The stability of acoustic extensometers designed to measure the thrust in a ship's propeller shaft
H I S Allwood, J W Drinkwater · Journal of Physics E Scientific Instruments · 1971
Acoustic (vibrating wire) extensometers are robust and accurate transducers which lend themselves to telemetry from moving parts. The factors that limit their stability have been studied and extensometers of high stability have been made. Experiments with an array of these extensometers mounted on the propeller shaft of a diesel-engined cargo liner showed that their residual instability could be considered as a creep of about 2 parts in 10 8 per day, together with a port-to-port variability of about 1 part in 10 7 . In a later trial a strictly linear relationship was found between the torque applied to the propeller shaft and the thrust generated by the propeller. The scatter of the points about the line indicated that a standard deviation of not more than 0.3% of maximum thrust would be generally achievable. Thrust measurements of this accuracy are more than adequate for measuring the performance of ships. Consequently the stability of the new extensometers is high enough for this service.