no sign of trouble. If the shaft is observably
causing the transmission to move, or you can feel the movement
by putting your hand on it, then there's definitely a problem.
- Many boat yards think that the way to check shaft alignment
is to disconnect the coupling and check the flange clearance
with a feeler gauge. That's only part of the story. Before doing
this, with the vessel hauled, check the position of the shafts
relative to the bearings. Cutless bearings that are worn more
on one side than another are a positive indicator that something
is out of alignment. Heavy shafts of 1.5" or more will
naturally compress the rubber bearing on the bottom side, but
not so much that there's an obvious gap at the top. Photo #1
illustrates a clearly misaligned shaft.
- Next, check the shaft-to-bearing alignment at both the front
and back ends of the bearing. If the shaft is off-centered,
either to one side, top or bottom at one end, but is off-centered
to the opposite side at the other end, then the shaft is not
parallel with the bearing bore. It is either up or down, or
off to one side. In this case, the whole system should be suspect,
at which point the entire system alignment must be redone, including
setting up target wire to make sure that the struts themselves
are properly aligned.
- Now check how the shaft is centered with the opening in the
stuffing box flange. If there is little clearance (shaft in
the opening), it should be exactly centered; if a lot of clearance,
it can be off a little without causing harm.
-
 Judging
by the large gap at the top, his bearing looks like it is worn.
Actually it is new but the shaft is badly misaligned with the strut.
This is determined because the gaps are on the opposite side of
the bearing at the front side of the strut. Thus, the shaft is "cocked"
in the bearing.
When there is significant misalignment involved,
the struts and strut bolts should be checked. To do this, I usually
pick a heavy piece of shoring found in most boat yards and give
the strut several good whacks. If the strut deflects or shudders,
its not stable. If water squirts out from the base, its loose. Also
note whether the whole bottom of the hull is defecting when you
hit the strut. If so, there's no point in realigning anything unless
you first do something to make the strut base stable. If the struts
are fluttering because the bottom is weak, the entire system is
unstable and must be corrected.
Dial Indicating Shafts Recently a number
of articles have appeared in boating publications about dial indicating
shafts while in the boat. I'll explain why this should never be
done. As noted above, boat hulls can and do change shape over time.
Further, the shape of the hull changes when hauled versus afloat.
The changing shape of the hull is likely to cause the shaft
to bend. Now, you may look at a 1.5" stainless shaft and think
that it's cannot be easily bent. In fact, if you suspend it on its
ends, even gravity will bend it considerably. Gravity will bend
a 15' shaft a lot.
Finally, when you rotate the shaft by pulling on
the propeller, you're moving the shaft against the rubber bearing,
further throwing your dial indicator readings off. As any trained
machinist will tell you, there's only one way to dial a shaft and
that is on a calibrated roller bed. One simply cannot dial a rotating
object without a reference base and expect accurate results. And
a shaft in a boat doesn't provide such a reference.
Only after all the above checks have been made
and proven that all else is in order, should one then procede to
use the feeler gauges on the coupling. The problem with using feeler
gauges is that this measurement can give a false indication that
all is well. Particularly smaller shafts can be out of alignment
at one end and yet still give a favorable reading at the coupling
end. This is because shafts with multiple bearings can bend it into
a favorable position while overall it is still badly out of alignment.
Weak Struts This is a very common problem
on small to midsize boats. If you want to understand just how important
strong struts are, just take a look at the huge struts used on larger
Bertrams, Hatteras or Viking yachts. They don't spend all that money
on massive struts for no reason. When there are hundreds of thousands
of dollars at stake in machinery, the drive support system must
be strong.
A futher fact to consider is that the smaller and
less the value of the boat is, the less the amount real engineering
has gone into its design. The drive system is one area that is often
shortchanged.
Take a close look at the strut arm length to strut
base ratio. Or, in otherwords, the ratio of the base footprint to
the strut length. Also look at the strut width and cross-sectional
thickness. In photograph #3, notice how long the strut arm is and
how narrow the width is. The arm to width ratio is over 3:1 while
the length to base ratio is more that 2:1. The extreme amount of
leverage that the arm applies to the base means that it can't possibly
hold the shaft steady. It will deflect and allow the shaft and propeller
to oscillate.
 The
strut in photo #3 carries the same size shaft and prop as in #2,
both on a 40' boat. But the later is double the thickness in both
width and cross-sectional area, with a substantiallly wider base.
There is no doubt that this strut will hold the shaft steady. Also
notice that the length of the strut bore is double the length of
that in photo #2.
This strut will hold the shaft very steady, and
because of this, correct engine/shaft alignment is much more critical.
If its not correctly aligned, something will have to give, and whatever
that is is likely to be expensive. The same holds true for shafts
with double struts. In fact, a double set of struts will hold a
shaft in even closer tolerance. As a general rule, shafts under
1.5" tend to be self-centering because they will bend so some
small degree. But shafts larger than this do not take well to bending.
They will not bend but work to tear the system apart if not correctly
aligned.
Another general rule is that the larger the shaft
diameter and the more bearings it has, the more critical are the
engine mounts. Unless the mounts are holding in the engine in place
and not permitting any movement, there's no point in trying to achieve
proper alignment of anything. If the engine moves, no other part
of the system will stay in alignment either.
 Still
another source of engine mount, strut bearing, vibration and shafting
problems results from engines that are mounted on weak stringers,
or on a hull bottom that is flexing. There are literally thousands
of boats that have this problem. During the course of sea trials
I frequently encounter engines that are shuddering and bouncing
around to such a degree that the entire drive system is being damaged,
from engine mounts all the way back to the strut bearings.
Engines mounted on weak stringers frequently results
in transmission failures. This is because the worse part of the
misalignment is being absorbed by the transmission coupling and
output shaft bearings. The problem is usually blamed on the transmission
when the source of the problem is really the hull itself. For boats
that have had a history of transmission failures, this is something
that should be checked first, before blaming the machinery or faulty
repairs.
Only after checking all of the above points and
making sure that all is in order should the transmission to shaft
coupling alignment be checked.
Posted in 1997
|
