Shaft Power Meters Coming of Age

Shaft Power Meter installation photo
By Duncan Gaskin, Project Manager, Vessel Technology
Duncan Gaskin
 At a time when there is an abundance of new digital tools and technologies to optimise operations, it can also be beneficial to re-valuate how existing tried and tested solutions can be applied to solve modern fleet challenges. One example is the use of shaft power meters. These have been installed on ships for many years, without being viewed as an efficiency tool. Following continuous adaptation, they can now play an integral role in improving fuel efficiency and whole-life vessel condition as well as contributing to achieving emissions reductions.

 

Shaft power meters measure the real-time power output of a ship's engine as transmitted through the propulsion shaft. By providing precise data on how much energy is being used to propel the vessel, these meters allow operators to optimise engine bad, avoid overconsumption of fuel, and identify inefficiencies in the propulsion system.

 

With a variety of shaft power meters available, it is challenging for fleet managers to assess and select an appropriate solution for the vessels they oversee, with the upfront cost of installing this technology representing an additional barrier to adoption. That said, with a short pay-back period, the proven long-term savings of these solutions make these a worthwhile investment for shipowners and operators.

 

Having precise control over a vessels energy consumption can make all the difference in meeting financial targets and environmental regulatory requirements. Also, with fuel costs typically representing around 50% of a vessel's operational expenses, which is set to rise with the onset of alternative fuels, even small improvements in fuel efficiency can offer considerable savings.

 

When comparing performance against baseline data, shaft power meters can provide an early indication of any degradation in operating conditions by highlighting increasing power usage. Examples of this could include issues with the engine, shaft components, or fouling of the hull and propeller. This information can then be used to adjust operating conditions and help schedule preventative maintenance.

 

Fouling of the hull and propeller is a formidable challenge faced by the shipping industry and can increase fuel consumption and engine emissions by over 50%. With a shaft power meter fitted, the effects of fouling can be spotted early, and preventative action taken to limit its impact. This also allows for the monitoring of anti-fouling technologies applied to the hull or propeller to determine their effectiveness.

 

Shaft power meters like Trelleborg's TSX5 system utilise strain gauge technology applied to the surface of the intermediate shaft. The engine and gearbox apply torque to the shaft, while resistance from the propeller applies drag. The combination of torque and resistance twists the shaft, causing the strain gauge to deform and change its electrical resistance, from which a measurement of strain is calculated.

 

The change in resistance from the strain gauge is measured by the rotor board unit, which is housed in the rotor ring clamped to the shaft. The rotor ring also protects the strain gauge from damage and contamination. The rotor board unit conditions and processes the information from the strain gauge and then wirelessly transmits the signal zo the processing cabinet for calculating shaft torque and power.

 

The TSX5 processing cabinet and power head are mounted on the stanchion unit, and the height of both can be adjusted to match the centre of the shaft and optimise performance. The stanchion unit is mounted on a pedestal, which is securely welded to the deck.

 

An electromagnet housed in the power head induces an electrical current in a copper coil wrapped on the rotor ring, and this powers both the rotor board unit and acts as a magnetic source for rotor-mounted inductors to measure shaft direction and speed.

 

The values for torque and power can be displayed on remote screens in the engine control room and on the bridge. Shaft speed is also displayed and thrust measurement can be provided as an option. Cumulative totals are reported, and the configuration mode allows for set-up, calibration, and fault finding.

 

Despite the clear benefits of installing shaft power meters, there are practical considerations when it comes to fitting them on a vessel. TSX5 shaft power meters can be fitted to a wide range of shaft sizes, from 150mm up to 1,000mm for the largest ships. 4-20mA and serial data outputs are available for connecting to the ship's IAS or DCS and for connecting to a ship performance monitoring system.

 

Most ships currently in operation that needed to comply with the EEXI rules for limiting engine power have completed the necessary modifications.

 

Shipowners and operators could choose between engine power limitation (EPL) or shaft power limitation (ShaPoLi) to reduce engine power. Many have chosen the EPL approach where the engine power is limited through a physical device on the governor to restrict fuel flow to the engine (for mechanical engines) or through a software upgrade in the engine control system for electronic engines.

 

Shaft power limitation on the other hand offers a different approach, where power is measured by a shaft power meter and is connected to an alarming device on the bridge. This device is activated when the measured power axceeds for the engine's EEXI limit. Unlike the EPL method, the engine under ShaPoLi does not have any limiting device fitted. This method, with its unique features, is gaining traction among ship operators.

 

As operators gain experience in managing engine performance and compliance with this regulation, some have identified that the EPL method poses operational and safety challenges, especially for ships with mechanical engines and significant power restrictions where engine responsiveness and performance during critical situations can be compromised. Consequently, some owners are switching from EPL to ShaPoLi systems.

 

The shift has been further driven a new policy directive (16670 CG-ENG Policy Letter 01-24) that was introduced in April this year by the US Coast Guard (USCG), requiring ships using an EEXI compliance method to 'override' the system while operating in US waters.

 

Although any ship using the EPL or ShaPoLi power limitation methods on the face of it will not comply with EEXT when in US waters, safety concerns for ships equipped with ShaPoLi systems are significantly lower than for those using the EPL method for mechanical engines as the engine remains unrestricted and in its original condition. This means that the existing pilot cards and wheelhouse chars remain applicable even when the ship operates within its EEXI limits as unreserved power can be accessed even without needing to override the system. However, to comply with this directive, the system must be overridden, and the captain can easily do that by selecting the override function on the ShaPoLi display and cancelling the EEXI alarm - a process that takes just seconds.

 

On the other hand, ships with mechanical engines using the EPL method are physically limited and cannot exceed the EEX1 limit without removing the limiter on the governor. Therefore, to override an EPL system will require the limiter to be removed prior to entering US waters. As a result, the existing pilot cards and wheelhouse posters will not apply if they have not been updated. In this context, the USCG's concerns regarding the safety of ships operating in restricted waters with these systems are valid.

 

As the maritime industry strives to simultaneously decarbonise and improve fleet operations, systems like shaft power meters are emerging as crucial tools for achieving these goals. The whole-life cost savings and operational efficiencies offered by shaft power meters far outweigh the initial investment, making them an essential component in the future of vessel monitoring and decarbonisation.