The Development of Hydrodynamics: 1860 – 2010
By Professor A F Molland FRINA and Professor P A Wilson FRINA
In the early 1860s, little was really understood about ship resistance and many of the ideas on powering at that time were erroneous. Propeller design was very much a question of trial and error. Te power installed in ships was oſten wrong and it was clear that there was a need for a method of estimating the power to be installed in order to attain a certain speed. However, over the following 150 years, significant development was made in estimating ship propulsive power.
The Early Years
In 1870, William Froude initiated an investigation into ship resistance using models. He noted that the wave configurations around geometrically similar forms were similar if compared at corresponding speeds, that is speeds proportional to the square root of the model length. He propounded that the total resistance could be divided into residuary, mainly wavemaking, resistance and skin friction resistance. He derived estimates of frictional resistance from a series of measurements on planks of different lengths and with different surface finishes. Specific residuary resistance, or resistance per ton displacement, would remain constant at corresponding speeds between model and ship. His proposal was initially not well received, but gained favour aſter full-scale tests had been carried out. HMS Greyhound (100 ſt) was towed by a larger vessel and the results showed a substantial level of agreement with the model predictions. Model tests had been vindicated and the way opened for the realistic prediction of ship power. In his 1877 paper, Froude gave a detailed explanation of wavemaking resistance which lent further support to his methodology. In the 1860s, propeller design was hampered by a lack of understanding
of negative, or apparent, slip, naval architects being not fully aware of the effect of wake. Early propeller theories were developed to enhance the propeller design process, including the momentum theory of Rankine in 1865, the blade element theory of Froude in 1878 and the actuator disc theory of Froude in1889. In 1910, Luke published the first of three important papers on wake, allowing more realistic estimates of wake to be made for propeller design purposes. Cavitation was not known as such at this time, although several
investigators, including Reynolds, were attempting to describe its presence in various ways. Barnaby, in 1897, goes some way to describing cavitation, including the experience of Parsons with Turbinia. During this period, propeller blade area was based simply on thrust loading, without a basic understanding of cavitation.
32 By the 1890s the full potential of model resistance tests had been
realised. Routine testing was being carried out for specific ships and tests also being carried out on series of models. A notable early contribution to this is the work of Taylor, closely followed by Baker. It is a tribute to Taylor that the results of his work, including the re-analysis by Gertler, are still in common use today.
The Middle Era
Te next era saw a steady stream of model resistance tests, including the study of the effects of changes in hull parameters, the effects of shallow water and to challenge the suitability and correctness of the Froude friction values. Tere was an increasing interest in the performance of ships in rough water and the need to assess this performance. Several investigations were carried out to determine the influence of waves on motions and added resistance, both at model scale and from full-scale ship measurements. Following the earlier work of Michell and Havelock, Wigley presented his first of several papers on the mathematical calculation of ship resistance together with supporting experimental results. Te 1920s saw much interest in improving propeller efficiency
and, in 1928, vortex theory applied to propellers was introduced by Perring. Tis was basically a combination of momentum and blade element theories, generally following the approach for airscrews used by Lanchester, although there were shortcomings due to the wide blade of the marine screw. In 1927, Telfer introduced a fundamentally new method of
extrapolating model resistance values to full-scale ship values which does not entail breaking down the resistance into its components. Experiments are carried out on a family of models, which Telfer termed ‘Geosims’, and the slope of the extrapolator determined experimentally in the region of the model values. Te method has a sound scientific basis and is valuable as a research tool, but is found not to be cost effective
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