In a ship’s Speed/Power Trial, the relationship between ship speed, shaft power, and propeller speed is measured under specified ship conditions.

These measurement results are used to verify that the contractual speed and power have been achieved, and also to determine the speed and power used for the Energy Efficiency Design Index (EEDI) established by the IMO.

Shaft horsepower meters are used not only during sea trials of newly built ships, but also for monitoring shaft power and understanding the operational performance of ships in service.

This article outlines the definition of shaft power and the basics of measurement using shaft horsepower meters, with reference to the ITTC guidelines for speed/power trials.

4.1 What Is Shaft Power?

Shaft power is the net power supplied to the propulsion shafting from the propulsion machinery after passing through the reduction gearing and other transmission equipment, and after deducting the power used by auxiliary machinery.

In contrast, the power obtained at the output coupling of the propulsion machinery, before passing through the reduction gearing or transmission equipment, is called brake power.

Shaft power does not represent the power generated by propulsion machinery such as the main engine itself, but the power supplied to the propulsion shafting after the effects of the transmission equipment and auxiliary machinery.

4.2 What Is a Shaft Horsepower Meter?

A shaft horsepower meter is a measuring device used to measure shaft torque and shaft speed in a ship’s propulsion shafting and to determine shaft power from these measured values.

The main measurement items in a speed/power trial include shaft torque or shaft power and shaft speed.

Shaft power P [W] is calculated from shaft speed n [rpm] and shaft torque Q [N·m] using the following equation.

4.3 Measuring Shaft Torque

The following devices are used to measure shaft torque.

・Torsionmeter with a calibrated permanent torque sensor
・Strain gauges attached to the shaft
・Alternative measuring devices with proven accuracy equal to or better than the above

The ITTC guidelines require that the measurement system used for speed/power trials be certified to have a bias error in power measurement on the test shaft of less than 1%, and be capable of achieving an overall bias error of less than 2% on board the actual ship.

If shaft torque cannot be measured directly, an alternative power measurement method recommended by the main engine manufacturer and approved by the shipowner and the verifier may be used.

4.4 Measuring Shaft Speed

Shaft speed is a main measurement item used together with shaft torque to determine shaft power.

The following devices are used to measure shaft speed.

・Pickups
・Optical sensors
・Onboard tachometers

Shaft speed is expressed in revolutions per minute (rpm).

4.5 Role of the Display Unit

A shaft horsepower meter may include a display unit for checking measurement results such as shaft torque, shaft speed, and shaft power.

The display unit is not a sensor mounted on the shaft to directly detect torque or speed, but a device that displays the measurement results received from the measurement system.

It is used not only during sea trials but also after the ship enters service, to check measurement results such as shaft torque, shaft speed, and shaft power.

The displayed items, connection methods, communication protocols, and data recording functions vary depending on the configuration and product of the shaft horsepower meter.

Therefore, when replacing or selecting a display unit, it is necessary to confirm compatibility with the existing sensors, signal processing unit, power supply, and communication specifications.

About This Article

Reference
・International Towing Tank Conference, Preparation, Conduct and Analysis of Speed/Power Trials, ITTC Recommended Procedures and Guidelines, No. 7.5-04-01-01.1, Revision 08, 2024.
(Last accessed: September 25, 2026)

※This article was prepared with reference to the source listed above and organized based on the author’s understanding.