Propellers are one of the most critical components within a vessel propulsion system, turning the rotational energy from the vessel engine into forward thrust through the water. Their designs are carefully considered to maximise efficiency of this energy transfer, but their location within open water means they are susceptible to wear and damage in service.

Given criticality of the propellors, they are a key item to be assessed by Classification Society survey – and require regular inspection.

Propeller

Rotating in open water hundreds of time a minute, with large vessel shaftlines easily reaching 100 tonnes or more, there is every chance that in contact with lines, sand, rocks and submerged debris the propeller will continue to rotate. The resulting contact forces with foreign objects are enormous and capable of structural damage to the propeller.

In situations where the propulsion system is not fully optimised, a phenomenon known as cavitation can occur. This is when gases held in solution within the water are released as the propeller lowers the local pressure. Gas bubbles form along the propeller surface and are at risk of collapsing rapidly causing localised damage through pressure spikes. Over time this will remove material from the surface of the propellor and can contribute to stress failures

Propeller Assessment

There are a number of different methods by which the condition of a propeller can be assessed, and are suited to different features and whether the vessel is still in, or out of the water. These include:

Visual inspection – can be carried out while still in the water with the use of divers and cameras

Basic measurement – can be done in the water using divers and taking some basic reference dimensions and comparing blade values to a drawing.

Detailed measurement – usually done out of the water and multiple measurements are taken across the whole component and compared to drawn surface references.

Dye penetrant inspection – done out of the water where a strong dye is drawn by capillary action into small cracks or voids and wiped from the surface. A developing spray is then applied which draws any dye within the propeller to the surface giving a clear dye bloom identifying a defect.

Electromagnetic inspection – done out of the water by applying an electromagnetic field to the propeller and an indicating powder. Similar to how iron filings will align in a magnetic field, the impressed magnetic field within the part will vary in the presence of defects and manifest as a change in texture or clear line in the indicator powder.

X-ray inspection – done out of the water by locally photographing the part using electromagnetic rays. Defects are shown in the developed image like a hospital x-ray of broken bones.

Detailed interface inspection – done out of the water and after removal of the propeller. A variety of the above techniques are applied to the propeller bore and shaft interface to identify any signs of damage or stress cracking.

Propeller Damage

Even minor damage to the tips of a propeller can cause significant vibration and damage to associated systems and be to the detriment of fuel efficiency by as much as 5%. It can usually be seen relatively easily if one or more tips have sustained damage by visual inspection.

Propellors are coupled to drive shafts in a number of different ways – from press fitting of a tapered cone, to hydraulic coupling, mechanically key-ing or bolting. The forces being transferred from the vessel drive to the propeller are enormous, and as such the quality and condition of the interface between shaft and propeller are critical to ensuring they remain connected. Inspection of the propeller to shaft interface can only be carried out by removing the propeller.

Highest stresses in the propeller are generally towards the centre of the hub and at the transition from the blade to the cylindrical hub of the component (root). Significant changes in cross section around holes and other features will also see higher local stress. Design dictates that these areas are thick and transition smoothly in section but can sometimes be at higher risk of damage. Most of these areas can be checked by visual inspection initially, followed by dye-penetrant, electromagnetic or x-ray inspection.

AtZ can provide all of the above during maintenance or dry dock, and assure appropriate repairs are undertaken if necessary to address any issues.


FAQ’s

One of my propeller tips has damage – can it be locally repaired underwater?

Usually – it can.
If the damage is small, it can be locally straightened or cropped to minimise the impact of the damage on the propeller performance. Careful thought has to be given however to the overall balance of the propeller – it is usual that where one blade is cropped to remove a damaged section, the other blades are cropped similarly to retain equilibrium.
This is usually considered a temporary repair which would be addressed in a future dry dock.

There is a crack in my propeller – will I need to replace it?

This depends very much on the size of the crack, its location and the opinion of the surveyor who ultimately will have to approve your plan.
If the crack is considered small and in a low risk position, it would be common for the crack to be locally ground and dressed to remove the crack altogether or minimise sharp transition at its extremities which may lead to it propagating. Measuring and photographing it with reference to size and position will allow it to be monitored over time – Classification Societies will often require this and place a condition on the defect to be regularly monitored and to insist on immediate repair should it worsen.

Our organisation has a spare propeller – can we simply swap it during a dock?

Yes – you can swap an entire propeller for an identical one – but it must be correctly prepared for fitting.
In the case where there is a simple mechanical bolted coupling – it can be a straight swap. In the case of a taper fit however, the fit must be checked by ‘blueing’ the interface so that near perfect contact of the two surfaces is assured. Where the percentage of contact is not acceptable, the taper must be locally dressed (usually by hand) before rechecking the fit and contact.
Fitting a propeller to a shaft is done preferably in a vertical orientation with the shaft first removed so that the influence of gravity is even around the shaft circumference for the fitting. Horizontal fitting is possible, but requires some additional preparation and process.

Your Industry Is Our Industry

Download Our Latest Brochure

The experts in marine projects and mechanical engineering. 

Our team of highly experienced technical engineers offer industry knowledge gained throughout their time at several multi-national marine propulsion design and service providers.

Free Download
Top