Not all damage to the propeller occurs only in an emergency. Many problems develop gradually, for example due to electrolysis, fouling, exposed teeth, a lack of maintenance or careless handling of the lines on board.
You can find out more about the effects of vegetation in Antifouling: Propeller fouling protection ensures efficiency, on the safe handling of ropes in Seamanship: Ropes – how to splicing, securing and throwing them correctly and to get ready at the jetty in Harbour manoeuvres: What you should bear in mind when mooring and casting off.
Text: Nico Krauss; Updated by Antonia von Lamezan on 7 August 2026
Apart from damage caused by foreign objects in the propeller and shaft system – whether self-inflicted or through no fault of one’s own – there are several other reasons why a machine may no longer be able to utilise its power for propulsion. Electrolysis can be a major threat to the propeller and the shaft. In this process, electrically charged subatomic particles – electrons – migrate from one metallic component of the propulsion system to another.
This is because the boat’s drive shaft is made of stainless steel. At its end, the propeller is mounted on a cone; the propeller itself is made of cast bronze, an alloy of various metals. In the battle of the electrons, the bronze loses out, resulting in imbalance and cracks, and ultimately the material breaking under full load.
Don’t underestimate anodes
Galvanic currents cannot be entirely avoided. The solution lies in the use of sacrificial anodes on the shaft and on the propeller itself, which, as the weakest link in the chain of dissimilar metals, so to speak, eliminate the destructive energy through self-corrosion. The necessity and effectiveness of anodes are often underestimated by boat owners. “A missing or half-worn anode should definitely be replaced,” explains Manuel Keinberger, managing director at Kiesow Schiffsmotoreninstandsetzung in Kappeln. Keinberger knows from experience that an expensive branded product is significantly more effective here than a bargain from the no-name section.
Corrosion isn’t always caused by us
In the Flensburg Fjord, the owner of a steel yacht was asked by the harbour operator to ‘earth’ his yacht to prevent ‘galvanic problems’ from arising with neighbouring vessels. On his jetty neighbour’s yacht, the hull had become destabilised and porous within a short space of time. However, the cause of the strong stray currents was ultimately not the steel hull, but a faulty electrical system on the jetty, which was conducting electrical currents into the water.
These stray currents creep through the yacht’s conductive parts, causing anodes to age more quickly and resulting in significant corrosion and consequential damage to the shaft system. In modern marinas, this scenario is virtually impossible, but as the example shows, it can certainly occur in outdated harbours with dilapidated cabling and inadequate protection.
Fouling, interlocking parts, assembly faults
According to propeller manufacturer SPW, in addition to electrolysis and external factors such as floating debris, fouling – and the resulting loss of efficiency – is also a problem.
Another risk of losing manoeuvrability whilst under power is the propeller not being securely fastened to the shaft, which often results in the complete loss of the propeller. “If, for example, the connection between the shaft and the propeller comes loose, the propeller spins freely on the shaft or flies off entirely,” explains Ragnar von Winterfeld, an expert in yacht construction and marine engineering. A lack of maintenance of the key retaining components, unnoticed wear and tear, and incorrect installation can lead to this undesirable outcome. “Boat owners without prior technical knowledge should leave the repair and overhaul of these sensitive areas to a specialist firm,” says von Winterfeld, adding: “However, sailors can carry out maintenance and upkeep themselves.”
The crew and the gear system are also part of the preventative measures
Even when everything is running like clockwork from a technical point of view, mistakes can still occur amongst the crew despite all due care, caution and foresight. During standard manoeuvres such as mooring and casting off, lines accidentally left floating in the water are less common. However, if, for example, the boat is moored to skerries or long lines are used to provide additional mooring to the shore, the situation can become confusing.
If the wind then changes direction and makes it necessary to cast off at night, things get complicated. The situation then becomes more complex than expected; loose lines can end up in the water and, drawn in by the propeller wash, block the wave. “The helmsman should, of course, keep an eye on this, but in difficult situations – and particularly in the dark – a crew member should explicitly take charge of checking the lines,” suggests von Winterfeld. “Essentially, however, it’s about raising awareness amongst the entire crew by asking the following questions: How does the propulsion system work, how do I prevent damage, and how do I respond to a potential problem?”
For von Winterfeld, it all starts with how the gear lever is used: “From forward to reverse in one go – ouch! That really takes its toll on all drive units and shortens the service life of the components, if it doesn’t actually cause damage straight away.” In his workshops, the expert responds to the practice of moving the lever straight through with a simple explanation of the underlying physics. This can save propellers’ lives.
Mindful of the powertrain
- Regular visual inspection for cracks and corrosion
- Remove any loose threads and bits of twine
- Cleaning and greasing the propeller
- Clean open gear teeth, remove fouling
- Replace the shaft seal every six to eight years
- Check the shaft end bearings – replace them if there is 0.5–1.0 mm of play on the shaft
- Replace anodes once they are half worn
- Securing the propeller, depending on the design, with a crown nut and an additional split pin
- Training and raising awareness amongst the crew regarding objects in the water and the use of the gearshift
- Single-handed sailors need to pay particular attention during harbour manoeuvres involving mooring lines
Series: Propeller emergencies and propulsion protection
This series explains what to do immediately if a screw becomes jammed, what damage this could cause to the drive, and how to prevent such problems.
Book recommendation:
The book Seamanship in pictures starts with the basics that are crucial in everyday life when it comes to safe rope handling on board. The book illustrates knots, the correct handling of ropes and key seamanship procedures in a clear and practical way. For readers concerned with the issue of ‘lines in the propeller’, this is particularly useful as a reference book, as it helps to avoid typical mistakes when handling and securing lines.
Nico Krauss
Freier Fotograf