In hull shape studies we use fully appended models with a reference set of rudders, keels and foils. The position of each appendage shifts to match the shape of each candidate hull. For example, the rudders will change angle slightly to remain perpendicular to the hull surface on each hull. For appendage studies we use a set of reference hulls and change appendage concepts, shapes, positions and orientations. Each variation is evaluated by comparing the resulting forces (drag, side force, roll and yaw moments) and by comparing the flow characteristics using stream lines and other visualization techniques.
Our hull shape research program is quite extensive. The design spiral begins with the required transverse stability and waterline beam. We then obtain accurate sail force coefficients using our own aerodynamic simulations. Different sail sets are tested for upwind and downwind sailing in light and heavy conditions. This is important, especially for very beamy boats, because the sail forces can have a large effect on the longitudinal trim which changes the drag. Instead of attempting to match a given sail set to specific sailing conditions, which is difficult and error prone, we find the center of effort for the sails and apply a force vector at that point in the hydrodynamic model. Then the hydrodynamic simulation runs so the drag on the boat matches the force generated by the sails. The hull shape investigation continues with studies of volume distribution, prismatic coefficient, transom width and immersion, bow fullness, etc. Hull shapes are organized with parent hull shapes and their derivatives. This allows for easy analysis of trends and performance drivers and final selection of a hull shape.
After the final hull has been chosen and the lines have been sent to the builder, research continues with appendages and sails. We investigate the size, shape, and position of the keel, bulb, rudders and dagger boards. We adjust the transverse inclination, longitudinal inclination (sweep), alignment of the keel cant axis, pitch of the bulb, angle of attack of the dagger boards, etc. Different solutions and details for the attachment of foils to the hull are investigated, like the recess or ‘bubble’ in the hull where the keel attachment (a cylinder) rotates, and the fairings between hull and foils.
Candidate boats are then run through race simulations to determine which design is the best. VPP is used to analyze trade-offs, such as stability versus drag, and to determine optimum balance. Our Router program simulates the best course for each hull using statistical weather data for relevant parts of the world, and then compares the time needed to complete the course for each hull. Thus, the overall winner of the race is found in probabilistic terms. Other design variations, such as the position of a dagger board, are more straightforward to analy
se and usually it suffices to compare the amount of drag at a given side force to draw conclusions. In many cases the design and modification of appendages and sails is given a reality check when tested on the trial boat in real sailing conditions. During this on the water training period we are given valuable feedback from the real world that keeps us motivated and focused.