When thinking of propeller efficiency, the controlling factors are really
only the thrust and drag that is developed. However, these two factors are influenced by
the number of blades, area of the blades, and to a lesser extent, the shape of the
Pitch and diameter are also generally
related, as far as efficiency is concerned. Best efficiencies usually result from a
pitch to diameter ratio of between 0.8 and 1.5, with the optimum being about 1.2 to 1.3.
It is not necessary to go into this in any great detail since propeller
"sizing" is invariably based on experience and practical tests with alternate
sizes if necessary.
It can be said that any slip in the range of
20 per cent and 80 per cent can be acceptable, although the optimum working range is
likely to be between 20 per cent and 50 per cent. A figure of 25 per cent is a good
design goal. A slip rating of less than 20 per cent is not likely to be a design
target, because of the low thrust resulting from the low angle of attack of the blades.
Propeller design is largely based on
practical experience. The requirement that each blade of a propeller should be set
at an angle consistent with the geometric pitch required. This was explained earlier.
The actual angle for each part of the blade can be determined by simple geometry,
resulting in a progressive "twist" from the root to the tip.
Theoretically, at least the blade angle
required at the center is 90º, but the center is almost invariably covered by the boss or
hub, are relatively ineffective and so are cut away to a minimum width to reduce
drag. The hub provides both a fixing the propeller blades, the fixing of the
propeller to it's shaft and to form a fairing to smooth the water flow past the propeller.
How Many Blades?
As a generalization, where as a two-bladed
propeller is theoretically more efficient than a one with more blades this is commonly
offset in practice by the "masking" a propeller normally receives under working
conditions. it is carried on the end of a shaft, supported by a bracket or skeg,
masking part of the upper half of the propeller. The following picture shows the masking
by the strut or skeg using a two-bladed propeller.
This "masking" is more apparent with a two-bladed, as only
one blade is "unmasked" then the other is "masked". A
three-bladed propeller would always have two bladed "unmasked", and a
four-bladed propeller would always have three blades "unmasked". A
four-bladed propeller will thus usually be less efficient than a two-bladed propeller,
particularly in model sizes.
A three-bladed propeller would thus appear to
be preferred for all applications, although this does not necessarily hold true at high
speeds, particularly if higher pitches are also being used. So, what is better, two
or three blades? The lower form drag of the two-bladed propeller may have a distant
advantage, so there really is no real answer to this question. A change from a
two-bladed propeller to a three-bladed propeller of the same diameter and pitch can have a
marked effect on performance in many cases; and in others, have very little effect.
It's best to say that two-bladed propellers are to be preferred for high speed work and
racing; and a three-bladed propellers are for general applications with both gas engines
and electric motors.
Blade shape can very a lot with individual
design. The general purpose propeller usually has a generous amount of blade area.
A "sweptback" form with the leading edge longer than the trailing
edge also appears to be the most efficient form. With this shape it is often
possible to reduce actual blade area while maintaining the thrust developed and increasing
efficiency by reducing the drag. High speed and racing propellers commonly sacrifice
some blade area to reduce drag, with further variations on blade shape.
Although inherently less efficient than full
sized propellers, model propellers are generally designed with less blade area than their
full sized counterparts, although they may follow the same blade shape. This is
because with the small diameters involved there is a likely to be considerable
interference between the blades, further reducing efficiency. To combat this the
propeller disc is given more "open" area, generally at least 40%. This is
also desirable since model propellers normally operate at rpm figures far higher than
their full sized counterparts, again favoring more open area to minimize interference
between the blades.
This can be as important as blade shape.
In fact, a good blade section can often compensate for poor blade shape.