The paper describes the improvements made to the aerodynamic and hydrodynamic force models of the Offshore Racing Council (ORC) International Measurement System (IMS) sailing yacht velocity prediction program (VPP) since 1990. The paper explains the mechanisms of the force modeling used in the IMS VPP to provide a framework within which the modifications and revisions to the VPP are described. The major revisions fall into three categories.

1) The hydrodynamic model, where a revised formulation for residuary resistance and characteristic length has been introduced, which includes modifications to both the residuary and viscous resistance components. Revisions to the drag due to heel and induced drag formulations are described that more accurately reflect the behavior observed from towing tank tests.

2) The added resistance in waves module was introduced to assist with the equitable handicapping of diverse bull types and construction methods, which affect the behavior of sailing yachts in waves. The mechanism by which the sea state is charaterised and the added resistance calculated is described.

3) The aerodynamic force model, which derives complete rig lift and drag coefficients from standard sail types is described. The behavior of the VPP force model is compared with wind tunnel test results, and the recent additions of coefficient sets for a range of different sail types is described. Finally the components of a more detailed model of hull and rig windage are outlined.

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