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Hydrodynamics of Three Slender Models Resembling Polynesian Canoe Hulls
Towing tank tests were carried out on three slender models in the Towing Tank at Newcastle University at fixed sink and trim in order to obtain more information on the hydrodynamics of such shapes, and in particular, how the shapes could generate side force when operating at a leeway angle. The research was motivated by a study of ancient Polynesian multi-hull vessels which did not appear to have keels, and so the side-force had to be generated by the hulls. The authors speculate that the earliest vessels had rounded hulls (from trees) and were probably used mainly for sailing downwind. However, it appears that evolution has caused a change in shape from circular to Vee, presumably because such shapes are better able to generate side force to enable the vessels to also sail across the wind. A CFD study with ANSYS-CFX using three different hulls was carried out as suggested by the first author and it showed that sharper Vee sections were better at generating side-force than a rounded hull. The purpose of the present tests was to investigate whether such behaviour could also be observed in physical testing. Three models were manufactured and were tested in the Towing Tank at Newcastle University in July and August 2013. It was found that there was good agreement between the CFD and tank test results, and that indeed the hypothesis that narrower Vee-shaped hulls would generate more side-force when at leeway than a rounded hull was proved.
A Comparative Study of Program FloSim Results against SYRF Wide Light Project Data
In November, 2015, the Sailing Yacht Research Foundation (SYRF) published the tank test data from their “Wide Light Yacht Project” for the hydrodynamics of a modern, high performance, semi-planing yacht. This comprehensive data set, comprising canoe body with and without appendages in upright, heeled and yawed conditions, provides an important validation base for CFD codes; previously, such data were not readily accessible mainly due to proprietary issues. The SYRF report includes a number of comparative results from commercial CFD codes, the RANS program Star-CCM+ results in general showing the best correlations with the measured data, albeit with some significant departures.
In this paper, we present results computed using an advanced Boundary Element Code, FloSim, these calculations being compared against the test matrix of “Wide Light” measured data and also Star-CCM+ calculated results. Times for computer model preparation and case execution are discussed together with computer requirements.
An outline of the FloSim method is presented; it is an unsteady, time-stepping code with a coupled integral boundary layer analysis for viscous effects such as skin friction resistance and boundary layer displacement effect. The free surface wave development uses a non-linear mixed Eulerian-Lagrangian treatment at each time step. FloSim has free convection and rollup of vortex wake elements providing non-linear lift characteristics and includes a number of modeling techniques for treating “real world” effects, such as flow separation and wave breaking. For the higher speed cases in the SYRF data set that have a breaking bow wave crest, FloSim’s Wave-Breaker treatment is applied to convert excessive energy in the bow wave to a “dead-weight” pressure applied on the free surface; this effectively attenuates downstream wave amplitudes consistent with the loss of energy at the breaking crest. The empirical Foam Density Factor in the model has been established here as a simple function of Froude number based on the measured results at the Fn 0.8 upright case.
FloSim, already used in America’s Cup and Volvo racing yacht analyses, was developed specifically to bridge the gap between basic potential flow panel methods and RANS codes, with the objective of providing accurate, practical solutions on a laptop computer within a reasonable turnaround time and cost. In essence, the results presented in this paper demonstrate these objectives have been achieved. The comparisons of FloSim’s essentially “low-order” results against test data and Star-CCM+ RANS calculations, provide a measure of tradeoff between calculation accuracy versus cost and turnaround time for a case. Throughout the discussions presented below, the reader should keep in mind that this was not a “blind” comparison as it was for the original Wide Light Project participants who published their results prior to the tests.
Experimental measurement and simplified prediction of T-foil performance for monohull dinghies
The rise in interest in large foiling yachts, such as those in the America’s Cup, has spurred a corresponding interest in foiling applications in monohull sailing dinghies, both for boats designed specifically for foiling, and through retro-fit foil kits. The present study considers the assessment of foil systems for such boats, and specifically the prediction of the necessary foil performance of T-foils with flaps.
The main lifting foil for a moth dinghy with flap was tested at full-scale in a towing tank at a range of speeds, trim angles and flap angles, and immersions, to measure vertical and lift and drag. Results are then compared with two simplified models typical of those utilized in Velocity Prediction Programs for preliminary design. The first model uses section lift and drag data obtained using the well-known XFOIL code allied to a simple correction for 3D effects while the second model deploys a numerical lifting line theory in conjunction with section data.
A number of practical conclusions are drawn regarding the set-up and sailing of foiling dinghies with flapped T-foils. Results show that whilst the simple and rapid models typically used in basic VPPs may not accurately represent the relationships between angle of attack / flap angle with lift and drag, the predicted relation between lift and drag is reasonably accurate in most cases.
Statistics are provided for number of sailing catamarans and approaches to craft dimensioning are reviewed. Styling trends and typical catamaran arrangements are featured. Weight components are studied for number of catamarans of different sizes and levels of comfort on board. The effect of catamaran architecture on performance is studied by combining VPP predictions with CFD modeling of various deck/cabin configurations. A summary of safety requirements specific to catamarans is given. Case studies are presented of a number of cruising catamarans designed by AMD and a new prospective concept of 44' catamaran featured.
To sail a 470 faster, authors consider the sailing performance of the 470 from the various measured data and the simulated results of Velocity Prediction Program (VPP). This is a summary of TOBE-470 [1] presented by the author.
