Hull and machinery insurers, P&I clubs and classification societies are reporting increased cases of stern tube damage because of incomplete propeller immersion, writes Glen Ralston
The use of low-immersion propellers on some vessels with low-speed fixed-pitch installations is not new. As such, increasing incidences of stern tube damage caused by incomplete propeller immersion may suggest that operational considerations need to be better addressed. Such propellers are used on a diverse range of vessels, such as bulk carriers, container vessels and large tankers, due to draught restrictions at certain ports; they also come into play when vessels are in a lightship condition.
Owners and operators need to recognise when these situations will occur and make a voyage plan before entering or manoeuvring in restricted rivers and ports where these conditions exist.
Stern tube failure can happen quickly when low immersion conditions occur. Its main cause is the loss of the hydrodynamic lubrication film within the white metal bearing, causing a rapid temperature rise and overheating. This wipes the bearing surfaces, rendering the stern tube ineffective and causing delays and expensive repairs that the owner and operator must address quickly.
“Stern tube failure can happen quickly when low immersion conditions occur”
Conditions in which stern tube failures occur vary and can be affected by the overall condition of the propeller, rudder, stern tube and main engine. When looking into the factors that cause specific failures, a number of issues must be considered.
A main consideration is the detrimental effect of wiping the bearing due to loss of the oil film, causing shaft and bearing contact. Hydrodynamic lubrication failure will lead to excessive temperatures and other symptoms including reduced engine performance. Although this failure may not be a direct result of low immersion, the effect of cyclic loading and shear stress on the stern tube bearings over time may lead to failures.
Pressure conditions within the bearing surfaces should also be considered. The stern tube manufacturer’s recommended oil supplier should be consulted, as it will advise the oil grade to be used with the correct viscosity during service. It will also have information about service life suitability, but regular sampling should be carried out to monitor water contamination.
The propeller shaft diameter and weight determine the contact area on the bearing, but shaft misalignment tolerances will affect the overall bearing load distribution. This will also be influenced by the propeller’s weight, which will cause bending moments along the shaft leading to localised pressure variations.
When the propeller is only partially immersed, its performance is usually another main contributory cause of stern tube problems. Because it is not operating to its optimum and designed thrust and torque parameters, the effects of eccentric thrust, cavitation and vibration will create uneven load distribution and inadvertently lead to bending monuments and localised stress areas, especially in the aft part of the bearing where edge loading may develop and increase over time. Apart from the stern tube being exposed to damage, cavitation damage on the propeller may also occur as a result of operating the vessel in these conditions over an extended period of time.
Large rudder angles when the propellers are in a low-immersion condition can also play a part in increasing the risk of damage, as propeller wash efficiency is reduced causing the vessel to respond to helm orders more sluggishly. In some instances, this can cause the master to increase main engine power and speed, exacerbating the situation.
Foreign materials on the water surface around the propeller area can also contribute to stern tube bearing failure. Logs, ropes or other sizable objects can contact the propeller blades as they break through the water, leading to localised damage and increased cavitation and vibration.
To avoid problems when operating vessels in these conditions, ship operators should consider limiting the main engine’s speed and the power output of the engine and arrange for the crew to closely monitor the stern tube bearing temperature. These actions, along with regular oil sampling and draining, can reduce the risk of such damage occurring, but shipowners and operators should also seek advice from their stern tube manufacturers, in order to implement procedures that can be adopted on board.
Finally, they should use manoeuvring tugs in ports or areas where these conditions may occur and reduce the demands for large rudder angles.