There are few products in such common use as marine propellers that
are often so little understood. We shall try to provide in non-technical
fashion, an understanding of how a propeller functions.
power developed by any marine engine is available at the propeller
shaft in the form of torque, or "twisting effort." We
need something to convert this twisting effort to thrust or "push"
which can be used to drive the boat. The propeller does this job.
It will be helpful to become familiar with propeller geometry
diameter is the 'diameter' of a circle circumscribing the tips
of the propeller blades. It is equal to twice the distance from
the shaft centerline to the tip of one blade.
pitch is a linear dimension usually expressed in inches, feet,
millimeters, or meters, and is equal to the advance
of the propeller in one revolution at "zero slip". It
is exactly the same thing as the pitch of a machine screw if one
imagines the propeller to replace the screw and the water to replace
the nut into which the screw is threaded.
commonly have three blades, but may be built with two, four, five
or more blades for special purposes.
are either right hand or left hand turning, depending on the direction
of rotation of the shaft. Direction of rotation has no special
effect upon performance as far as speed or engine load are concerned.
Theory of Propeller Action
function of a propeller is to convert torque to thrust. Thrust
is another name for force, and a basic axiom of mechanics tells
us that Force (thrust) is equal to Mass times Acceleration. In
other words, if we impart an acceleration to a mass of water,
we will generate a thrust or push which will accelerate the boat
forward while the mass of water is moved in the opposite direction.
propeller blade is given a shape such that when rotated in water,
it acts like a pump and pushes a mass of water astern. It is,
in other words, a pump without a casing, operating submerged in
the fluid it is pumping.
propeller and an oar blade do the same job--both impart an acceleration
to a mass of water, except that the oar blade does so intermittently
whereas the propeller does so continuously.
have shown that the propeller, in order to generate thrust must
accelerate or move a mass of water astern. Now the propeller,
being shaped like a screw might conceivably, as it rotates, slide
through the water as a machine screw would into a nut, without
displacing any water aft. If this happened the propeller (and
the boat) would, in one revolution of the shaft, advance an amount
equal to the propeller pitch. This would be called zero slip.
But in order to produce a thrust,
we must accelerate or move some water aft, and therefore it is
apparent that the propeller will not advance the full
amount of its pitch in each revolution, but will advance some
lesser amount, depending upon how much water it accelerates astern
in the process of producing enough thrust to offset the resistance
of the boat to being driven ahead.
If the boat were tied to a dock, the propeller would not advance
at all but would generate maximum thrust because full engine power
would go into accelerating water astern. This would be called
operation at 100% slip.
term "apparent slip" is used to indicate the difference
between the theoretical speed that the boat would obtain on the
propeller pitch and the rpm of the propeller shaft, and the actual
speed of the boat.
must not be confused with efficiency that is a measure of the
percentage of engine power converted to thrust by the propeller.
We have seen that we must have slip in order to generate thrust
and the amount of slip will be proportional to the amount of thrust
required by the boat.
runabouts and fast cruisers require relatively low thrust and
therefore operate at low slip whereas tugs and other heavy vessels
require high thrust and therefore operate most efficiently at
Selection of Efficient Shaft Speed
force (thrust) is equal to mass times acceleration, it would seem
that we might get the same force whether we gave a large acceleration
to a small mass of water (small propeller turning fast), or a
small acceleration to a large mass of water (large propeller turning
slowly). In practice, however, there are other factors such as
the relation of the propeller pitch to its diameter and the energy
losses due to friction between the accelerated water and the surrounding
water that make a proper relation between boat speed and propeller
shaft speed essential to an efficient installation.
general, horsepower available and shaft speed determine the propeller
diameter, while shaft speed and boat speed determines propeller
pitch. The pitch of a propeller divided by its diameter is a term
called "pitch ratio." For example, a propeller of 20"
diameter and 20" pitch has a pitch ratio of 1.0, a diameter
of 20" and pitch of 15" a pitch ratio of .75, etc. For
best efficiency, the pitch ratio of boat propellers should be
in the range of .55 to .80 for tugs and trawlers, .65 to 1.0 for
heavy and average cruisers, .80 to 1.2 for medium and fast cruisers
and .90 to 1.5 for exceptionally fast cruisers and runabouts.
Pitch ratios outside these ranges generally will indicate an unsuitable
evolution of blade contour to its present high state of efficiency
has followed the development of marine steam turbines, high-speed
marine gasoline engines, marine diesel engines, and marine electric
increasing pitch ratios have necessitated continual advances in
blade contour and section design. The refinements, which came
as a result of this work, have been responsible for the development
of the marine propeller into the high speed, precision mechanism
that it is today.
styles have elliptical blade contour, with the maximum blade width
located from 1/2 to 2/3 the radius; both leading and trailing
edges are symmetrical. These propellers have maximum blade widths
ranging from 25% to 40% of the diameter, except over 50"
diameter, in which case widths are usually not in excess of 33
of the elliptical blade contour are employed for specific applications.
For extremely heavy work boats using low speed heavy duty engines,
modification of the blade contour with a square end places the
maximum blade width at approximately 3/4 the radius with the leading
and trailing edges practically symmetrical.
blade types usually are the choice for four and five blade propellers,
selected to avoid vibration. The total developed blade area of
such a propeller is comparable to that of a conventional three-blade
propeller. Special designs of blade contour have been developed
for light high speed, high rpm craft.
corrosion (sometimes called dissimalar metal corrosion) is the
process by which the material in contact with each other oxidises
or corrodes. There are three conditions that must exist for galvanic
corrosion to occur. First there must be two electrochemically
dissimalar metals present. Second, there must be an electrically
conductive path between the two metals. And third, there must
be a conductive path for the metal ions to move from the more
anodic metal to the more cathodic metal ( salt water for example
). If any one of these three conditions does not exist, galvanic
corrosion will not occur.
Select metals listed as closely as possible to each other in the
Insulate between dissimilar metals.
Locate dissimilar metals as far apart as practical.
Never use fasteners of less noble material than the metal parts
they are used to secure. For example, while bronze bolts might
be used to secure an iron skeg, steel bolts would corrode rapidly
if used to secure a bronze skeg.
Rules of Thumb (Reprinted Courtesy of the Propeller Handbook by
are countless rules of thumb floating around about propellers.
Some are useful and others are worthless. We will take a brief
look at a few of them.
inch in diameter absorbs the torque of two to three inches of
pitch. This is a good rough guide. Both pitch and diameter absorb
the torque generated by the engine. Diameter is, by far, the most
important factor. Thus, the ratio of 2 to 3 inches of pitch equals1
inch in diameter is a fair guide. It is no more than that, however.
You could not select a suitable propeller based only on this rule.
higher the pitch your engine can turn near top horsepower and
RPM, the faster your boat can go. This is also accurate as far
as it goes. The greater the pitch, the greater the distance your
boat will advance each revolution. Since top engine RPM is constant,
increasing pitch means more speed. Then, why aren't all propellers
as small in diameter as possible, with gigantic pitches? The answer
is simply that when the pitch gets too large, the angle of attack
of the propeller blades to the onrushing water becomes too steep
and they stall. This is exactly the same as an airplane wing's
stalling in too steep a climb. Within limits it is worthwhile,
on high-speed craft, to use the smallest diameter and the greatest
little pitch can ruin an engine. This is quite true if the pitch
and diameter combined are so low that it allows the engine to
race at speeds far over its designed top-rated RPM. Never allow
your engine to operate at more than 103 to 105 percent of top-rated
RPM. If your engine exceeds that figure, a propeller with increased
pitch or diameter is indicated.
two-inch increase in pitch will decrease engine speed by 450 RPM,
and vice versa. This is a good rough guide for moderate- to high-speed
pleasure craft, passenger vessels and crew boats. Like all rules
of thumb, though, it is no more than a rough guide.
square wheel (a propeller with exactly the same diameter and pitch)
is the most efficient. This is not true. There is nothing wrong
with a square wheel; on the other hand, there is nothing special
about it, either.
same propeller can't deliver both high speed and maximum power.
This is true. A propeller sized for high speed has a small diameter
and maximum pitch. A propeller sized for power or thrust has a
large diameter. For some boats you can compromise on an "in-between"
propeller, but for either real speed or real thrust there is little