the prop.) A more accurate method is to
record the vibration or noise on a small
cassette recorder, using a contact mike
capable of recording quite low frequencies.
Play the cassette back into an oscilloscope or
frequency analyzer, and you literally can get a
better picture of what’s happening. This
sounds rather high-tech, but it’s simple in
principle.
Say the unwanted noise is occurring
at 2,200 revolutions per minute (rpm), and
the boat has a 2:1 gear and 3-bladed props.
The frequency for shaft noise would simply
be 2,200 rpm ÷2, or 1,100 shaft rpm, which
would be 1,100 cycles per minute (cpm).
Divide by 60 to get 18.33 cycles per second
(cps), or hertz (Hz).
Propeller tip noise would be 1,100 rpm
shaft speed ×3 blades =3,300 cpm, or 55 cps
or Hz.
You can see that there’s a big difference
in frequency, so usually it’s fairly easy to dis-
tinguish between shaft or bearing and tip
noise. The exception would be if one prop
blade was damaged. Then the frequency for
this vibration would be the same as shaft
speed. Regardless, this isn’t advanced electri-
cal engineering. All you need is a simple
shareware oscilloscope emulator program on
a laptop computer.Number of Blades and
Curing Prop Noise
When you run into propeller vibration prob-
lems due to inadequate tip clearance or poor
water flow to the prop, often the solution is
to increase the number of blades, in addition
to reducing diameter to increase tip clear-
ance. Increasing the number of blades offsets
the loss in blade area from the loss in diame-
ter. Having more blades also reduces the
vortex energy from each blade. Roughly, each
tip vortex from a 4-blader will have just three-
fourths the energy of a tip vortex from a
3-blader, while a 5-blader would have three-
fifths the energy of a 3-blader and four-fifths
the energy of a 4-blader in each tip vortex.
Since the tip vortices dissipate their energy
as the square of the distance from the tips,
the combination of reducing diameter some-
what and increasing the number of bladescan make a significant difference. In addition,
more blades generate higher-frequency noise,
which resonates in the hull structure less
effectively.
Of course, the new propeller has to be
fully calculated using standard methods. A
reduction in diameter would mean a corre-
sponding increase in pitch—in many cases, too
much. In such instances, a new gear may be
required. Actually, the calculations of propeller
dimensions, including pitch, diameter, power,
rpm, blade area, and boat speed, are all inex-
tricably interrelated. The rough rule of thumb
is that for every inch (25 mm) you decrease di-
ameter, pitch should be increased by 2 inches
(50 mm), and vice versa. If in an existing
installation you reduce diameter by 2 inches
(50 mm), this simple rule of thumb would indi-
cate a 4-inch (100 mm) increase in pitch.
This, however, could very well increase
the pitch ratio of the propeller (pitch divided
by diameter) beyond what is suitable for the
operational speed of the boat. In this case the
blades would enter the water flow at too
steep an angle of attack and therefore stall
(cease to generate lift). This is exactly like
an airplane wing stalling in too steep a climb.
Accordingly—though pitch is not the subject
we’re dealing with here—you must keep in
mind that the smaller-diameter propeller
must be recalculated using standard meth-
ods, and a significant change in diameter,
pitch, or both (they are related) may require a
different reduction gear to keep the pitch, di-
ameter, power, rpm, blade area, and boat-
speed relationship in balance.Propeller Tunnel and Pocket
Noise
Another cause of propeller vibration and
noise is propeller pockets and tunnels in hull
bottoms. I make a distinction between a
pocket and a tunnel, the former being a fairly
shallow recess into an underbody, while the
latter is quite deep, with more than 50 percent
of the propeller tucked up into it. Figure 1-6
shows the exceptionally deep tunnels in one
of my ultra-shoal tunnel-drive motor cruisers.
Twomblydraws just 22 inches on 42 feet LOA
(56 cm on 12.8 m LOA) and is fully beachable,
yet ocean capable.PART ONE: DRIVETRAIN INSTALLATIONS