Are Good Vibes"
How to eliminate excessive engine and drivetrain vibration.
By Capt. Alan Ross Hugenot
August 2002 Sea
The Beach Boys celebrated "Good
Vibrations" in song. But for power boaters, there's no such
thing as "good" vibration.
Not only is excessive vibration
irritating (nobody likes the experience of uncontrollable
chattering teeth), but it also indicates problems with your
propulsion system or running gear. Getting rid of this unwanted
shaking is so important to boaters, many companies have sprung
up that specialize in anti-vibration systems - and sell nothing
The first thiing you need to do
to eliminatevibration is to determine exactly what is causing
it. We can do all kinds of things inside the hull to quiet
down an engine and shaft. We can add resilient mountings,
constant velocity joints, flexible feet and couplings - but
if the "knock" is coming from a wrong-sized propeller, we
will not get rid of those vibration problems until we go outside
the boat and change the wheel.
Figuring out what is causing the
vibration involves a simple process of elimination. Vibration
is almost always caused by one of four things: the engine,
the shaft, the shaft supports or the propeller. In my experience,
I have found that most boats' vibration problems stem from
the propeller - but it is easier to look at the other areas
Shake and Shimmy No More
Getting to the root of a vibration
problem often requires some detective work. You may have checked
your prop and your engines - but did you look UNDER them?
Old or worn engine mounts just
might be your problem. New flexible engine mounts (available
from Vetus, Aquadrive, R & D and other manufacturers) are
designed to be rigid enough to absorb forward and reverse
propeller thrust, yet are soft enough to isolate drivetrain
and engine vibration from the hull.
Next, take a look past the propeller,
at the prop shaft. Flexible shaft couplings, such as those
made by R & D, fit between the transmission output flange
and the existing shaft coupling, helping to eliminate vibration.
A flexible coupling disc creates a barrier that absorbs vibration
and compensates for temporary misalignment of the shaft and
Constant velocity (CV) joint axles,
such as those used in the Aquadrive anti-vibration system,
can also be used to dampen engine vibration. They also eliminate
the need for absolutely precise alignment. Aquadrive uses
a thrust unit on the load-bearing hull section to reduce stress
on transmission and engine mounts, so that the propeller's
action pushes the boat instead of the engine.
Manufacturers report that these
tactics can produce some remarkable results. N.A. D'Arcy Co.,
which sells the Aquadrive anti-vibration system, claims that
most installations result in a 50 percent or greater reduction
in cabin or cockpit noise and vibration.
Is this where your problem is?
Here's how to find out. First, run a dockside test. With securely
tied spring lines, after the engine is warmed up under a partial
load, reduce the speed and shift into neutral. Then, run the
engine up to maximun rpm in neutral and check for vibnition.
By now, you will have found out
whether your engine is misfiring. If it is misfiring, that
means it needs a tune-up (it it is a gasoline engine) or it
needs to have its injectors cleaned (if it is a diesel).
The motor mounts are the next places
to look. Just one loose motor mount can cause engine vibration,
so you must make sure they are all tightened. Unless they
are loose, motor mounts are not usually where the vibration
If the engine vibrated when it
was under load and did not vibrate when it was in neutral,
the vibration problem is not coming from the engine. There
are only three other things that could be causing it: engine
misalignment, the propeller shaft or the propeller.
Engine and Shaft Alignment
To test your engine alignment,
run the engine at idle and shift it in and out of gear. While
doing so, observe the rotating shaft coupling.
What you should see is smooth turning,
with no "wobble." If it wobbles when you place it in gear
with any kind of "out-of-round" movement, you need professional
engine and shaft alignment.
The alignment is usually to within
.003 inch. If your boat has "settled" over the years, you
may need flexible couplings, a constant velocity joint or
even a thrust bearing installation, to overcome your boat's
For a further check, loosen the
coupling bolts. If the shaft coupling flange does not appear
parallel to the transmission coupling flange, then it is definitely
out of alignment.
Bent Shafts And Bad Bearings
With the engine off, turn the shaft
by hand. If you see more than a 1/4 inch wobble, the shaft
With modern short shaft installations,
most boats no longer have thrust bearings. If you DO have
thrust bearings, here's how to find out if they're still good:
If the shaft has more than 1/16 inch of "play" movement, it
is time for a new bearing.
In my experience, if the bearing
is worn, it is usually because the shaft is bent. I usually
measure for a bent shaft with a flashlight. When the shaft
is turning under load, I watch to see how much wobble there
is in it.
If the bearing is absorbing more
than a 1/16 inch movement, the shaft is bent - and the bearing
probably needs to be replaced, as well. If a bent shaft is
indicated, remove the coupling bolts and hack off the coupling
from the transmission -just enough to insert a feeler gauge.
Rotate the shaft and measure the clearance with the feeler
gauge at every 90 degrees.
Be sure to recheck the original
location, once you get all the way around, to verify that
you did not back the shaft out farther during the rotation.
If you found any of the test locations to be tighter or looser,
you have a bent shaft.
Cutlass bearings can be checked
while the boat is in or out of the water. Simply push up on
the shaft and see how much play it has. If it moves more than
1/16 inch, your cutlass bearing will need to be replaced.
Worn cutlass bearings result in
vibration, which can cause damage to the shaft itself, the
shaft coupling and the keyway.
The prop is where 80 percent of
all vibration problems occur. Vibration at the prop can be
caused by one of several different problems. The propeller
can be out of balance, have a worn hub, have inadequate blade
area or have insufficient tip clearance.
Out-of-balanec propellers and props
with worn hubs wobble on the shaft as they turn. The easiest
way to observe this is under water - with a dive mask and
fins - while the boat is in relatively clear water. Be careful
to stay away from that propeller while it is turning.
Another good test is to remove
the propeller, then run the engine up to its full rpm. If
the vibration disappears, the problem is definitely with your
Take your poor-performing prop
to a professional propeller shop, where it can he balanced
and the huh can he rebuilt to tlie original specs. If you
have a feathering propeller, the only way to inspect it is
to remove it and take it to a propeller shop.
Cavitation is another common cause
of vibration. The most overlooked area of propeller design
is adequate blade area. If there is too little area, the propeller
blades become overloaded - causing cavitation, blade erosion
Propeller tip noise is also common
- as well as shaft noise from bearings, and poor alignment.
Tip noise comes mostly from having
insufficient blade tip clearance. A propeller actually works
just like a sail, creating lift on the leading edge, which
"pulls" the propeller forward. The blade tip has
a tip vortice just like any airfoil or hydrofoil - and this
excess energy is swept away by the passing water. However,
if the tip is too close to the hull, the energy will "slap"
the hull and make a real "knock."
The distance from the tip of the
blade to the hull should be 0.15 to 0.2 times the blade diameter.
If it is less than that, you will almost always experience
knock and vibration. Have your teeth stopped chattering now?