Assessment of bulbous bow performance over operational profiles using full scale data

A method of calculating the effects of a bulbous bow on delivered power has been developed. The method takes the full scale measured shaft power of a vessel with a bulbous bow, and compares this to a calculated calm water shaft power of the same vessel without a bulb. This enables the performance of the bulbous bow to defined in terms of a change in shaft power. The shaft power of the bulb-less vessel is calculated using the Holtrop and Mennen statistical prediction. Many of the inputs to Holtrop and Mennen are draft dependent, therefore these variables are calculated for the measured draft of the full scale data, using geometrical and empirical formula. Propulsive efficiency is calculated via draft corrected wake and thrust deduction (empirically) and assuming a Wageningen B-series propeller. The effective power prediction of Holtrop and Mennen is then converted to shaft power via the calculated propulsive efficiency. The performance of the bulbous bow is defined as the change in residual shaft power due to the addition of the bulbous bow. This change is made non-dimensional to remove the effects of hull shape and a similar performance formula to that of Kracht [1]. Two power effecting bulbous bow parameters, as defined by Kracht [1] ,the depth parameter coefficient and vessel Froude number are shown to widely range over the operational profile of 7 sister LNG vessels. The performance of the bulbous bow is calculated over these ranges. It was found that the bulbous bow performance improves in general with increasing Froude number, and that for a given Froude number range there is an optimal bulb-depth parameter for ballast and laden conditions. Once the performance of a bulbous bow over an operational profile has been identified, it can enable the vessel operator to make operational changes, in order to ensure the bulb is working in optimal power-reducing conditions. Additionally, this performance indicator could be used to decide as to the requirement of a re-design of the bulbous bow. The performance of this bulbous bow is found to be dependent on the full-scale measurement accuracy and the accuracy of Holtrop and Mennen to predict effective power. A future sensibility analysis of this method is needed to understand the reliability of the bulb performance value. Despite this, this method still shows the expected trends of a bulbous bow from the literature, and can therefore be used for operational optimisation, via ensuring the bulb depth parameter is in an optimal condition for the given Froude number range.