Conventional transom-hung rudders are often used on small sailboats because of their simplicity compared to rudders mounted under the hull; however, they present substantial performance penalties, including (1) the rudder is more likely to ventilate by drawing air down from the free surface, (2) the effective aspect ratio, and therefore the lift-to-drag ratio, is not increased by the mirror-plane of the hull bottom and (3) there is additional spray and wavemaking resistance that arises as a result of the rudder passing through the free surface. This case study focuses on a means to mitigate the last of these penalties, the increased spray and wavemaking resistance. While many transom-hung rudders are essentially parallel, or tapered with the maximum chord at the top where it meets the tiller handle, the reader will recognize that having the largest cross section of rudder at the free surface will generate significant spray and wavemaking resistance, especially when the rudder is turned. This study investigated the use of minimizing the rudder chord length where it passes through the free surface, demonstrating the findings by full-scale towing tests of a series of rudders designed for a Fireball-class dinghy. Running the tests at full-scale, therefore matching Reynolds number and Froude number, eliminated questions on scaling. Experimentation on the effects of sweep angle, section shape and chord length at varying angles of attack and velocities showed a noticeable increase in lift-to-drag ratio of foils with reduced chord length at the free surface and by sweeping the rudder forward. To complete the case study, a velocity prediction program was used to estimate the change in speed around a notional race course.
Two boats sailing in close proximity may inevitably compare their boat speeds, perhaps even “race” each other. Over the last few centuries there have been numerous handicap systems designed to estimate the performance of different boats relative to one another. Corollary to handicapping is another arcane art, that of scoring races. Scoring methods have both technical options, in part determined by handicap rules, as well as “human engineering” options in the sense that different solutions can work best for different constituencies, be they race organizers or sailors. Options may include single vs. multiple ratings, time on distance vs. time on time, pre/during/post race handicapping, attempts to predict the environmental conditions on the race course, constructed courses, pursuit vs. staggered vs. fleet start racing, and performance curve scoring. The underlying assumptions and motivations for these choices are presented along with the consequences of adopting them. The expectations of the competitors, and indeed their ability to intuitively grasp the fundamentals of how elapsed times are transformed into race rankings, are discussed with a view towards finding solutions that achieve a successful balance of fairness, transparency and acceptance.
Sailboat Routing with Multiple Objectives for Sailing Races
Sailboat routing consists in computing the best route for a sailboat taking into account the characteristics of the vessel and environmental data such as weather forecast. In the context of sailing races, the best route computation is usually based on the isochrone algorithm, a sub-optimal solution to optimize the time to destination (TtD) criterion by computing a route as a sequence of waypoints. In this paper, we propose to compute a set of possible routes by considering two criteria : the time to destination and the stress. The time to destination is evaluated according to weather forecast and boat polar diagrams. The stress function is a combination of human and environmental factors. The set of possible routes is then obtained by using an iterative multiple objective optimization algorithm. Isochrone algorithm is used for initializing the set of routes. Then mutation operators are used to explore alternative solutions. Applied to realistic test cases, our search strategy allows to obtain routes with very different characteristics in terms of time to destination and stress values, asserted by experimented sailors. Concerning the main objective of minimizing time to destination, we obtain very similar results by comparison with commercial softwares such as MaxSea or Adrena.
In this paper, we propose a new control method for the next generation of autopilots. These new systems will need to manage more actuators to control the hydrofoils, which is going to significantly increase the energy requirements. So, this method is aware of the autopilot power consumption. It uses a model predictive controller to manage the actuators (position control - appendage angle control). This controller uses a dynamic model of the actuator, running in real time, to anticipate the future behavior of the system. Once the predictions are made, it determines the future control sequence to apply in order to follow the reference trajectory. To do so, it minimizes a cost function which includes the quadratic error according to the behavior prediction and the associated energy consumption. So, it takes into account two criteria: the precision/rapidity of the system and the energy. With the proposed control method, skippers can weight each criterion in order to focus on one or the other depending on their goals and the boat’s energy balance. We apply this method to one of the autopilot’s subsystems, namely the rudder control. The electric actuator intervening in this control loop and the load representing the force opposed to its motion are modelled to design the control law. The first results of that method are compared with a standard autopilot. We increase by 40% the precision level and we are able to reduce the consumption by at least 20%. This work provides the first necessary components of a future autopilot that will control the whole appendages to a three-dimensional piloting. Moreover, this type of management is a first step towards possible fossil fuel free sailboats.
Over the past two decades, the numerical and experimental progresses made in the field of downwind sail aerodynamics have contributed to a new understanding of their behaviour and improved designs. Contemporary advances include the numerical and experimental evidence of the leading-edge vortex, as well as greater correlation between model and full-scale testing. Nevertheless, much remains to be understood on the aerodynamics of downwind sails and their flow structures. In this paper, a detailed review of the different flow features of downwind sails, including the effect of separation bubbles and leading-edge vortices will be discussed. New experimental measurements of the flow field around a highly cambered thin circular arc geometry, representative of a bi-dimensional section of a spinnaker, will also be presented here for the first time. These results allow to interpret some inconsistent data from past experiments and simulations, and to provide guidance for future model testing and sail design