The effect of weather, particularly short sea waves, on ship speed performance

Abstract

A survey of literature concerning methods of predicting the effect of weather on ship speed performance at sea has been made covering the following topics: - - Added resistance due to regular waves, - Wave diffraction, - Wave drift force, - Added resistance due to irregular waves, - Added resistance due to wind, - Wind and wave conditions at sea, - Involuntary speed loss and power increase for a given added resistance, - Effect of rough weather on ship speed performance. The survey revealed that it is difficult to determine numerically the speed loss and power increase due to inclement lather. A principal reason for this is that none of the existing theoretical methods available for determining the added wave resistance are convincingly accurate, in particular over the range of wavelengths which are short compared to the length of a ship. An approximate method for calculating added resistance due to regular wave reflection has been established by the Author. The basic steps of the method are as follows: - (1) To find a mathematical model of a ship by making use of the NAG subroutine based on a minimax polynomial fit method. (2) To evaluate the wave drift force due to very, short waves(i. e. When it may be assumed that the incident wave potential is the same as the potential due to body disturbance) for a vertical axis cylindrical body having infinite draught, and for which the waterline shape is the same as the actual ship, using the mathematical formula obtained at 1(1)1 and Bessho's formula. (3) To correct the result of '(2)' with a correction factor for the effect of wave scattering, based on the wave scattering coefficient derived by Jones et al. using ideas developed in the shady of geometrical optics. (4) To correct the result of '(3)' for the effect of finite draught considering the orbital motions of water particles. (5) To correct the result of '(4)' for the effect of forward speed with the correction factor given by Fujii-Takahashi but modified for the case of oblique waves by the Author. In order to confirm the applicability of this method, an experimental work was carried out by the Author using a Series 60 model with oscillations in the 6-degrees of freedom restricted. Particular attention was paid to the case of the shorter wavelength range where the effect of wave reflection is dominant cared to the effects of the ship's motions (Wave steepness = 10.6 ~ 101.0, A/L = 0.23 ~ 1.18, Fn = 0.10 ~ 0.25). When comparing the measured and the computed resistance due to wave, reflection in a head sea, good agreement is shown. To modify the results of most conventional methods for added resistance due to regular waves, it is assumed that the total resistance increase of a ship in regular waves can be approximated by the sum of the resistance due to wave reflection and the resistance due to the ship's motions. Modifications of results determined by Gerritsma's method and by Maruo's method were made using the Author's routine. The modified results were compared with results using Fujii-Takahashi 's method, that of Gerritsma, and that of Maruo as well as with some experimental data measured by Strom-Tejsen et al., van Sluijs et al., Loukakis, Shintani, and Fujii et al. for Series 60 and tanker forms in head and oblique regular waves. Fran the comparisons, the Author concludes that his method of determining added resistance due to regular waves provides a good approximation for practical purposes, bearing in mind, however, that an increased error may be found with longer wavelengths and higher speeds in the range Fn > 0.25. Using the Author's method and the linear superpositiari technique, added resistance due to irregular waves was calculated for a Series 60 model in several experimental spectra used by Sibul. The computed results were compared with the model data measured by Sibul in irregular waves. The comparison reveals that the results of the Author's method agree well with the measured data. A comparison was made by the Author to find differences between the results of added wind resistance calculated using methods due to Isherwood, van Berlekan, Aage, Wilson et al., Tsuji et al., Wagner, Gould, and Shearer et al.. The mean difference between the results of Isherwood's method and the others quoted above was 7% for a given relative wind speed for a tanker in head winds where the added wind resistance may be larger than that for oblique winds. Using the following routines :- - the Author's for added wave resistance, - van Berlekan's for wind resistance, - van Berlekan's for speed loss and power increase due to a given added resistance. and using the I. T. T. C. standard spectrum and particular sea conditions, various effects of weather on ship speed performance at sea were investigated. The following topics were covered: - - Effect of weather intensity and ship type on added wave resistance (Tanker, Containership), and comparison of the result with full scale data (Tanker). - Caparison of the result of the Author's method with those based on experimental data for the thrust increase due to waves (Tanker). - Effect of ship size, ship type, and weather intensity on the ratio of added wave resistance to the total added resistance (Tanker, Containership, Passenger liner). - Comparison of the results of the Author's method with full scale data for speed loss (Ore carrier). - Effects of ship speed and draught on power increase in certain cases (Tanker). - Effect of weather intensity on the additional energy expenditure per nautical mile (Tanker). - Comparison of the result of van Berlekan's formula for speed loss due to a given added resistance with the result of Townsin's empirical foi hula based on full scale data (Tanker). - Establishing approximate formulae (the Townsin/Kwon formulae) for speed loss due to the effect of head wind and waves (Tanker, Container-ship). - Comparison of the relative effects of weather and hull roughness on power increase and speed loss (Tanker). To conclude, the Author considers that the thesis provides an improved understanding of the effect of weather on ship speed performance, particularly in short sea waves. The Author expects that his work will facilitate the following: - - Analysis of sea trial data. - Improved estimation of service power margins. - Accurate determination of optimum speed for fuel economy etc.. This thesis is based on research done by the Author as a member of the Ship Performance Group in the Department of Naval Architecture and Shipbuilding of the University of Newcastle-upon-Tyne in England, under the supervision of Dr. R. L. Townsin.