Propeller school
We will here explain the most common propeller terms, their shape and how they work. There are a lot of literature to be found in the subject for the ones who would like to go deeper, but we will try to keep it simple and understandable for everyone out there.
Common
Although a propeller looks like a screw, known for many as a popular
explanation, it does not work like one. The propellerblades are shaped
like wings, and like those three crucial properties, pressure, thrust and
resistance. The wings are mounted to a rotating hub where the common
thrust draws water upstream and then throw it out downstream of the
boat's wake. The law of action and reaction, Newton's third, makes the
boat run forwards. The opposition which the wings are in the water,
making the need for an enginepower to keep the revs up. The water
hits the blade leading edge in a certain angle, depending on the
propeller speed, and the boat's speed. Other parts affecting the
oncoming water are for instance the boat hull and the gearhousing.
Diameter
The diameter of the propellers normalit measured in inches (1 "= 25.4 mm) and is a circle formed by the tip of the propeller blade as it rotates.
Most propellers are marked with diameter x pitch (for example 13" x 21").
Pitch
The pitch tells you how far the propeller advances in one turn. The pitch
is measured in inches and decides the angle of which the twisted airfoil
surface is based upon. It should be noted that a propeller pitch does not
need to be the same over the blade, but it appears that the pitch varies.
Since it is the pitch which provides the blade angle of attack, it is not
unusual for the pitch to be reduced at the blade root, this because
oncoming water has lower velocity at the hub.
No of blades
There are today mainly 2, 3, 4 and 5 bladed propellers on the market. More blades for better grip in the water, but it also increases the resistance. The 2-blade propellers are used exclusively on the smaller engines in the range of 2-5 hp. 3 and 4 bladed, it is most commonly used in pleasure boats, while the 5-bladed mainly occurs on boats for water sports such as wakeboarding, water skiing and similar, they are considered to provide a better wake to ride on.
Rake
Cupping
Blueprinting
To get your new propeller blueprinted you send it back to the manufacturer
or a propeller specialist who performs fine adjustments to the form as it
have been stated in the manufacturing drawing. This is mainly valid for
propellers made of metal. Deviations in the shape of the blades and angles
are from the manufacturing process where as always occurring tolerances.
Typically, these tolerances are within an range that works just fine for
normal performance and usage.
Conclusion
As the previous discussion, it is the lift of the airfoil that provides the
propulsive force, while the resistance requires enginepower. There
are however, a characteristic of airfoils which may be used to maximize
the efficiency and it is having the right angle of attack to the water. For
a given speed and rpm is the propeller pitch which should be amended
to obtain a more economical propulsion.