Many of you might have often heard the discussion between Master, Chief officer and Chief Engineer with regard to vessel propeller immersion issues, due uneven load of cargo, lack of cargo or impossibility of ballasting/de-ballasting the vessel due shear forces or bending moments.
This is a very serious issue as propeller immersion less than 100% will result in loss of vessel performance, main engine over speeding and stress or damage to vessel machinery.
Recent operational experience suggests a tendency toward an increasing number of reported losses to vessels’ propeller shaft bearings. It is believed that the majority of the damages occurred during a relatively short period of time, ranging anywhere from a few minutes to an hour on average, depending on the operating conditions.
The trend that has been observed is not unique to a certain category of vessel; rather, it is attributable to the operation of the affected vessels in regions with restrictions on the draft of the vessel or loading conditions, without taking appropriate precautionary measures to limit the RPM or power of the engine.
For the purpose of complying with Class criteria, the fundamental design of stern tube and shaft systems takes into account the presence of a propeller that is completely submerged.
When the propeller tip is somewhat close to the surface of the water, design margins account for a small amount of minor eccentric propeller loading.
The immersion of propeller is defined as the ratio of the distance between free surface and propeller blade tip to propeller diameter, as shown on the image below.
If propeller is not completely immersed, it will result in:
- excessive eccentric thrust
- increased downward bending moment at the aft end of propeller shaft, leading to higher edge loading of stern tube bearing.
- breakage of oil film and ineffective hydrodynamic lubrication in the aft stern tube bearing.
- increased shaft system vibrations
- increased cavitation of propeller
When propeller and shaft lines are operated outside the design criteria there is a risk of:
- Stern tube seal leakage
- Increased wear of stern tube bearing
- Fatigue failure and subsequent damage of stern tube bearings.
- Wear and damage to shaft line bearings
- Cavitation and wear of propeller
When the shaft comes into direct physical contact with the material of the bearing, the temperature of the bearing will rise, and in most situations, this will happen at an exponential pace.
Bearing damage was seen even with a slower rate of temperature rise when it occurred in a singular instance that involved lengthy operation beyond the alert limit, which is generally set at 65 degrees Celsius. The damages that were documented led to repairs that were both expensive and time-consuming.
Failure of the bearings can increase the chance of the main propulsion function capacity being lost entirely or significantly reduced, and in rare instances, it can be harmful to the propeller shaft in the event that steel-to-steel contact occurs. In the event that long-term operation with incomplete propeller immersion does not result in an immediate failure, the risk of fatigue-related bearing failures arising out of excessive cyclic loading and associated shear forces on the bearing will co-exist. These failures are caused by excessive cyclic loading and associated shear forces on the bearing.
The idea behind shaft alignment accounts for an adequate distribution of loading across the shaft bearings while also taking into account the forces and related bending moments that are created by the propeller while it is in operation. The weight of the propeller as well as the forces exerted by the hydrodynamics have an effect on the angular misalignment of the shaft by way of the aft bearing (relative slope), and this, in turn, has an effect on the shaft-bearing contact area.
The rate of rotation per minute (RPM), the diameter of the shaft, the viscosity of the oil, the net effective contact area of the shaft in way of the bearing, and the bearing load are the primary factors that determine hydrodynamic lubrication conditions. The local surface pressure that is applied to the bearing can also be regulated by the contact area.
Under normal circumstances in order to avoid the above mentioned issues, the minimum draft aft must be:
- Draft required for min. 100% propeller immersion (as per Trim & Stability book) + 0.6 meters.
During navigation in stormy conditions, a ship can think about postponing or eliminating trim optimization altogether, bringing the ship to an even keel instead, or adjusting the trim by the stern as necessary depending on the severity of the weather.
If the propulsion shaft system is experiencing an abnormally high amount of vibration, you may want to consider increasing the aft draft in order to reduce the level of vibration.
When the propeller is only partially submerged during operation, this can result in an excessively eccentric force on the propeller and, as a consequence, a downward bending moment on the shaft. Because of this, there is a possibility that the aft bearing will experience increased localized loads (edge loading), as well as surface pressure, as a consequence of the increased relative slope and lower bearing contact area.
Because the design criteria do not account for localized bearing stresses operating on a reduced contact area, this can result in the complete or partial loss of an effective hydrodynamic oil layer with a minimal thickness. As a result, there is a possibility that the bearings will be damaged in the future as a consequence of the incomplete propeller immersion when unusual operating conditions are present.
The degree of lack of propeller immersion, revolutions per minute (RPM), and power all have a role in the generation of the additional bending moment.
To provide further clarification on this topic, the bending moment is related to the thrust force, which in turn is proportional to the square of the RPM. As a consequence of this, increasing the RPM in a circumstance where the propeller is partially submerged adds an exponentially greater degree of risk.
In exceptional cases it may not be possible to achieve 100% propeller immersion + 0.6m, for example:
- Vessel going in/out of dry-dock
- Phasing in/out of a certain trade
- Low cargo load
- Vessel trading in areas with limiting factor e.g. minimum water depth and/or port restrictions on maximum vessel draft.
In such cases vessel superintendent is to be informed to ensure that appropriate measures are planned, and following risk mitigation measures are put in place:
- All options to increase propeller immersion to greater than or min. 100% must be considered, and cargo planner may be contacted if any concerns with ballast intake and/or stress & stability limits.
- At propeller immersions between 87% to 100%, the maximum load on main engine should not exceed ME power corresponding to “Half Ahead”.
- It must be ensured that all stern tube and intermediate bearing temperature alarms are checked and slow down functions (Manual or Automatic) are tested.
- Vessels equipped with ‘Manual Slow Down’ require immediate attention during a high temperature alarm.
- In general, temperature alarms for stern tube bearings are recommended to be set at:
- High Alarm setting 62 ºC
- High High Alarm and Slow Down 65 ºC
- Other settings may have been applied originally and should only be changed in agreement with the superintendent.
On some vessels additional alarms and checks are available in order to ensure stern tube safety and proper functioning.
- Temperature rise max. 5 ºC/min (Slow Down)
- ΔT Max differential temp. between SW and S/T temp. (Slow Down)
- Increased monitoring of stern tube bearing temperatures, stern tube seal drains and LO water content during the entire low draft operation.
Vessel crew must ensure efficient stern tube cooling by always keeping the cooling water tank around the stern tube filled with fresh water.
As mentioned above LO water content should be checked regularly due the entire low draft operation, as in case of stern tubes with white metal bearings, water in the lubricating oil can cause severe damage with considerable repair expense and time loss. On the other hand, Wartsila Railko stern tube bearings can work with a limited amount of sea water in the lubrication oil without damage to the bearings.
In case that high temperatures occur in the stern tube bearing:
- Reduce shaft revolutions immediately to Dead Slow. In case protection system is only set up to give an alarm or manual Slow Down, it is of high importance that duty officer immediately reduce rpm on the ME telegraph;
- Keep rudder position at mid-ship position as far as possible;
- Monitor stern tube bearing temperatures rise, if temperature is stabilizing keep RPM and monitor that temperature is gradually going down.
- In case temperature is continuously decreasing, continue with Dead Slow RPM until temperature is stabilized below sea water temperature + 30 ºC;
- At above stages never stop the Main Engine, as this could result in the tail shaft being bent due to spot heating of the propeller shaft.
If stern tube temperature does not decrease or rises above 85°C with above procedures, then:
- Stop the Main Engine;
- Engage the turning gear immediately and start turning of shaft to avoid spot heating of the propeller shaft;
- Monitor cooling down of stern Tube;
- Turning gear must not be stopped during this process.
Obviously vessel must liaise with the superintendent to coordinate on further actions in case of reduction in shaft revolutions due to abnormal conditions in the stern tube system and above checks are to be initiated subject to safe navigational conditions.
If high temperatures have occurred, check the filter in the oil system for impurities from the bearings, and for a “phenol” types smell (Railko bearings only).
If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.
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