Propeller Selection at Antares
Diameter
When you consider the power the prop is expected
to transmit in relation to the shaft rotation speed
(rpm - revolutions per minute), you get the "ideal"
diameter - the higher the rpm for a given power, the
smaller the propeller diameter. For an exaggerated example
of this, consider a 300 h.p. (horse power) tug boat
with a 53" diameter propeller turning 400 rpm in
contrast to a 300 h.p. ski boat with an 18" propeller
turning 2500 rpm. Both propellers absorb the same power,
in this case, 300 h.p.
Obviously the shaft rpm is critical. The shaft rpm
is a function of the engine's ideal operating rpm (as
viewed on the instrument panel tachometer) and the reduction
ratio of the gearbox. What the gearbox does is allow
forward, neutral and reverse operation, but it is also
equipped with gears to "slow down" the propeller
shaft to an ideal speed for the propeller. As gearboxes
come in different ratios, it is important to select
the best ratio / prop diameter for the vessel's intended
operation. Typical for the type of vessels we build,
the ratio is between approximately 2:1 and 3:1.
In the example given above, the tug combination has
been maximized for best thrust at virtually no vessel
speed (low shaft rpm, large diameter), whereas the ski
boat has been maximized for the best thrust to obtain
a high vessel speed (high shaft rpm, small prop diameter).
A good analogy is the selection of choosing the right
gear in your car when climbing a hill as opposed to
cruising at highway speeds. As we have no multiple gears
to select from, we need to judge a good ratio / diameter
that will move us along efficiently under most circumstances.
Having decided on an engine and reduction ratio, the
diameter can then be approximated from simple charts.
Pitch
 As
a propeller rotates, it "screws" its way through
the water. The angle of the blades determines how far
we would expect the prop to travel forward in one rotation
of the shaft - a greater angle creates more travel.
If our props could move through solid material, the
geometry and rpm would be related directly to vessel
speed. However, as we operate in water, there is a degree
of "slippage".
Imagine the tugboat with no forward speed as the propeller
continues to move through the water, a condition of
virtually 100% slip. At the other end of the spectrum
the high-speed ski boat would operate below 20% slip
(there must always be some water pushed aft as the boat
moves forward, due to Newton's laws). Taking into account
the degree of slip appropriate for our vessel, we select
an angle for the blades and specify it in inches of
forward travel per revolution in a perfect geometric
sense. This inch dimension is the "pitch"
of the propeller.
The available power is dictated by the engine specification.
We try to select a pitch that allows the engine to operate
close to its maximum designed rpm and output. In situations
of higher vessel loads, such as headwinds, sea conditions
or extra cargo, the inability of the available engine
power to push the vessel to top speed will load the
prop in excess of our "ideal". In this situation
the operator can pull back a little on the throttle
and relieve the engine of a large percentage of its
load. This is because the effort required to turn the
propeller increases exponentially in relation to the
rpm, so in effect, a small drop in rpm results in a
large drop in engine loading.
Direction
of Rotation
With twin-engine installations, it is generally desirable
to rotate the shafts in opposite directions or "counter
rotating". This is intended to assist in maneuvering
astern primarily, and counters a tendency for the stern
to move to one side with the engines both in reverse.
The top portion of the prop operates in less dense water
than the lower portion, a phenomenon which results in
the prop trying to "climb" to the side. When
the props turn opposite to one another, this tendency
is cancelled out.
We are less concerned with maneuvering problems in
the catamaran as the drivelines are so widely spaced
- their respective thrust directions are more effective
in spinning the vessel than is the case with relatively
narrowly spaced monohull drivelines. Nonetheless, when
gearbox options permit it (the engines themselves all
turn the same way), we install counter-rotating drivelines.
The current powerboat gearboxes will permit either rotation,
so the starboard prop turns clockwise as viewed from
the stern ("right hand"), and the port propeller
turns counterclockwise ("left hand"). When
servicing the props note which prop goes on which side.
(If you find yourself facing the stern of the vessel
scratching your head, put the prop stamped "RH"
on the right side).
Number of Blades
"Why don't I have four-blade props? Wouldn't more
be better?"
The issue of blade numbers is related to two factors.
The first is the blade area ratio, which
defines the portion of the disc - defined by the diameter
- that is filled in with blade surface. In higher thrust
applications like the tugboat, the disc is filled. The
drawback is high surface drag and loss of rotational
efficiency. We select a lower blade area ratio in higher
speed vessels to balance the tradeoff and that leads
us to three blades of moderate area.
Another factor to consider is the natural vibrations
that may be set up in the rotating driveline. The powerboat
has a vertical fin shape or skeg ahead of the propeller.
The water trailing off from the skeg will have some
turbulence through which the prop blades have to travel
above and below the shaft line. It is advantageous if
one blade of a three-blade prop traverses a turbulent
zone at a time, rather than two blades of a four-blade
prop simultaneously crossing and potentially setting
up a vibration of greater magnitude. [Five blades in
this situation may be better, but will the vessel go
faster if more power is expended rotating the prop itself?]
The sailboats have the added factor of minimizing drag
while sailing. If the prop is limited to the smallest
possible blade area ratio (that offered by two skinny
blades), the loss in thrust efficiency is balanced against
not having to drag a disc through the water. When cost
and complication are not seriously limiting factors,
feathering or folding props are used to improve the
situation. The folding prop has two blades that hinge
aft and with a compromise to blade shape, are made to
nest in a semi-streamlined shape. The feathering prop
has two or three blades that twist on their axis to
knife through the water in position. The feathering
prop has the minimum compromise in the blade area but
requires a relatively complicated hub.
Shaft Size
There are three things that determine the shaft
diameter: the power it is expected to transmit,
its length between supporting bearings, and the material
it is machined from.
Power - The power load is composed of the torque
or twisting effort, multiplied by the rotational speed.
We are primarily concerned with the torque when we look
at the diameter, as the rpm has been decided when we
chose an engine and reduction gearbox. Although a calculation
might suggest that a smaller diameter would be adequate,
we don't go below 1" for practical reasons.
Lengths Between Supporting Bearings - As all
shafts have some slight runout from true (imagine a
slightly wobbly bicycle wheel), the goal is to prevent
that runout from being exaggerated by a "floppy"
or misaligned installation. We use charts as guidelines
to suggest bearing spacing for given diameters to prevent
the shaft from throwing itself around out of balance.
Machined Shafts - We are fortunate that the
Society for Automotive Engineers (S.A.E.) has established
a standard specification for the machining of the shaft
ends and propeller hubs. This means that props are interchangeable
and shafts can be ordered by their overall length with
confidence that things will fit together. The propeller
fits the shaft on an accurately machined taper. The
taper locks the propeller perfectly to the shaft and
the retaining nut then secures the assembly. If you
attempt to remove a prop by just undoing the nut, you
will find the assembly remarkably rigid and you'll need
a prop puller tool to break the propeller off the shaft
taper.
Conclusion
Going back to the original question, we hope that it
has become apparent why a bigger prop doesn't necessarily
result in going faster.
Propeller selection is a process that involves many
variables. Diameter, pitch, and reduction ratio are
interdependent and must be considered as a function
of the expected vessel speed and power. In practice,
the pitch is the factor that requires the most initial
judgment to estimate, but it readily lends itself to
later adjustments. For tuning to a specific vessel's
operating condition, props can be re-pitched (have their
blades bent to a different angle) at a service facility.
This is generally the only change that may be suggested
in the field.
Also, changes to weight loads and fore / aft trim have
a significant effect on how the installed power is used.
The less we expend on making waves and hull friction,
the more we have to contribute to speed or fuel economy.
A propeller that is specified to meet an estimated normal
operating condition will not be perfect for all conditions
but it is capable of providing good all around reliable
service.
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