Let’s Get into the Theory

First of all, we must know that the pitch of the propeller is ​​the distance theoretically travelled in one complete revolution by the propeller – neglecting the compliance of the fluid – and corresponds to the space that the same would travel if it moved inside a solid body, such as would make a screw in the wood.

 

A = start position

B = real position after one revolution

C = Theoretical position after one revolution

Since the propeller moves in a yielding substance, the ship’s shifting, after one revolution, will not be equal to the geometric pitch but will cover a smaller space called advance or pitch. The difference between geometric pitch and advanced pitch is called slip.

• The slip is equal to the speed of the mass of water pushed by the propeller in the opposite direction of the motion.

If the slip were null, there would be no current rejected by the propeller.

We have the greatest slip when the ship is stationary and when we “start the engine” and the propeller starts to turn, the slightest slip when the ship proceeds at cruising speed.

In formula:

• Sl = (Vt-Vr) / Vt * 100

Where:

Vt = theoretical speed

Vr = real speed.

We must make a first significant distinction between fix-pitch propellers and adjustable-blade propellers, commonly known as “variable-pitch”.

Let’s see its peculiar characteristics:

Fix-pitch propellers are very common because they are economically advantageous, have blades keyed to the hub, and it is not possible to adjust their orientation.

To reverse the direction of the ship’s motion is necessary to uncouple the axis from the engine, stop the latter, restart it in the reverse direction and re-couple the axis.

 

 

The propeller, integral with the shaft, follows it, also reversing the direction of rotation.

If a clutch does not assist the propeller shaft for the inversion of the motion, the engine is subject to starts, and there is always the possibility that these will fail; it can also happen, on old naval units, that the propellers need to have a considerable volume of air for every single start and that the available air reserve is sufficient only for a certain number of these, consequently reducing the room for manoeuvre; considering that if you lose the steering, it is necessary to intervene with a new start, it is easy to understand how limiting this can be.

Ships equipped with a clutch on the shaft solve the problem. They use one or more clutches to reverse the axis rotation direction without stopping the engine. Often, however, to allow the axis to stop rotation and switch from forward to reverse – and vice versa – it is necessary to wait a few seconds, a time that can be important in the dynamics of the manoeuvre, and it is essential to know when manoeuvring a ship with a fix-pitch propeller; whether this is a modern ship equipped with a clutch or an old-fashioned ship that has a fixed number of starts, how many seconds we need to calculate to give the command in the right times and how many starts we can use before running out of air.

The fix-pitch propeller has the undeniable advantage that, by interrupting its rotation with the engine stop, the danger of catching a mooring line is cancelled, and the mooring-men, when working with their boats near the stern of the ship, do not have to fight against the wake, with an evident increase in safety. A further positive feature is that there is, in reverse, good power and, consequently, excellent effectiveness in stopping the headway.

Furthermore, for the practical purposes of manoeuvring, a ship with a fix-pitch propeller is particularly appreciated for its course stability when it proceeds with the engine stop towards the mooring position.

When we can change the orientation of the blades utilizing servomechanisms, we have the variable pitch propellers.

In other words, the blades of the variable pitch propellers can be rotated around their longitudinal axis, thus causing a variation in the angle with which they cut through the water.

To vary the speed and reverse the motion is sufficient to change the orientation of the blades concerning the hub.

 

 

The engines always work in the same direction at constant revolutions; consequently, we will not have engine start-ups for reverse gear or problems related to the air reserve or waiting times between forward and reverse, and we will be able to adjust the speed easily, that is a handy feature when manoeuvring; think, for example, of the delicate unmooring manoeuvre carried out using the spring line to open the stern of the ship and the possibility of adjusting the workload on the mooring lines.

On the other hand, since the evolutionary effect due to the continuous rotation of the blades is accentuated, proceeding with the engine stop while maintaining course stability, continuing to have a little control with the helm – as occurs when the ship is equipped with a traditional propeller – is generally not possible. Still, it is necessary to keep a “minimum” ahead or compensate with short engine kicks to achieve the same result. Another defect of the variable pitch propeller is linked to the loss of steering, which generally occurs when the pitch is reduced too quickly and in the fact that there is a particular difficulty in finding the so-called “zero pitch” or in identifying the position of the strictly neutral pitch. The performance in reverse gear is lower than that offered by the fix-pitch propeller. Finally, since the propellers are in continuous motion, the mooring lines being sucked in during the mooring manoeuvre or unmooring remains high.

Due to their characteristics, the variable pitch propellers have found wide diffusion on Ferries, Passengers, Ro-Ro and on all those ships that need to manoeuvre frequently. In contrast, large cargo ships generally tend to prefer the economy of fix-pitch propeller systems.

Let’s see in this short video the difference between pitch-propeller and fix-propeller.