Uncertainty in the real-time estimation of ship speed through water

The ship speed through water (STW) is a critical variable for evaluation of ship performance. With a cubic relation to the expected shaft power, even small inaccuracies in measured STW amplifies to more considerable inaccuracies in the expected shaft power. STW is traditionally measured using a speed log, more specifically by the Doppler acoustic speed log principle. The stability and precision of this technique is however questionable, particularly when ships are exposed to waves. For ships equipped with sensors and instrumentation for measuring propulsion related data, the STW can be estimated from in-service measurements on the propeller shaft. An accuracy of this STW estimate similar to the speed log will increase the overall confidence in the ship performance evaluation. In this paper we study the uncertainty in the estimated STW using propeller loading measurements, and identify the most critical components of ship instrumentation in order to achieve an estimate of STW with sufficient accuracy. The uncertainty analysis includes a fixed pitch and controllable pitch propeller, influence from including a thrust sensor and influence from waves on the expected uncertainty in estimated STW. The uncertainty in estimated STW is found to be similar to the uncertainty provided by manufacturers of Doppler speed logs. The propeller pitch angle is found to have a significant contribution to the total uncertainty in STW. Including thrust measurements decreases the uncertainty in STW by 34%, and the uncertainty is found to be not very much affected by the presence of waves.