Enhancing Marine Engine Efficiency: A Solution for Low-Speed Operation

Author: Daniel G. Teleoaca – Marine Chief Engineer

Marine engines are the unsung heroes of the shipping industry, tirelessly powering vessels across vast oceans and seas.

However, these workhorses face a unique challenge when it comes to low-speed operation. Low speed operation can cause various problems for marine engines, such as increased fuel consumption, reduced power output, higher emissions, and more wear and tear. The inefficiency of marine engines at lower speeds can have significant economic and environmental implications.

Preceding the implementation of emission-limiting regulations, some of the ships, especially containers, were generally engineered to achieve maximum cruising velocities of 30 knots. Presently, operators are obligated to comply with regulatory frameworks such as the carbon intensity indicator (CII) and the energy efficiency existing ship index (EEXI).

As a consequence, cruising veers off at approximately 18 knots, which is roughly two-thirds the speed for which the engines were originally designed. As a result, engines operate extremely inefficiently at low loads, consuming significantly more fuel and emitting significantly more CO2 than is required.

Without intervention, Wartsila predicted in 2022 that by 2023, over one-third of container ships would be non-compliant, based on an analysis of the global fleet. Moreover, in the absence of intervention, 80% of container ships will be classified under the lowest CII category by 2030.

In this article, we’ll explore the reasons behind this inefficiency and the options available to improve marine engine performance when running at low speeds.

Understanding the Inefficiency

Marine engines are designed to operate at a certain range of speed and load, depending on the type and size of the engine, the ship’s hull form, the propeller characteristics, and the operating conditions. When the engine operates outside this range, it can suffer from inefficiency and performance loss. There are several key reasons for this inefficiency:

  • Reduced Combustion Efficiency: A cause of marine engine inefficiency at low speed is the incomplete combustion of fuel in the cylinders. The combustion process in a marine engine depends on many factors, such as the fuel quality, the air-fuel ratio, the injection timing, the compression pressure, the ignition temperature, and the combustion duration. When the engine operates at low speed and load, some of these factors can be adversely affected, resulting in incomplete combustion of fuel. Incomplete combustion can lead to lower power output, higher fuel consumption, higher emissions of carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM), and smoke, and more carbon deposits in the cylinders and turbocharger.

  • Mechanical Losses: At low speeds, the engine’s mechanical components, such as pistons, bearings, and crankshafts, experience higher frictional losses. This additional resistance leads to decreased engine efficiency. Moreover, the turbocharger is a device that uses the exhaust gas from the engine to drive a compressor that increases the air pressure and density in the intake manifold. The turbocharger improves the engine performance by allowing more air and fuel to be burned in each cylinder. The turbocharger efficiency depends on the pressure ratio between the exhaust gas and the intake air, which is called the boost pressure. The boost pressure is highest at high engine speed and load, when there is more exhaust gas available to drive the turbocharger. When the engine operates at low speed and load, there is less exhaust gas available, and the boost pressure drops. This means that less air is supplied to the cylinders, resulting in lower power output, higher fuel consumption, higher emissions of nitrogen oxides (NOx), and more turbo lag.

  • Propeller Inefficiency: One of the main causes of marine engine inefficiency at low speed is the mismatch between the engine and the propeller. The propeller is a device that converts the rotational energy of the engine into thrust force for propulsion. The propeller efficiency depends on the ratio of the propeller speed to the ship speed, which is called the advance ratio. The propeller efficiency is highest at a certain advance ratio, which corresponds to a certain engine speed and load. When the ship operates at low speed, the advance ratio increases, and the propeller efficiency decreases. This means that more engine power is wasted as friction and turbulence in the water, rather than converted into useful thrust.

Therefore, the effects of marine engine inefficiency at low speed can be summarized as follows:

  • Lower power output: The engine produces less power than it is capable of, resulting in lower ship speed or lower reserve power for maneuvering or emergency situations.
  • Higher fuel consumption: The engine consumes more fuel than it needs to produce a given amount of power, resulting in higher operating costs and lower profitability.
  • Higher emissions: The engine emits more pollutants than it should, resulting in environmental damage and potential non-compliance with emission regulations.
  • More wear and tear: The engine suffers from more stress and damage due to friction, corrosion, erosion, vibration, overheating, fouling, etc., resulting in higher maintenance costs and lower reliability.

Options to improve marine engine efficiency and performance at low speed

The inefficiency of marine engines at low speeds is a persistent challenge, but there are several innovative solutions available to mitigate this issue. Some of these options are:

  • Variable Geometry Turbochargers (VGTs): VGTs are turbochargers that can adjust their geometry to optimize airflow at different engine speeds. They help maintain higher combustion efficiency, even at low speeds, reducing fuel consumption and emissions.

  • Slow Steaming Strategies: Slow steaming involves deliberately operating a vessel at reduced speeds to conserve fuel. It has become a popular strategy in the shipping industry, allowing ships to run more efficiently at lower RPMs, thus saving fuel.
  • Dual-Fuel Engines: Dual-fuel engines are designed to run on a combination of natural gas and diesel fuel. These engines offer improved combustion efficiency and emissions control, making them an attractive option for low-speed operation.

  • Waste Heat Recovery Systems: Waste heat recovery systems capture and reuse the heat generated by the engine’s exhaust. They can be used to produce additional power or drive other ship systems, enhancing overall energy efficiency.

  • Upgraded Propellers: Shipowners can consider investing in more efficient propeller designs, specifically tailored to their vessels’ operating profiles. Modern propeller designs are more adaptable to a wide range of ship speeds.

  • Improved Hull Design: The vessel’s hull design can also impact its performance at lower speeds. Optimized hull shapes can reduce hydrodynamic resistance and improve overall efficiency.

  • Hybrid Power Systems: Some vessels employ hybrid power systems that combine traditional diesel engines with electric propulsion. This setup allows for efficient power delivery at various speeds, including low-speed operation.

  • Engine derating: Engine derating is a method of reducing the maximum power output of an engine by adjusting its settings or components. Engine derating can improve the engine efficiency at low speed by reducing the mismatch between the engine and the propeller, and by optimizing the combustion process and the turbocharger operation. Engine derating can also reduce the emissions of NOx, CO, HC, and PM. However, engine derating can also reduce the reserve power of the engine, and may require the approval of the engine manufacturer and the classification society.
  • Turbocharger cut-out: Turbocharger cut-out is a method of disconnecting one or more turbochargers from an engine by closing a valve or opening a bypass. Turbocharger cut-out can improve the engine efficiency at low speed by increasing the boost pressure and the air supply to the cylinders. Turbocharger cut-out can also reduce the emissions of CO, HC, and smoke. However, turbocharger cut-out can also increase the emissions of NOx, and may cause the turbocharger to overheat or surge.

In conclusion, addressing the inefficiency of marine engines at low speeds is critical for both economic and environmental reasons. The shipping industry has made significant strides in developing technologies and strategies to improve engine efficiency during slow steaming and low-speed operation. These solutions not only reduce fuel consumption but also contribute to lower emissions and a more sustainable maritime industry. As technology continues to advance, marine engines are likely to become more versatile, making them more efficient across a broader range of operating speeds, ultimately benefiting the entire global shipping industry.

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Exploring the Intricacies of a Vessel’s Stern Tube Lubricating Oil System

Date: August 29, 2023

By: Daniel G. Teleoaca – Marine Chief Engineer

When it comes to the complex machinery that powers maritime transport, every component plays a vital role. One such component that often goes unnoticed but is crucial for the smooth operation of a vessel is the stern tube lubricating oil system. This intricate system ensures the longevity and efficient functioning of a vessel’s propulsion system, making it an indispensable element of maritime engineering.

The stern tube, which is oil lubricated, serves as a bearing support for the propeller shaft and is sealed at both ends with lip seals, and the shaft is supported by two bearings situated between the seals. Seals are installed at the stern tube’s outer and inner ends. The seals at the stern tube’s aft end prevent water from entering and oil from escaping out to sea. Seals at the forward end of the stern tube keep oil from flowing into the machinery space.

Oil lubricated stern tube. Source and credit: Eagle Industry Kemel

Understanding the Stern Tube Lubricating Oil System

Lubrication for the stern tube (tail shaft) serves to discharge waste heat from the shaft, reduce friction in the shaft bearings and seals, and provide corrosion prevention. At the same time, the oil maintains pressure equilibrium at the seals and determined by the size and specified draught of the vessel, the lubrication oil system is supplied as a loss system with gravity tank or as a circulation system with circulation pump.

In case of smaller systems with only minor variations in the specified draught, the oil-filled chamber between the outer and inner seals is connected to a tank above the water line and as a result, there is a static pressure inside the chamber. The pressure is somewhat higher than that of the surrounding saltwater, preventing seawater infiltration into the vessel. Oil is delivered to both the radial seal ring and the bearings at the same time. The waste heat is removed by free convection via the oil and water in the tank through which the stern tube travels and the bearing temperature is determined solely by external stimuli with this sort of loss lubrication. Two gravity tanks are arranged at separate heights if necessary to prevent major pressure changes at the ship’s seals with two distinctly different draughts.

The forward seal has, in general, two sealing rings and oil pressure for the seal is supplied by two pumps. One of the pumps operates as the duty pump and the other pump is selected as the standby pump, which will be started automatically should the duty pump fail to maintain the LO pressure. The pumps are selected at the local pump selector switch for OFF/STANDBY/RUN. The pumps take suction from the stern tube LO sump tank via suction filters and deliver the oil under pressure to the space between the two sealing rings. The aftermost sealing ring seals the lubricating oil in the stern tube bearing and both sealing rings face outwards (aft). The oil outlet pipe is connected to the top of the seal housing and the oil flows back to the stern tube LO sump tank via the return line which is fitted with a sight glass which allows the oil flow to be monitored.

The aft seal consists, usually, of three parts:

  • Four rubber lip sealing rings.
  • The metal housing which carries the lip sealing rings.
  • A chrome steel liner which rotates with the propeller shaft.
Aft shaft seal arrangement

The aftermost sealing ring is No.1 seal ring and this faces outwards (aft), as does No.2 sealing ring. No.3 and No.3S sealing rings face inwards (forward). No.3S sealing ring is a backup sealing ring and can be brought into operation should No.3 sealing ring become damaged. In the event of No.3 sealing ring becoming damaged the valves on the oil lines to the space between No.3 and No.3S sealing rings are closed. The oil supply for the aft seal and the stern tube bearings comes from the stern tube aft seal and LO line pump, which takes suction from the stern tube LO sump tank.

Oil is supplied to the after seal and the stern tube bearings and the system gravity tank ensures that the correct oil pressure is applied to the aft seals. The gravity tank is fitted with a level alarm and the oil supply line to the space between No.3 and No.3S sealing rings is equipped with a sight glass, so that oil flowing to the space can be monitored. There is also a sight glass on the oil supply line for the space between No.2 and No.3 sealing rings and on the supply line to the stern tube bearing.

Challenges and Maintenance

Failure of the after seal rings can result in sea water entering the stern tube and lubricating oil can also leak into the sea if the after seal rings fail. The space between No.2 and No.3 sealing rings is connected to the lower header tank and this space contains oil but the oil is not circulating. In the event of the after seal rings failing sea water will enter the space between No.2 ring and No.3 ring and this water will find its way into the LO header tank.

If No.3 sealing ring fails, oil will leak from the bearing into the space between No.2 and No.3 rings. This oil will flow to the lower header tank and the level will rise. The header tank has a level alarm and if this is activated it indicates leakage of water past the No.1 and No.2 sealing rings or oil past No.3 sealing ring. If No.3 sealing ring fails the valves which connect with the space between No.3 and No.3S rings must be closed and No.3S ring will then act to seal the stern tube bearing.

Maintaining a stern tube lubricating oil system presents challenges due to the harsh maritime environment. Factors such as water contamination, temperature variations, and extreme pressures can impact the system’s efficiency. Regular maintenance, including routine oil analysis, filter replacement, and seal inspections, is essential to ensure the system operates optimally.

Nowadays, the repair and replacement of the stern tube aft seals can be done, in emergency situations, while the vessel is in service as the system is designed and in such way. Below there is a video example of Wartsila stern tube aft seal repair during vessel in service.

Warstila stern tube aft seal underwater replacement. Source and credit: Wartsilacorp and mySealTV

During engine operation the following checks must be performed:

  • Check the pressure gauge readings daily
  • Check the stern tube LO temperature daily
  • Check the forward seal LO temperature or casing temperature daily
  • Check for any discoloration of the LO and for the presence of water daily
  • Check the operation of LO filters and clean as required, or at least every month.
  • Check that the air control unit is functional and is working correctly.

It is important to note that the oil in stern tube system must be sampled and analyzed at intervals suggested by the oil supplier. Also, when the vessel is in dry dock the oil supply to the bearings and seals must be switched off and the stern tube drained. When the dry dock is being flooded the stern tube lube oil system must be restored.

Environmental Considerations

With growing environmental awareness, the maritime industry is under pressure to reduce its ecological footprint. Stern tube lubricating oil systems can potentially lead to oil leaks and spills if not properly maintained. As a result, ship designers and operators are exploring environmentally friendly lubricants and innovative seal technologies to mitigate these risks.

Nowadays, on the new modern vessels, the aft main seal is an air type seal operation, clean filtered compressed air is used as the means of controlling and maintaining the seal differential pressure.

Simplex airspace sterntube seal. Source and credit: BAHR visuelle Kommunikation

Air guard seal. Source and credit: Wartsilacorp

As can be seen air is supplied to the space formed by no.2 and no.3 sealing rings which then flows into the space formed by no.1 and no.2 sealing rings before flowing out to the sea. The air is supplied from a control unit and the flowrate is adjusted so that there is always the same differential pressure no matter what the draught of the vessel. The space formed by no.2 and no.3 sealing rings is open to the stern tube sump tank, so if the oil sealing rings no.3 and no.3S are damaged, any oil entering the space formed by no.2 and no.3 sealing rings is drained to the stern tube sump tank.

Stern tube bearing lubricating oil is supplied by one of the stern tube LO circulating pumps which take suction from the stern tube sump tank, which on modern vessel is maintained under pressure by air from the air control unit. The pressure in the stern tube LO tank provides the same effect as gravity tank fitted in the LO system, which is for emergency use if the air supply to the stern tube tank will fail.

The air control unit is equipped with flow regulators which are adjusted to provide constant air flow rate to the stern tube seal. Any alteration in the vessel draught is automatically compensated for by the changes in pressure of the air supplied.

The leakage past no.1 and no.2 sealing rings will be dependent on the general condition of the seals and the condition of the shaft liner. Any oil and sea water that may be present in this space is drained into the drain tank which should be checked regularly as it warns the duty engineer of any seal leakage problems. If the tank contains sea water, no.1 and no.2 seals are leaking, but if contains oil, then it is the seal no.3 that is leaking.

If the air supply fail or there is a disruption in the air supply from the air control unit to the stern lubricating oil tank the aft seal can be converted to the emergency seal condition. This is done by bypassing the lubricating oil tank and using the return pipe system to maintain a positive head on the oil in the stern tube. In the event of this kind of failure the system will trigger an alarm and system valves should be changed in order to ensure that an oil supply is maintained at the aft seal. In this case the stern tube bearing system must be converted to the gravity tank system in order to maintain the desired lube oil pressure, as this is necessary because there will no air pressure acting on the system.

The aft seal space between rings no.1 and no.2 may be flushed through with fresh water when necessary as the water is supplied at the connection to the air control unit.

In conclusion, the stern tube lubricating oil system might be concealed beneath the surface, but its role in maritime operations is undeniable. From enabling smooth propulsion to safeguarding against wear and tear, this intricate system is a testament to the marvels of maritime engineering. As technology advances and environmental concerns grow, the industry continues to refine and innovate this system to ensure safer, more efficient, and more sustainable maritime transportation.

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What you need to know about Energy Efficiency Existing Ship Index (EEXI)

In June of 2021, the IMO Marine Environmental Protection Committee (MEPC) held its 76th meeting, where they adopted resolution MEPC.328(76) containing amendments to MARPOL Annex VI concerning mandatory goal-based technical and operational measures to reduce carbon intensity of international shipping. Developed under the framework of the Initial IMO Strategy on Reduction of GHG Emissions from Ships agreed in 2018, these technical and operational amendments require ships to improve their energy efficiency in the short term and thereby reduce their greenhouse gas emissions.

From 1 January 2023 it is mandatory for all ships to calculate their attained Energy Efficiency Existing Ship Index (EEXI), to measure their energy efficiency and to initiate the collection of data for the reporting of their annual operational carbon intensity indicator (CII) and CII rating. The attained EEXI shall be calculated for each ship and for each ship which has undergone a major conversion.

The required EEXI value is determined by the ship type, the ship’s capacity and principle of propulsion and is the maximum acceptable attained EEXI value.

The amendments to MARPOL Annex VI are in force from 1 November 2022. The requirements for EEXI and CII certification came into effect on 1 January 2023. This means that the first annual reporting will be completed in 2023, with initial ratings given in 2024.

Vessels impacted by EEXI must demonstrate compliance by their next survey – annual, intermediate or renewal – for the International Air Pollution Prevention Certificate (IAPPC), or the initial survey before the ship enters service for the International Energy Efficiency Certificate (IEEC) to be issued, whichever is the first on or after 1 January 2023.

A ship’s attained EEXI indicates its energy efficiency compared to a baseline. Ships attained EEXI will then be compared to a required Energy Efficiency Existing Ship Index based on an applicable reduction factor expressed as a percentage relative to the Energy Efficiency Design Index (EEDI) baseline. It must be calculated for ships of 400 gt and above, in accordance with the different values set for ship types and size categories. The calculated attained EEXI value for each individual ship must be below the required EEXI, to ensure the ship meets a minimum energy efficiency standard.

The CII figures out the yearly reduction factor that is needed to make sure that a ship’s operational carbon intensity keeps getting better while staying within a certain rating level. The annual operational CII that was actually reached must be written down and checked against the minimum annual operational CII. This lets us figure out the operational carbon intensity grade.

The carbon intensity of a ship will be graded A, B, C, D, or E, with A being the highest. The rating indicates a performance level of major superior, minor superior, moderate, minor inferior, or inferior. The performance level will be documented in a “Statement of Compliance” that will be expanded upon in the ship’s Ship Energy Efficiency Management Plan (SEEMP).

A ship rated D for three consecutive years, or E for one year,  will have to submit a corrective action plan to show how the required index of C or above will be achieved. Administrations, port authorities and other stakeholders as appropriate, are encouraged to provide incentives to ships rated as A or B.  A ship can run on a low-carbon fuel clearly to get a higher rating than one running on fossil fuel, but there are many things a ship can do to improve its rating, for instance through measures, such as: hull cleaning to reduce drag, speed and routing optimization, installation of solar/wind auxiliary power for accommodation services, installing main engine power limiters etc.

The easiest way to get the energy efficiency index down is to reduce engine power, as vessels’ fuel consumption and emissions, respectively, increase as speed increases. The propulsion power, thus CO2 emissions, is approximately proportional to the cube of the speed. This means that reducing speed by 20% can drop the emitted CO2 by 50%. Slow steaming, therefore, is a more carbon-efficient way to transport goods. The engine power limitation systems can be bypassed, but only if required for the safe operation of the ship, for example, in harsh weather conditions.

Example of mechanical EPL developed by MAN

The Engine Power Limiter (EPL) must be overideable and will limit engine power by restricting the fuel index to a calculated set value. This restricts the total amount of fuel that can be injected into the engine and thereby limiting the power the engine can produce. For correct installation, the EPL must limit the fuel index to match the engine power for MCRlim.

The Engine Power Limitation (EPL) as such does not alter NOx critical settings or components of the engine.

The calculation of the EEXI follows the calculation of the well-known EEDI. It is based on the 2018 calculation guideline of the EEDI, with some adaptations for existing vessels. In principle, the EEXI describes the CO2 emissions per cargo ton and mile. It determines the standardized CO2 emissions related to installed engine power, transport capacity and ship speed. The EEXI is a design index, not an operational index. No measured values of past years are relevant and no on-board measurements are required; the index only refers to the design of the ship.

The emissions are calculated based on the installed power of the main engine, the corresponding specific fuel oil consumption of the main engine and of auxiliary engines (taken from the engine test bed), and a conversion factor between the fuel and the corresponding CO2 mass. The transport work is determined by capacity, which is usually the deadweight of a ship and the ship speed related to the installed power.

The calculation does not consider the maximum engine power, but for most ship types it is 75% of MCR or 83% of MCRlim (in case of an installed overideable power limitation). Specific fuel oil consumption of the main engine and ship speed are regarded for this specific power.

In conclusion, the EEXI is applied to almost all ocean going cargo and passenger ships above 400 gross tonnage. For different ship types, proper adjustments of the formula, through correction factors have been introduced to allow a suitable comparison. Several correction factors are defined to correct the installed power, such as for ice-classed ships, as well as to correct the capacity, for instance to consider structural enhancement. From a technical perspective, all ship owners and shipbuilding stakeholders must consider and assess how they will support compliance with EEXI. Depending on the vessel age and prospects, some owners and operators may even be scrapping vessels earlier than envisioned.

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What you need to know about vessel propeller immersion and stern tube cooling

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.

Example of propeller not fully immersed into the water

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.

Example of propeller shaft removal for stern tube bearing inspection

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

Example of damaged stern tube bearing due propeller immersion issue

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).

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Source and Bibliography:

  • DNV-GL

Ship Energy Efficiency

The overall energy efficiency of a vessel is determined by the choices made throughout its lifecycle, from the planning and new build phases to the final recycling phase, and is measured by the total amount of energy consumed in relation to a specific output, the total energy or fuel consumed per nautical mile.

Improving energy efficiency is one of the strategic plans of most shipping companies because the benefits include: lower fuel costs, which have risen significantly in recent years and are expected to rise further in the future, environmental protection, and compliance with current and future regulations.

The IMO has introduced mandatory requirements to encourage energy savings and reduce greenhouse emissions, which are currently centered on the SEEMP – the Ship Energy Efficiency Management Plan. The SEEMP’s goal is to create a mechanism for a company and a ship to improve the energy efficiency of ship operations.
The SEEMP aims to increase a ship’s energy efficiency in four steps: planning, implementation, monitoring, and self-evaluation and improvement.
These components are essential in the continuous cycle of improving ship energy management. The pursuit of energy efficiency requires more responsibility than the ship owner and operator can provide.
The list of all the parties involved in the efficiency of a single voyage is lengthy. Designers, shipyards, and engine manufacturers for optimal ship design, charterers, ports, and vessel traffic management services, and, of course, the crew for the specific voyage are obvious parties. All parties involved should consider incorporating efficiency measures into their operations, both individually and collectively.

Everyone onboard is responsible for using the least amount of energy necessary to do their job effectively and safely, because increasing energy efficiency benefits the ship, the operator, and the environment.

Key development areas should be identified and considered early in the business planning process. Comparing the performance benefits and lifetime costs of different investment opportunities can be a difficult task, and decision makers must find technically and economically optimal improvements.

It is critical to understand that a well-designed new ship can save up to 50% in operational costs when compared to an older ship. Up to 30% of this can be saved through fuel savings, as fuel consumption accounts for up to 50% of the total cost of operating a commercial cargo vessel. The easiest to assess and monitor is fuel usage reduction, as long-term forecasting of fuel price fluctuations is difficult.
Fuel prices were relatively low in the 1960s and 1970s. Many ships were built with little regard for energy efficiency. Newer vessels are designed to be more fuel efficient. The value of fuel efficiency will continue to rise and initial investments are required for energy efficiency. The profitability of this depends on a variety of factors, including the type of cargo being transported, interest rates, fuel prices, and the expected lifetime of the ship, among others.

Generally speaking, a ship’s life expectancy on regular routes is expected to be greater than that of a ship on irregular chartering in unstable financial conditions. The time it will take to recoup the initial investment should be considered when calculating the investment benefit. Short-term measures could save up to 10% of fuel consumption. Long-term measures should reduce it by at least 20% to 30%.

Ship design is the art of selecting a vessel configuration that corresponds to the mission, intended operational profile and route, as well as the operator’s performance requirements. This necessitates a thorough understanding of the vessel type, as well as new technologies and how they integrate with the vessel.
The first step toward improving energy performance is to recognize that the ship’s current performance is heavily influenced by the ship’s design. The hull shape and bow design, propeller and propulsion system, ship automation and crewing, heat recycling, and power generation are all important design factors in a vessel’s energy efficiency.
The above are normally chosen during the ship’s design and construction stages, but new developments in maritime design have made it possible for retrofitting or later design to deliver greater fuel economy.
The shape of the hull determines the ship’s water resistance and thus its fuel consumption.

Example of long slim hull design

A long, slim ship will provide less resistance than a short, beamy ship. Fuel consumption is also affected by the shape of the bow and stern.
The benefit of the bulbous bow has been well known for many years. Later research shows that this is also true for smaller vessels. Fuel savings of 25 to 50% can be obtained by increasing the length/beam ratio from 4.5 to 6.5. The optimal length/beam ratio is a trade-off between cargo capacity, harbor dimensions, and fuel consumption.

Example of bulbous bow replacement for efficiency purpose

Experience has shown that increasing the total length of the ship has little effect on the energy required to maintain the same speed as before.

Approximately half of the energy consumed by the ship’s main engine is wasted, primarily through cooling water and exhaust gases.
Heat recovery devices reduce energy losses by passing the hot exhaust gas through a heat exchanger and converting it to steam. Steam is then used to pre-heat fuel, turbo generators, and other heating devices. The benefits of installing such devices vary depending on the type of equipment that needs to be installed. However, in most cases, the amortizing time, or time to recover costs, will be around two years.


Exhaust gas recovery system example

A fuel gauge and a speed log are the bare minimum for better fuel economy. Onboard computers calculate fuel consumption in relation to the distance traveled and the vessel’s speed. Instruments used correctly can reduce fuel consumption by nearly 10%.

Example of mass flowmeter installed onboard vessel

Many ships have relatively high propeller speeds, ranging from 300 to 400 rpm. Cutting the speed in half could reduce fuel consumption by up to 25%. Larger propellers necessitate more space. This is limited by the shape of the stern and the ship’s draught. Longening the propeller shaft and the overhanging stern is one option. A new reduction gear box should be installed in addition to the new propeller. Ships with slow speeds and high propeller loads will benefit from the use of a nozzle around the propeller. However, most cargo ships traveling long distances will benefit little, if at all.

Example of high efficiency propeller

Diesel generators, turbo generators, and main shaft generators can all generate electricity. The type of engine and the ship’s management profile will determine which type is the most energy efficient. Shaft generators should be considered for fast and continuous-running engines.

Example of shaft generator

Small changes in operating conditions can result in significant changes in energy consumption, making it critical to continuously optimize operations throughout the ship’s lifecycle.

Optimal navigation can save up to 25% of fuel consumption, resulting in significant savings. Economical routing, time spent in port and at sea, speed optimization, continuous load output, optimal propeller pitch, engine efficiency, total efficiency, optimum draught and stability, reporting and computerized logging are all potential areas for efficiency improvement in ship navigation.
With such a diverse set of variables to consider, high performance in vessel navigation is possible if interrelationships are well understood and utilized.
The careful planning and execution of voyages can result in optimal routing and increased efficiency. Thorough voyage planning takes time, but there are a variety of software tools available for planning purposes. Routing is the calculation of the most efficient path. Land masses, soundings, prevailing currents, prevailing winds, and tide effects are all factors to consider. Once the route is established, it is the navigator’s responsibility to stick to it unless unusual weather conditions dictate otherwise. In general, excessive rudder use is not recommended because it slows down the vessel unnecessarily. In most cases, an autopilot will keep you on course better than manual steering.

The most significant energy savings can be achieved by reducing the ship’s speed, but unfortunately will also reduce its transportation capacity. As the ship’s speed increases so does the fuel consumption and even a small speed increase results in large increase in fuel consumption.

Based on experience operation at constant power can be more efficient than continuously adjusting the speed through engine rpm, but if the arrival time is the priority and the weather uncertain it is wise to allow for a sufficient time margin.

Harbour maneuvers should be done with progressive and moderate accelerations if the conditions allow for it, as any hard maneuver is a strain on the engine and a waste of fuel.

Some ships have a tendency to bury of lift their bow and the speed should take this into account, as in some ships it is possible to asses and adjust for optimal trim condition which will improve fuel efficiency continuously throughout the voyage.

Ballast should be adjusted taking into consideration the optimum trim and steering conditions and optimum ballast conditions should be achieved through good cargo planning. Ballast conditions have a significant impact on steering conditions and autopilot settings and it needs to be noted that less ballast water does not necessarily mean the highest efficiency.

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.

If you like my posts, please don’t forget to press Like and Share. You can also Subscribe to this blog and you will be informed every time when a new article is published. Also you can buy me a coffee by donating to this website, so I will have the fuel I need to keep producing great content! Thank you!