Victoria Propellers |
Retail Outlet
1650 Belford
Nanaimo, B.C. Canada |
Repair Shop
3385 Trans Canada Hwy
Cobble Hill, B.C. Canada |
Phone: We take telephone orders and ship anywhere.
Cobble Hill: 250.743.8151
Nanaimo: 250.753.9700
Toll Free: 1.800.665.0616 |
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Propeller
101
General Considerations
Theroy of Propeller
Action
Propeller Slip
Selection
of Efficient Shaft Speed
Blade Contours
Galvanic Corrosion
Good Practice
Galvanic Series
Rules of Thumb
General
Considerations
There are few products in such common use as
marine propellers and yet often so little understood.
We shall try to provide in non-technical fashion, an
understanding of how a propeller functions.
The 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 a form of effort, namely 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 and dimensions.
- Propeller diameter is the diameter of a circle circumscribing the tips of
the propeller blades. (See Fig. 1) It is equal to twice the distance from the shaft
centerline to the tip of one blade.
- Propeller pitch
is a linear dimension usually expressed in inches
or feet, 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.
Propellers commonly have three blades, but may
be built with two, four, five or more blades for special
purposes. Propellers are either right hand or left hand
turning, depending on the direction of rotation of the
engine. Direction of rotation has no effect upon performance
as far as speed or engine load are concerned.
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Theroy
of Propeller Action
The 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.
The propeller blade is given a shape such that when
rotated in water, it acts like a pump and pushes a mass
of water astern. Actually it sucks water from ahead,
gives it an acceleration as it passes through the propeller
disc and discharges it astern. It is, in other words,
a pump without a casing, operating submerged in the
fluid it is pumping. A 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, because it is submerged and is
a rotating device, does so continuously. Top
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Propeller Slip
We 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.
The 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.
Slip 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.
High-speed 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 high slip.
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Selection
of Efficient Shaft Speed
Since 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.
In 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 shaft speed.
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Blade Contours
The 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
drives.
Constantly 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.
Some 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 1/3%.
Modifications 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.
Narrower 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.
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Galvanic Corrosion
Galvanic corrosion results from the
fact that when dissimilar metals are in contact with
one another in salt water, one of them suffers excessive
corrosion. It follows that marine propellers, rudders,
shafts, fastenings and other underwater fittings may
be subject to galvanic corrosion.
In recognizing galvanic corrosion as distinguished
from the ordinary corrosion of metals in salt water,
it is important to remember two requisite conditions
for corrosion due to galvanic action: There must be
dissimilar metals located near to each other, and there
must be an electrical (metallic) connection between
them.
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Good Practice
- Select metals listed as closely as possible to each other in the galvanic series.
- 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. Top Of Page
Galvanic Series
Corroded End (less noble)
Magnesium
Zinc
Aluminum
Cadmium
Steel
Cast
Iron
Stainless (active)
Lead Tin
Nickel
Brasses
Copper
Bronze
Monel
Stainless (passive)
Silver
Gold
Protected End (more noble)
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Rules of Thumb (Reprinted
Courtesy of the Propeller Handbook by Dave Gerr)
There 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.
1.One 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.
2. The 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 pitch possible.
3. Too 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.
4. Every 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.
5. A 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.
6. The 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
common ground.
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