Experimental and Theoretical Study of Ship Resistance with Unsteady Forward Speed

Whilst there is a huge body of literature related to the theoretical prediction and experimental measurement of ship resistance, the vast majority if it relates to resistance at steady forward speed. In contrast, relatively few studies have been made of the influence of acceleration on ship resistance. This is an interesting and challenging problem of fundamental ship hydrodynamics which has several practical applications. In ship resistance model testing, it is desirable to achieve steady conditions rapidly to make best use of a limited length of towing tank; improved understanding of the unsteady effects resulting from the acceleration at the start of a run can lead to more effective use of expensive facilities. Some particularly complex effects are expected when ships accelerate in shallow water. Understanding of these effects may help to improve planning of operational patterns for high-speed ferries. In particular types of vessels (e.g. rowing shells) the propulsion is unsteady, leading to periodic unsteady forward speed. Surge accelerations as high as 1.5g have been recorded. It has been suggested that unsteady effects increase mean resistance by 5% or more, so this phenomenon is clearly of interest if it is wished to optimise the hull form of a rowing shell or canoe.The current proposal is aimed at enabling the continuation of an existing collaboration to research in this area. Prof. Lawry Doctors, from UNSW, in Sydney Australia, visited the UK in June 2006 to carry out a research programme at the Universities of Glasgow and Strathclyde (UGS). It is proposed that this research is continued through a four week visit in Autumn 2007.Prior to Prof Doctors visit to Glasgow in 2006, a numerical method was developed which allows the time history of resistance to be predicted for any velocity history for a ship in water of finite width and depth. Two areas of interest were identified. The first area was an examination of the unsteady effects typical of a towing tank test, in which a period of steady velocity is preceded by a relatively rapid acceleration. The second area of interest was chosen as a preliminary investigation of the resistance of ships travelling with a periodic unsteady motion.In the UGS Acre Rd facility it is possible to carry out a towing test with any velocity profile desired, subject to upper limits on run length, velocity and acceleration. During Prof. Doctors visit to Glasgow in 2006, two innovative experiment studies using a standard hull form, and the results compared to theoretical predictions.The first study examined the impact of different acceleration profiles on the unsteady resistance effects on a towing tank test. This has now been completed and written up. Agreement between experiment results and numerical prediction was extremely encouraging and some very interesting phenomena were discovered. For example, the general form of the unsteady resistance predicted by Wehausen was followed; however, there were significant effects of the tank width and depth on the frequency, amplitude, and most importantly, the decay rate of the unsteady variations. The second study, examining the impact of periodically varying velocity on resistance has now been analysed. The results for the wave resistance prediction are also extremely promising, and it is expected that the results will be submitted for publication in 2007. The preliminary study of the oscillatory velocity case has highlighted some additional interesting challenges; in particular (a) the separation of unsteady wave effects from unsteady frictional resistance, and (b) the prediction of unsteady effects close to critical depth Froude Number. Additional experiments would create a more complete data set allowing a fuller insight to be gained into these interesting phenomena. These experiments are the reason for the proposed visit in 2007.