Dave Gerr - Boat Mechanical Systems Handbook-How to Design, Install, and Recognize Proper Systems in Boats

Bent Shafts

Oh yes, shafts can be bent, too. If you sus-
pect this, the simplest way to check is to
back off the coupling from the transmission
just enough to use a feeler gauge. Align as
usual; then rotate the shaft in 90-degree in-
crements. You should see no appreciable
change in the feeler gauge clearances. If you
do, either the shaft is bent or the coupling
itself is skewed.

Shaft-Whirling Vibration

There is one final calculation that’s worth do-
ing on the shaft itself: check it for whirling
vibration. Whirling vibration (also called lat-
eral vibration) occurs when the shaft rpm
matches the natural frequency of the beam
that is the shaft. This natural frequency is
largely controlled by the distance between
bearings and by the shaft diameter.
All vibration analysis is complex. Vibra-
tion occurs in frequencies that have harmon-
ics or modes. Small changes in mass and
location of components can have large
effects on the frequency (vibration period),
so all such calculations—with the limited in-
formation available for most small craft—are
approximations. Still, whirling vibration is a
useful check.
An approximate formula for estimating
the natural whirling frequency of solid, round
steel shafts (which yields accurate enough
results for all common shaft materials,
including bronze) is as follows.

Formula 1-5. Natural Whirling
Frequency of a Propeller Shaft

or

Where

cpm =cycles per minute

C = 4 .78 for first mode

C = 19 .2 for second mode

C = 43 .2 for third mode

Example: In almost all cases, the fre-

quency from the higher modes won’t be

relevant. We can quickly check the first-

mode-vibration shaft-bearing spacing from

our twin 450 hp (337 kW) sportsfisherman.

Say its engines run at 2,800 rpm, and it has a

2 :1 gear. Shaft speed is then 1,400 rpm, and it

has an Aqualoy 22 shaft, 1^3 / 4 in. (45 mm)

diameter.

Using the long form of the bearing-

spacing formula, we find that we can place

the bearings 75 inches apart. Checking this

for whirling frequency, we find

or

This is rather close to the 1,400 shaft

rpm speed—a potential vibration problem.

We can reduce the bearing spacing or in-

crease the shaft diameter to increase the

first-mode frequency and reduce the likeli-

hood of whirling-frequency vibration. For

example, if we decrease bearing separation

to 68 inches:

or

This no longer falls almost dead-on at

top-speed rpm, and cruising rpms will pro-

duce lower frequency still, so this bearing

spacing should be OK. Again, increasing shaft

diameter to, say, 2 inches (50 mm) would also

raise the natural frequency, though not as

much as this decrease in bearing spacing.

Intermediate Shaft Bearings

What happens when the shaft is too long

to be supported simply by the stern bearing

at the end and by the engine (through the

shaft coupling) forward? You have to install

Chapter 1: Propellers and Shafts

Frequency, cpm C 10

Shaft.Dia.,in.

Beari

=× ×^6

( nng Spacing,in.)^2

Frequency, cpm C 10

6 25.^4 Shaft.Dia.,mm

=× ×

×

(BBearing Spacing , mm)^2

4. 78 10

1. 75 in.

75 in.spacing

××^6 =1,48

()^2

7 7rpm

4. 78 10

25 .445mm

1,905 mm spacing

6

()

××

×

22 =1,505 rpm

4. 78 10

1. 75 in.

68 in.spacing

××^6 =1,809

()^2

rpm

4. 78 10

25 .445mm

1,727 mm spacing

××^6 ×

()^2

==1,831 rpm Formula 1-5.