next time you have a vibration problem, before you automatically
go sending the props off to the prop shop, or call for an engine
alignment, check out the entire system so you don't end up fixing
what doesn't need to be fixed.
alignment is a subject that is little understood and often neglected.
Most boat owners aren't even aware that engine alignment is one
of those things that requires occasional maintenance, and is one
of the last things to be considered when some kind of drive system
problem develops. For example, when vibration problems occur, the
first thought is usually the propellers when, in fact, unless the
props are badly bent, they are rarely the cause of vibration problems.
More often than not, the source of the problem lies elsewhere. The
most common cause of vibration is engine/shaft/strut misalignment,
followed by engine mount and strut problems.
The subject of alignment is really not very complicated,
but it does involve several other factors which most people are
not aware of. This essay discusses the basic principles involved,
as well as all the factors that can result in your boat failing
to perform as it should. Anyone can understand it, and by taking
the time to study the issue, you will not only know what to look
for, but can save yourself a lot of money by heading off problems
before they develop. Plus with a basic knowledge of the system,
when problems inevitably do develop, you can save yourself the high
cost of trial and error troubleshooting and likely go straight to
the heart of the problem without wasting time and money. Here's
a short list of the problems that can be caused by engine/shaft
- Rapid cutlass bearing wear.
- Misaligned strut galls shaft, requiring shaft replacement.
- 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.
- Vibration causes damage to other systems.
- 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, flooding or sinking the boat.
- Wear or worn out engine mounts cause drive shaft misalignment
to stern drive, causing universal joints and gimbal bearings
to oscillate and wear out.
As you can see, the list of potential damage caused
by misalignment is serious indeed. The good news is that drive systems
tolerate a lot of abuse and are very forgiving. The bad news is
that a lack of general understanding of these systems often translates
into more abuse than the systems can withstand.
|This shaft is badly misaligned
in the bearing.
||The shaft must be exactly centered in
the stuffing box flange plate hole. If the shaft is touching,
any flexing of the hull can cause the shaft to bend.
The Nature of Inboard Drive Systems
It is a common belief that engines and shaft couplings
have to be aligned to tolerances of a few thousandths. There is
some misunderstanding about this. Yes, the shaft coupling to transmission
coupling needs to fit within several thousandths, but what were
talking here is the coupling fit, not the shaft alignment.
This is an important distinction that is often confused. I'll explain
A conventional shafting system is essentially a
free-floating, semi self-aligning system. How's that? Well, because
the engine is mounted on rubber mounts, and the shaft is mounted
in rubber bearings. Of course rubber being soft, that means that
both the engine and shaft can and do move. See my point here? Since
neither the shaft nor the engine is held rigidly in place, there's
not much point in attempting to perfectly align the shaft with the
engine, is there? No, because if everything is approximately aligned,
the shaft will tend to self-center as a result of centrifugal force.
The fact is that conventional shafting systems will tolerate a great
deal of intolerance because of these factors.
The truth is that it is virtually impossible to
correctly align engine and shaft with the methods that are commonly
used. Because of the rubber mounts, the engine will not be in the
same position when running as it is when stopped, when the alignment
is made. Propeller thrust and engine torque will cause the engine
to change position. And since the weight of the shaft sitting on
rubber cutless bearings causes the rubber to compress, the shaft
is not in alignment with the bearings anyway. When the boat is running
and the propeller spinning, the shaft will align itself (but not
if the basic alignment is out).
The last influencing factor stems from the reality
that boat hulls themselves are rarely ever completely rigid; bottoms
flex and hulls change shape. After all, they are plastic and they're
not supposed to be completely rigid. Even if the engine stringers
are completely rigid (which they usually aren't) the shaft goes
through the bottom of the hull, which flexes, and most likely the
point where the struts are mounted does to. The end result is that
the entire drive train on the average boat moves around a lot, often
in tolerances as great as =/- 1/4" for a total movement as
much as 1/2" !!!
So now you see why when you pay all that money
to a guy with a feeler gauge trying to make an alignment within
a few thousandths of an inch is a waste of time. It can't be done.
Add to this the factor of the boat hull not being rigid and its
not hard to see the futility of it all.
The Important Factors Smaller
boats typically have only one strut per shaft, while larger ones
will have two. Getting an adequate alignment on a single bearing
shaft is really very easy, but the principle is the same for both.
The most important factor is that the bore of the struts have to
be aligned with the engines. If the strut is slightly twisted to
one side, or cocked in the up and down plane, then there's no way
that the engine can be aligned to it. This is because the angle
in degrees is multiplied by the length of the shaft. Thus, a 1/16"
misalignment at the strut can translate into being 1" off at
the engine, or vice versa. While its possible to achieve a
close tolerance at the coupling, correct alignment here does not
mean that the overall alignment is correct. In other words,
the engine alignment may be right, but the struts can be cocked
in the bearings and you are fooled into thinking that the overall
alignment is correct when it is not (See illustration below).
But that's not all. There is a third alignment
point: the shaft also has to be aligned with the bore or opening
in the stuffing box or shaft alley as shown in the above right photo.
This is the opening where the shaft exits the hull. Typically there
is about 1/4' total clearance here (1/8" all around). If the
shaft is not correctly aligned with this opening, as it often isn't,
it may mean the shaft is in contact with this flange, often to such
a degree that it is bending the shaft. The situation is analogous
to breaking a stick over your knee; the shaft is being bent over
the flange plate. Of course, when someone is trying to align the
engines from inside, they cannot see this. This is why attempts
to align the engines can end up causing more problems than it solves.
Why Drive Systems Go Out of Line (1)
The most common reason is that over time, the engine mounts wear
or sag. Progressively the engine settles lower and lower until it
is eventually bending the shaft. (2) Hull changes shape. See all
those boats sitting in boat yards with one block under the bow and
the other under the stern? Supported at only the ends, would you
suppose that the hull is sagging just a bit? Sure it is, and that's
throwing the system out of alignment. (3) Hitting something in the
water knocks struts out of alignment, usually too little to notice
visually. (4) Engine mounts are too weak and permit too much engine
movement. (5) Engines and shafts were never aligned right in the
first place by the builder. This is more common than you might think.
A General Rule The longer
the shaft and the larger the diameter, the more critical proper
alignment is. That's because larger shafts tend less toward self-centering
than small ones because they're more rigid. Smaller boats with 1-1/4"
and under shafts can tolerate quite a bit of misalignment so long
as the shaft is not being bent around the hull opening flange. At
2" the system should be aligned within 1/16"; if that
sounds like a lot, consider that that is 0.0625" which quite
difficult to measure at a cutlass bearing. Its also very difficult
to align a strut any closer than this. The real beauty of these
systems is that the shaft will align itself as it wears into a new
bearing, eliminating this meager 1/16" intolerance. Another
aspect of this rule is that two bearing shafts require more exact
alignment than single bearing shafts.
|This type of mount is one of the least
effective types. Simply a threaded stud in a rubber doughnut,
it allows 360 degree movement. Note also that the long stud
can create a substantial lever if the engine is mounted up
toward the top. Over time they sag substantially, throwing
the engine our of alignment. It is supplied standard with
Mercruiser and Caterpillar engines.
||This is what often happens
with this type of mount. Note that its badly cocked to the
left. When the engine is shifted, the mount allows it to move
a full one inch. Not much point in bothering with alignment
under these conditions.
Weak Engine Mounts Top quality
engine mounts are expensive, hence the motivation by the builder
to use the cheapest mount possible. Weak engine mounts mean that
your engines will never align properly because they constantly permit
engine movement, so how can a shaft be aligned to a moving engine?
The procedure for determining whether the engine mounts are holding
the engines steady is very simple, it is called the Back Down Test.
If the mounts are the vertical stud type set in
rubber in an aluminum base, these are the kind that are prone to
rocking back and forth, particularly with heavy diesel. 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 photos above ).
Conduct a back down test. It takes two people to
do this, one person operating and one observing the mounts and engine.
Observe the point where the engine bracket attaches to the mount.
One engine at a time, start from neutral, put the engine in gear
and then accelerate hard, up to about 1/2 throttle for several seconds.
Stop, then repeat this process in reverse, all the while watching
the mounts for movement. Repeat several times. 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.
While underway, another thing to check is the shaft
runout at the stuffing box. 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 (you are observing only one end of the shaft).
On the other hand, a badly wobbling shaft means something's wrong.
Runout up to 1/8" is acceptable on shafts 1-1/2" and under
at idle speeds but not at cruise. If shaft wobble is visible at
1200 RPM and over, suspect a problem. Again, that's because rotating
shafts tend toward self-centering. At high speed even bent bent
shafts can straighten out and show no sign of trouble. Shafts 1-1/2"
and over won't tolerate bending and can cause transmission damage.
If the shaft is observably causing the transmission to move (oscillate),
or you can feel the movement by putting your hand on it, then there's
definitely a problem.
||A preferred style, the Ace mount permits
very little engine movement. At the least, this type should
be used for the rear engine mounts. The engine bracket should
be as low on the stud as possible so as not to provide excessive
leverage to the mount.
||This puny little mount on a diesel
engine is totally inadequate and was responsible for transmission
and shaft damage.
With the boat out of the water, you can check the
shaft alignment yourself by checking how the shaft aligns to the
cutlass bearing. Needless to say, you can only do this with a bearing
that is not worn. In anything but a brand new bearing, there is
likely to be a gap at some point where the shaft meets the rubber.
Inspect the bearing from the front and back, noting the size of
the gap and where that gap is. If the strut is correctly aligned,
the gap should be at the top at both ends. If the gap appears at
opposite positions -- either side-to-side, or top-to-bottom -- the
the strut is cocked and out of alignment. See the illustration below.
The reason a slight gap appears at the top on a correctly aligned
shaft is due to the weight of the shaft compressing the rubber.
If that gap is excessive, and if the rubber at the bottom is badly
compressed, then the bearing is either worn or the rubber is very
Vee Drives Boats with vee
drives are the least intolerant to misalignment or weaknesses in
the drive system. This is because the shafts are very short and
reverse direction. Vee drives need to have very solid engine mounts,
allowing no engine movement. Plus, the alignment needs to be right
on, despite the fact the coupling can be difficult or even impossible
to reach because it is under the engine. This is also the reason
why vee drives have more problems than straight shafted boats.
|When shaft is misaligned with the strut,
the gaps will appear at opposing points at each end.
Testing for Bearing Wear This
is very easy. Simply lift up on the end of the shaft as hard as
you can. Use a long board as a lever if necessary. If the shaft
moves up and down, its worn and is ready for replacement. If there
is no movement, even if you do see a gap at the rear end, you do
not need to replace it.
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,
particularly when it is hauled. 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. Therefore, aligning engines
to the shaft must always be done with the boat afloat.
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 proceed 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.
Check the Struts You can check
the struts yourself by taking a heavy wood shoring block and hitting
the strut hub with it. Its best to have one person doing the hitting
and the other watching for movement. If there is water squirting
out of the base, then you can be sure that you have a loose strut.
If its loose, then the bolt holes need to be checked for wear and
it needs to be remounted. Also note whether the hull bottom is deflecting
when you hit the strut. If it is, you have a problem.
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 tens of thousands
of dollars at stake in machinery, the drive support system must
A further 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
Take a close look at the strut arm length to strut
base ratio as shown below. In other words, the ratio of the
base footprint to the strut length. Then look at the strut width
and cross-sectional thickness. In the photograph at left, 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.
no way this long, thin strut is going to prevent the propeller
from fluttering and causing vibration. The shaft is also
too long and extends beyond the strut by 3". Distance
between strut and prop hub should be no more than 50% of
||Both these boats have
the same size propeller. The strut on this Hatteras is three
times larger with a mount base the same width as the strut
The strut in photo at right carries the same size
shaft and prop, both on a 40' boat. But the later is double the
thickness in both width and cross-sectional area, with a substantially
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 the other, which allows a much longer bearing that
will wear more slowly.
That large strut will hold the shaft very steady,
and because of this, correct engine/shaft alignment is much more
critical. That's because the bigger struts create a more rigid system.
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
Weak struts that are fluttering while underway
are one common cause of vibration problems that don't go away, no
matter what you do. Keep in mind that the two propellers are pushing
the entire weight of the boat and some beef is needed there to hold
them steady. The one shown at left in the photo above is what a
weak strut typically looks like. Of course the big strut costs a
whole lot more than the skinny strut, and that's why some builders
cut the corner on this critical component. There is no solution
for this short of replacing the struts with stronger ones.
Other Unsolvable Problems
Now comes the bad news for folks that own boats that have chronic
drive system problems. Chronic problems, such as frequent transmission
failure, rapid bearing wear, vibration and stuffing boxes that continue
to leak no matter how often you repack them, are usually related
to structural hull weaknesses. There are a fairly large number of
boats that have weak bottoms. A boat with a weak bottom usually
translates to engines that are moving up and down because the bottom
is doing the same. This can be checked by taking the boat on some
rough water and measuring stringer deflection.
This can usually be accomplished by using a rigid
measuring device and placing it on top of the stringer and then
gauging the amount of movement relative to the deck above. Don't
be shocked when you see as much as 1/4" deflection, for this
is common. But when the amount of deflection rises to a 1/2"
or more, that's the amount that the moving engine is throwing the
shaft out of alignment. And when its moving that much, its not hard
to understand why transmissions break down, stuffing boxes leak
and bearings wear out quickly. Short of a massive job of strengthening
the stringers, the only thing to be done is to pass the boat onto
As you can see, inspecting your drive system takes
a bit of time, but is simple enough for just about anyone to do.
The next time you have a vibration problem, before you automatically
go sending the props off to the prop shop, or call for an engine
alignment, check out the entire system so you don't end up missing
the source of the problem and fixing what doesn't need to be fixed.