A velocity prediction program (VPP) is a computer program which solves for the performance of a sailing yacht in various wind conditions by balancing hull and sail forces. VPPs are used by yacht designers, boat builders, model testers, sailors, sailmakers, also America's Cup teams, to predict the performance of a sailboat before it has been built or prior to major modifications.

Background

The first VPP was developed at the Massachusetts Institute of Technology during the early 1970s when Commodore H. Irving Pratt funded research to predict the performance "of a sailing yacht, given knowledge of its hull, rig and sailplan geometry".

Methodology

VPPs are iterative programs which require educated guesses of initial parameters to begin operating. Generally VPPs are composed of two mechanisms, a boat model and a solution algorithm. Initial guesses of parameters including boat speed (Vs), heel angle (Φ), number of reefs and sail flatness are input into the boat model. Using these input parameters the solution algorithm calculates the difference between the propulsive force of the sails and the resistive force of the hull. It also calculates the righting moment created by the hull and the operational heel angle. Since the propulsive force and the resistive force are not likely to be equal on the first iteration, the solution algorithm has the responsibility of adjusting the input parameters and balancing the forces until it produces the maximum possible speed at each true wind angle.

Hydrodynamic force model

The resistive forces acting on a hull and its appendages (keel, rudder and other fins) can be broken down into a number of smaller components.

Aerodynamic force model

Solving and optimization

VPPs solve for the performance of a yacht by resolving all forces and moments acting on the yacht. Pitching and yawing moments can be assumed to be zero for simplicity.

Presentation

VPPs produce a great deal a data, so the presentation of this data requires special consideration. While tabular output can be valuable for identifying specific values, the most common way to present VPP output is with a polar plot.

Uses

Sailing yacht design

Sailing yacht handicapping

VPPs are used by a variety of sailing organizations to assess theoretical boat performance and then assign "handicaps" to allow boats of different styles and sizes to race against one another. The IOR and IMS handicapping rules were some of the earliest adopters of VPPs. In the United States, the most common handicapping rule is the PHRF rule, developed and promoted by the United States Sailing Association. While most other rules do not take past performance into account, PHRF differentiates itself by allowing skippers to request handicap reductions after a series of poor racing results. In Europe, the most common handicapping system used today is the IRC rule.

Commercial VPPs