There are two main principals that help us understand the action of propellers;
the theory of momentum, and the theory of lift. The theory of momentum simply states
that accelerating a substance in one direction will create a force in the opposite
direction. And the theory of lift from Bernoulli’s principal, because a propeller
is essentially a circular wing or more accurately a number of wings propelled in
a circular motion, so they create forward lift.
The efficiency of a propeller is simply power output divided by power input and
can be as low as 40% for slower displacement hulls and in the high 70% for faster
planing craft. Efficiency however, varies at different speeds and RPM for the same
installation. Except at high speeds, the larger the
boat's propeller diameter and slower the RPM’S, with the least
amount of blades, is most efficient. Propeller efficiency can vary considerably
for the same vessel simply by varying the diameter and RPM. It is therefore important
to determine the most appropriate propeller and gear ratio for a particular vessel,
A propeller blade is essentially a wing, and the same theories of lift apply. The
reason for a propellers contorted shape, as opposed to a relatively straight wing,
is because it is moving in a circular motion, so the further you move out along
the blade the faster that slice of the blade is moving and therefore less pitch
is needed this results in twisting the angle of attack into a helix, see boat propeller
pitch for details.
To explain lift there are two main forces we’ll look at, high and low pressure.
For this we’ll cut a slice of the propeller blade and have a look at the cross section.
The cross section is an aerofoil comparable to the cross section of an airplane
wing. The undersurface is flat or even cupped and the top surface is cambered, is
If you think of water particles as magnets that repel each other but are attached
together by rubber bands. When a liquid moves over the propeller the camber on the
top widens and it stretches the rubber bands as the water moves faster to fill a
larger space, creating low pressure “suction”. The flat underside with its angle
of attack works to slow the water down or squash it together creating a high pressure
which respells pushing the prop forward. Because of the low pressure created ahead
it’s easier to move forward providing more efficiency. The combination of these
two forces creates the forward movement. These principals of lift, incidentally,
apply to many things for example, sails, wings, venturies, propellers, sail boat
keels, helicopter rotors, ship rudders, hydrofoil boats, etc... This is also why
a helicopter can glide in autorotation.
In spite of the fundamental similarities of lift on an air and a water propeller,
why do airplane propellers and boat propellers look so different? Besides the fact
that they operate in different substances, there are practical limitations on boats
that limit the diameter of a propeller, and boat propellers are, for a number of
reasons, placed at the stern, which means they operate in an unstable flow, the
wake. In addition, in water, unlike air, there is a limit to how much low pressure,
lift per unit blade area, can be created before capitation occurs. Boat propellers
therefore have a smaller diameter to RPM, effectively slow the blade tips down and
therefore have far more blade area to make up for it. They also tend to have more
blades to compensate for limited diameter and perform more satisfactorily in this
unstable flow by reducing vibration.