The Windward Performance of Yachts in Rough Water

A 5 year research program to investigate the effect of realistic hull form parameters on the added resistance of a yacht in waves is nearing completion. Model experiments and theoretical predictions were carried out and the results are discussed in this paper. Five hull form parameters have been investigated so far, they are: 1. stern overhang; 2. LCB-LCF separation; 3. prismatic coefficient, 4. displacement length ratio and 5. beam draft ratio. NOTATION CP Prismatic coefficient Cts Total resistance coefficient, for calm water LCB Longitudinal Centre of Buoyancy LCF Longitudinal Centre of Flotation Lwl Length, waterline RAO Response Amplitude Operator V Yacht velocity, (m/s) VPP Velocity Prediction Program INTRODUCTION The aim of this joint Australian Maritime Engineering Cooperative Research Centre (AME CRC) and Murray, Burns & Dovell (MBD) project was to develop a VPP module which calculates the added resistance of a yacht in waves from theoretical predictions and towing tank experiments. The RAO of the added resistance is obtained by dividing the added resistance of the yacht in waves by the square of wave amplitude. The International Measurement System (IMS) handicapping system relies on a VPP, originally developed by Kerwin, 1978, to predict the performance of a yacht for a given hull shape and rig geometry. The predicted yacht velocities are used to determine, for different wind velocities, a handicap time allowance for the particular yacht in seconds per mile. This handicapping system therefore endeavours to allow the only variables between yacht performances to be the sailing abilities of the crew. The IMS VPP is available to yacht designers to use during the design procedure, however it has certain drawbacks, for example, inability to incorporate tank test results into its calculations. To rectify this a VPP has been developed by MBD, that has the facility to include calm water resistance data obtained from tank tests, thus giving a more accurate indication of the yacht’s calm water performance compared to using the regression based resistance algorithms. This enables the designer to determine how a yacht will perform in relation to its handicap, by comparing results from the AME CRC VPP with the IMS VPP. Gerritsma conducted a set of theoretical predictions on the Delft Systematic Yacht Hull Series using a strip theory seakeeping code. This enabled them to produce a polynomial expression for added resistance based upon the main yacht parameters: length, displacement, beam, draught and prismatic coefficient, see Gerritsma et al., 1993, pp 239-245. Using this polynomial expression it is possible to calculate the added resistance, as a function of wave amplitude, of a yacht for a given wave direction, wave frequency and Froude number. The research described in this paper was conducted to investigate further the parameters tested by Gerritsma. MODEL DETAILS A standard yacht series was designed by MBD and tested by AME CRC. The parent hull form in the AME CRC series, 004a, is an IMS type yacht based on the Delft Series 2 yacht form. A body plan of the yacht is shown in Fig. 1.