SPAN (Sailing Performance Analysis) is a program for predicting the performance of sailing yachts in a variety of wind conditions. Based on the published algorithms for the IMS Velocity Performance Prediction Program, SPAN solves the equations for lift and drag for hull and rig and finds an equilibrium velocity and angle of heel.
Integrated into SPAN is a hull form measuring module that performs the function of the IMS Lines Processing Program (LPP). This allows the designer to read in a Maxsurf design and automatically perform the measurements required for input into the performance analysis. Alternatively measurements may be input directly from an IMS certificate although only a small number of key values are required.
Once a hull and rig have been fully specified the designer may calculate an entire table of performance values from 6 knots through to 20 knots true wind speed. A variety of true wind angles are used and performance is calculated with both spinnaker up and spinnaker down.
The output from SPAN is in the form of a table of values for each wind strength and wind direction with results given for apparent wind direction and apparent wind strength, hull velocity, VMG, heel angle and the various lift and drag forces involved. Velocity data can also be viewed in the form of performance polar curves.
In general the results from SPAN are very close to those produced by the current IMS VPP. The speeds predicted by SPAN tend to be more conservative than those predicted by IMS, usually by approximately 0.1 knot, however the shape of the polar curves produced by both programs tend to be very similar.
SPAN provides a valuable tool to the yacht designer, both for the estimation of performance during the design process, as well as the ability to produce polar curves of performance for the yacht owner once the design is launched.