The second key feature along with the diameter that defines the size of a propeller is the pitch. Like the diameter, the pitch is measured in inches and it is the one we usually change so as to allow the engine to reach the maximum rpm suggested by the motor manufacturer.
The term “Pitch” stands for the theoretical distance a propeller moves forward when it makes a complete rotation. A propeller i.e. with a pitch of 21" will travel forward 21-inches in one complete rotation.
But this is the theoretical distance, because in the real world of the sea, the propeller “slides” as it rotates, mainly due to the resistances that it has to overcome, and finally the distance traveled is actually less than its “nominal” pitch indicates.
Thus, a propeller with a pitch of 21" does not ever actually cover a distance of 21 inches in one revolution but a shorter one, because of the slip. (Figure A).
The pitch is defined by the angles of the Blade Face related to the level which is defined by the front part of the hub. As you can see on the Scheme B, A is the Leading Edge of the blade that will cut first the water when the propeller is rotated, and B is the Trailing Edge.
All the above referred terms describe the same thing. The pitch of the propeller, which is the theoretical distance that it travels when it performs a complete rotation (i.e., acting as a screw screwed into wood) while the critical characteristic, that will determine how much forward the propeller will move in one complete rotation, is the angle of the blades in relation to its body.
Given that each propeller blades are attached to the hub, this angle is standard and determines:
- How quickly the propeller will move forward (acceleration) and
- how far the propeller will go at each rotation (speed).
Both these effects are inversely proportional to each other. That is, if we quickly move ahead we will not go far, and if we go far will not move fast.
In other words, if we'll have great acceleration we'll have limited top speed, while if we have high top speed we will accelerate moderately. Therefore there is no way to simultaneously have great accelerations and the highest speed at the same time.
So it's in our hand to choose!
This is because the greater the angle of the blades is, the more the blades are in parallel with the main body of the propeller, and so the blades “grip” a greater amount of water. The more water is pushed back, the longer distance the boat will go. On the other hand, they will move equally slower because of the greater resistance of the higher amount of water that they are “gripping”.
To make the operation of the angle of the blades with the propeller axis more comprehensible, we can compare the propeller with a screw.
The more vertical the threads of the screw are to its head level, the greater distance the screw will go into the wood in one full rotation. But it will also be screwed with greater difficulty and will require more power (heavier load on our hand that screws, or if you wish, in the case of a propeller, a heavier load on the engine).
On the other hand, the more parallel the threads of the screw are to its head, the less distance the screw will cover inside the wood in a single rotation. But it will be screwed more easily because it will require less power (lighter load on our hand that screws or - in the case of the propeller - a lighter load on the engine).