Service-performance trials carried out in the North Atlantic on s.s. “Cairndhu” 1

Abstract

This paper describes trials carried out by the British Shipbuilding Research Association in a dry-cargo vessel under normal service conditions in the North Atlantic. The object was to study the performance of the vessel under different conditions in order to provide accurate data for comparing and developing methods of analysing service performance, power allowances, etc.

An important item in work of this nature is the trials instrumentation and consequently this was given careful attention, the object being to obtain autographic records of as many items as possible. Shaft revolutions, wind speed and direction, rudder angle, angles of roll and pitch, and ship speed by pitometer log, were all recorded autographically. It was not possible to record propeller thrust and torque autographically and for these measurements a Michell thrustmeter and Siemens-Ford torsionmeter were used. Estimates of wave length and height during each observation were made visually and stereoscopic photographs were taken.

During each observation the power necessary to maintain the recorded speed was measured; this power was higher than that necessary to maintain the same speed in calm water; the percentage difference between these two powers has been estimated to give the percentage increase in power under the prevailing conditions. The effect of direction of encounter has been analysed in a similar manner. The loss of speed for a given shaft horse power from the speed for the same power in calm water has also been estimated for each observation. Details are given of the weather encountered, ship motions during the trials, and of the velocity distribution within the boundary layer measured by means of the pitometer log.

It was realized that on a single round voyage of this nature, it would be quite impossible to cover all the variations of weather and condition of hull surface normally encountered in service. Conclusions reached regarding power margins would naturally depend on the weather encountered by these vessel during the voyage and the condition of the hull. An appendix has therefore been prepared giving details of the weather encountered by the vessel during the first year of service, and a brief analysis has been made of the performance during this time from data supplied by the ship’s officers.