Technology: What to do if a line gets caught in the propeller

The problem can occur at any time: With a yacht underway and the engine running smoothly, engaged in forward or reverse, the engine suddenly stops working and cuts out abruptly. A shocking moment, especially if the engine failure occurs during a narrow passage, during a harbour manoeuvre or on a lee shore.

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Before starting again, the internal checklist rattles through his head to find the cause: fuel supply, engine damage, electrics? A quick glance aft sheds light on the skipper's dark hour: there is a rope lying on the cleat, wrapped tightly over the ship's side and further down the hull towards the waterline - right down to the propeller. The loose end has been sucked in, so to speak, wrapped around the shaft and then choked the rotation of the propeller from 2,500 to zero. Incredulous astonishment, swearing, recriminations - but none of this helps. On the contrary, the second of truth requires maximum speed of action: Check the surroundings, look at the nautical chart, raise the sails and immediately sail clear of a potentially dangerous situation.

First steps in rope winding

Anyone who has thus prevented further damage to body, soul and hull can say they have skill or luck and sail into the next harbour, first lie at anchor or request towing assistance - and then concentrate fully on the disaster at the propeller.

It's worth a try: disengage the engine and pull on the line that is causing the problem. Of course, it must still be possible to grasp it on deck. With a bit of luck, the prop can be freed and the engine restarted. If that doesn't work, you will need to dive yourself or have someone else do it to free the tangled line, net or plastic sheeting from the shaft or saildrive. And even if the ball of air is later painstakingly cut out from between the stern tube and the propeller bearing, it is still not clear how the propeller, shaft, clutch and gearbox are doing.

Whether it's sheets or mooring lines hanging in the water, invisible floating fishing nets, snagged traps, stray ropes or plastic waste - there are a number of dangers for the rotating drive under water. And no matter what has caused the engine to stop unintentionally, there is an urgent need for action: "The best thing to do after such an incident with foreign objects that causes the engine to stop is to head for the nearest crane and a workshop," explains Manuel Keinberger. The Managing Director of Kiesow Schiffsmotoreninstandsetzung in Kappeln continues: "The possible effects on the affected materials are not readily apparent and can lead to consequential damage and pose a safety risk if operation continues." However, the suspicion that the expert is trying to boost business with such statements is unfounded - the order books are full and new customers have little chance of being looked after.

Professional disassembly of the propeller and shaft

What Keinberger and his guild urgently recommend after an unintentional rope entanglement on the propeller is a professional disassembly and dismantling of the propeller, depending on the type of construction as a rotary blade folding or fixed pitch propeller, various external and "internal injuries" to mechanics, gears and bolts can occur here, and further use of the propeller can cause further damage, up to a final loss of propulsion power - with the associated risk of an accident.

In addition to the propeller, the shaft should also be removed and meticulously checked for deformation. This is not possible when the propeller is installed. Even if the shaft is visible in its entirety, damage would only be visible in the event of major deformation; minor deformation can only be verified using measuring equipment. Keinberger's employees clamp the affected shaft in a lathe and measure with millimetre precision whether there are any fluctuations. "Even small deformations lead to imbalances in the drivetrain, which can have a negative effect on the clutch and the gearbox," says Keinberger. In addition, the entire periphery, such as the shaft end bearing, propeller shaft seal and other seals, suffers, which causes the next damage in the medium term and, in the worst case, can mean water ingress. Complications can also occur with a saildrive if the drive is forcibly stopped by ropes: The shaft seals and bearings can be damaged and leak and the shaft and deflection can be deformed.

At first glance, it seems rare for sailing yachts to find themselves in the stranglehold of a tampon. At least, for yacht insurance specialist Pantaenius, this type of damage only accounts for a small proportion of less than one per cent of annual claims. "If a conventional drive on a sailing yacht is hit, it is very annoying from a financial point of view, but the damage is usually manageable. With a saildrive, it quickly becomes more complex, even in the periphery. It is much more serious with motor yachts, where the gearbox is also affected depending on the design," explains Holger Flindt, who heads the international claims department at Pantaenius. If, for example, a jet drive, sterndrive or Volvo's IPS system is damaged by flotsam or grounding, the costs for a repair can easily run into five figures.

Damage can never be completely prevented

In the area of water sports, lines or nets in the propeller are the third most common reason for the DGzRS to be called out. In 2023, this was necessary a total of 32 times. Compared to grounding (293 missions) and engine or gearbox damage (237 missions), this is not a significant number, but these figures do not provide any information about the actual number of incidents and damage caused by blocked propellers, as sailors also make it to port under their own steam or with self-organised towing assistance.

The mechanics at Kiesow, who alone deal with around 20 cases of propeller problems caused by ropes and flotsam every year, are also aware of this. "Damage to the propeller and shaft system can be avoided, but never completely prevented," says Keinberger. "But yacht owners and workshops can do a lot in advance to prevent the scenario of an inability to manoeuvre, thereby increasing safety and saving a lot of money."

Other reasons for damage to the propeller and shaft system

In addition to damage caused by foreign bodies in the propeller and shaft system, either caused by the machine itself or through no fault of its own, there are several other reasons why a machine can no longer utilise its power for propulsion. Electrolysis can be a major enemy of the propeller and shaft. In this process, electrically charged atomic components - electrons - migrate from one metallic component of the drive train 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, which in turn is made of cast bronze, an alloy of different metals. In the battle of the electrons, the bronze loses out, resulting in imbalance and cracks or even breakage of the material under full load.

Galvanic currents cannot be completely 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 different metals, eliminate the destructive energy through self-dissolution. The necessity and effectiveness of anodes are often underestimated by boat owners. "A missing or half-worn anode should definitely be replaced," explains Keinberger, who knows from experience that an expensive branded product is much more effective than a bargain from the no-name department.

Corrosion problems are not always home-made

In the Flensburg Fjord, the owner of a steel yacht was asked by the harbour operator to "ground" his yacht to prevent "galvanic problems" for his neighbours. The shaft of his neighbour's jetty had destabilised in quick succession and had become porous. However, the strong external currents were not caused by the steel hull, but by a defective electrical system on the jetty that was conducting electrical currents into the water.

These extraneous currents creep through the conductive parts of the yacht, anodes age more quickly and considerable corrosion with consequential damage is caused to the shaft system or saildrive. In modern marinas, this scenario can be virtually ruled out, but as the example shows, it can certainly happen in outdated harbours with dilapidated cabling and inadequate protection.

According to propeller manufacturer SPW, in addition to electrolysis and external influences from floating debris, fouling and the resulting loss of efficiency are also problematic. In addition, the open teeth of folding propellers can be damaged, and increasing marine fouling impairs the function and promotes wear. In rotary blade propellers, the mechanics are protected from fouling in a closed housing. To ensure a longer service life, it is important to grease the rotary blade propeller regularly to reduce wear on the gearing.

Another danger of no longer being able to manoeuvre under engine power is the lack of fastening of the propeller to the shaft, which often results in the complete loss of the propeller. "If the shaft-propeller connection comes loose, for example, the propeller turns freely on the shaft or flies off completely," summarises Ragnar von Winterfeld. Lack of maintenance of the important retaining elements, unnoticed wear and incorrect assembly can lead to this undesirable event. "Boat owners without prior technical knowledge should leave the repair and maintenance of these sensitive areas to a specialised company," says von Winterfeld. The mechanical engineer, expert and workshop speaker: "However, sailors can carry out maintenance and care themselves."

Sensitisation of the entire crew

Even when things are running smoothly from a technical point of view, the crew can still make mistakes despite exercising caution, prudence and foresight. When mooring and casting off as a standard manoeuvre, lines drifting in the water unintentionally are less common. However, when mooring to skerries, for example, or when long lines are used to provide additional shore protection, things can get confusing.

If the wind shifts and makes a night-time casting off manoeuvre necessary, things get colourful. Then the situation is more complex than expected, loose lines can end up in the water and, attracted by the screw water, block the wave. "The helmsman should of course keep an eye on this, but in difficult situations and especially in the dark, a crew member should explicitly take over tug control," suggests von Winterfeld. "Basically, it's about sensitising the entire crew to the following questions: How does the propulsion technology work, how do I avoid damage and how do I react to a potential problem?"

For von Winterfeld, this starts with the use of the gear lever: "From forwards to reverse in one stroke - ouch! That really hurts all drive units and reduces the service life of the material, if it doesn't lead to ad hoc damage." In his workshops, the expert responds to the direct passage of the lever with a simple explanation of basic physics. This can save propeller lives.

As important and convenient as it is to have a well-functioning jib on board, the sails should always be clear to set until the yacht has moored or the anchor is secure. Preparing for a worst-case scenario is simply good seamanship.

The typical enemies of the propeller

• Electrolysis, lack of anode protection
• Floating objects such as ropes, lines from the deck, towing lines from the dinghy (use buoyant ones!)
• Nets, fishing lines (can work their way through the oil seals towards the gearbox), plastic bags, canisters, wood
• Grounding (especially motor yachts)
• Fouling and the associated loss of efficiency
• Wear of the open teeth on folding propellers
• Lack of maintenance, ignoring wear
• Incorrect operation, insensitive seamanship
• Incorrect assembly

Mindful of the drivetrain

• Regular visual inspection for cracks and corrosion
• Remove fishing line and rope remnants
• Clean and grease depending on type and model
• Clean open toothing, remove fouling
• Grease rotorcraft
• Replace the shaft seal every six to eight years
• Check shaft end bearing
• Replace anodes when half worn
• Securing propeller with castellated nut and additional split pin, depending on design
• Training and sensitisation of the crew regarding objects in the water and use of the circuit
• Single-handed sailors special concentration during harbour manoeuvres with mooring lines etc.

Measures in an emergency

• Disengage the clutch if the engine speed drops significantly when travelling
• Do not restart the machine
• Probing the problem
• Always keep the boat ready to sail
• Set sail, sail clear of the leeward wall and leave the fairway
• Ask harbour master for towing assistance via VHF or mobile phone
• Prepare towing line, crew with line on foredeck
• In case of danger, call for help on VHF channel 16
• Foxtrot flag (white with red diamond)

Equipment just in case

Rope cutter

The idea: A series of cutting blades rotate on the shaft, and ropes, lines, plastic and seaweed floating in the water attracted by the rotation of the propeller are cut up immediately before they can wrap themselves around the shaft. The reality: Depending on the ropes, speed, propeller and inflow, success can be achieved. However, even in the low speed range, too much rope gets around the wave for an effective cut to be possible. Prices from 100 to 400 euros, simple installation. More information: www.spw-gmbh.de/de/produkte/tampenschneider; www.svb.de/de/tau-schneider.html

The "Spurs Line Cutter" from the USA promises more success in rope cutting with a different cutting tool and more protection against twisting. The device costs around 900 euros and installation is time-consuming. More information: www.spursmarine.com/shaft-cutters

Useful accessories

A replacement propeller saves a long delivery time and can be particularly useful on long journeys. A simple model is sufficient instead of a rotary or folding propeller. A diving knife, neoprene, snorkel and mask should be on board.