The marine propeller is commonly called a screw, waterscrew, or screw
propeller. This is largely because of it's apparent performance of
"screwing" it's way through the water. In fact, it really does nothing of
the sort. It produces it's thrust by changing the momentum of the water passing the
circle or disc swept by the rotating blades; and by hydrodynamic "lift"
generated by the blades themselves acting in a similar manner to airfoils.
The "screw" analogy is convenient,
however, for it enables a number of important factors concerned with propeller design and
performance to be explained simply. It can be used as a straightforward method of
propeller design, and propeller selection. On the other hand, the theoretical design
of propellers based on "correct" theory is extremely complex, and far from
complete. We are still at a time where the best propeller selection is still based
on water trials. This is true for our models and also for full size craft.
An ordinary machine screw (or bolt) screwing
into a nut or tapped hole will enter, or advance, in a specific and equal amount
for each revolution of the screw. It can not do anything else, for the medium into
which it is screwing is a solid, with a matching thread to prevent any slip.
The advance of the screw per revolution is known as the pitch of the
screw. As far as "advancing" is concerned, the screw only needs one complete
turn of the tread. This will be sufficient to grip the mating thread and advance the
screw by the amount equal to it's pitch with each revolution. Again the machine
screw (or bolt) can not slip.
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The blades of a propeller form, in fact a single-turn screw. The
difference between our boat propeller is not advancing into a machined thread, but working
in a fluid medium. As a result it can not get a full grip on the medium and so the
actual advance achieved per revolution is less than the distance calculated on the basis
of the actual or geometric pitch of the propeller. The difference is known as the
slip
(Click image to enlarge)
The geometric pitch of a propeller is a
theoretical value which can be calculated from the blade angle. The actual advance
per revolution, or effective pitch can also be called hydraulic pitch, is not know or can
it be calculated directly. Yet it is by far the most important part of a propeller.
At this point the screw analogy is no longer
helpful. Having used it to define pitch and slip we can now
virtually set it aside. It is now necessary to consider just how the propeller
blades are working in a medium that allows them to "slip".
Propeller efficiency is directly related to
it's slip. An increase in slip will increase as the angle of attack of the propeller
blades increases. Slip it's self is no direct measure of efficiency of the
propeller. Thus it may appear at first that the lower the slip the more efficient
the propeller, but this is not necessarily so. A low slip will mean a low angle of attack
on the blades and in turn will mean less thrust developed. Oh the other
hand, high slip will mean a high angel of attack on the propeller blades with loss of
thrust through "stalling" (in a similar manner to an airplanes wings) and high
drag. Somewhere between the two there is an optimum value of slip which will give
the best angle of attack for the blades. This is not necessarily critical, however,
for a propeller will continue to work and develop thrust over a wide range of slip.
Another way to look at this, a propeller will work over a wide range of working angles of
attack on the blades. |
