Velocity Prediction Program (VPP)
It is intention of any rating system to give a boat rating which depends on her characteristics in order to equalize her with other boats of different size and characteristics. Characteristics that are positive for boat speed, like length of the water line or sail area, and those that are negative, like beam, displacement and wetted surface, are the most common influences in any rating system. Establishing which of these characteristics influences boat speed is therefore the problem that has to be resolved. Thanks to the developments in modern science and technology, ORC gives the most accurate solution to this problem, and establishes important principles of boat measurement and calculation of corrected times.
The only possible way to rate boat fairly is to tell how fast she can sail in certain wind conditions or to predict her speeds. To do so, ORC has developed with continuous research and update an ORC Velocity Prediction Program (VPP) as mathematical model which calculates boat’s performance from the set of measurements and gives rating which is then applied for race scoring. ORC VPP is calculating boat’s theoretical speeds for various conditions: wind strength of 6, 8, 10, 12, 14, 16 and 20 knots, direction of true wind from the close-hauled to 52, 60, 75, 90, 110, 120, 135, 150 and 180 degrees. VPP program creates a computer simulation of the boat’s performance based on scientific research of boat hulls in hydrodynamic basins, sails in aerodynamic tunnels and measurements taken on real boats as well as computer fluid dynamics (CFD) tools available nowadays.
The VPP has a two-part structure comprised of the solution algorithm and the boat model. The solution algorithm must find an equilibrium condition for each point of sailing where the driving force from the sails matches the hull and aerodynamic drag, and the heeling moment from the rig is matched by the righting moment from the hull.
The VPP determines the steady state conditions by satisfying 2 equilibrium equations: Firstly the net force along the yacht’s track (its direction of motion) must be zero:
Driving Force – Drag = 0
Secondly the aerodynamic heeling moment produced by the mast & sails must be equal and opposite to the righting moment produced by the hull and crew.
Heeling Moment – Righting Moment = 0
The boat model may be thought of as a black box into which boat speed, heel angle, and the sail trim parameters, reef and flat are input. The output is simply four numbers:
The solution algorithm iterates to a solution by interrogating the boat model with each new guess of the input values until a set of conditions is found that produces a match of thrust and drag and heeling moment and righting moment. The solution algorithm also seeks to find the highest speed on each point of sailing by adjusting the sail trim parameters for optimum performance.
Aerodynamic model is
calculating forces for each type and combination of sails. It has
aerodynamic co-efficient coming out from various test form wind
tunnels which in combination with sail area and center of effort
height can give the total driving force on sails. This force is then
reduced for the drag of hull, mast and crew windage as well as
Hydrodynamic model use results of tank testing and computed fluid dynamics in constant research of these values comparing it with real boat performance. It was shown that latest changes in the residuary resistance made the ORC VPP the most accurate so far.
Second equilibrium to match is
one between heeling moment and righting moment. Heeling moment is
calculated from the aerodynamic heeling force and sail plan centre
of effort height taking also in account hull windage, mast windage,
rigging wire and crew windage heeling moment. Righting moment as
result of stability measurement is reduced for the stability loss
due to forward speed and increased for effect of the shifting crew.