Even though sailboarding is a popular sport with many thousands of participants worldwide the forces experienced whilst sailboarding on water have not been documented. Harness line force is one of the key forces acting when sailboarding. The aim of this paper is to outline a method for measuring this force and to document its magnitude when sailboarding in a range of wind strengths (12 knots to 30 knots) using two different sail sizes (6.5 m2 in lighter winds and 5.0 m2 in stronger ones). A sailboard harness spreader bar was instrumented with a force transducer, amplifier and datalogger, to enable measurement of harness line force while sailboarding. Using this instrumented spreader bar an expert sailboarder sailed in a figure of eight fashion around two buoys lying across the wind. Average harness force measured during each leg of the figure of eight course was 381.6 +/- 43.2 N when using the 5 m2 sail and 415.0 +/- 66.7 N when using the 6.5 m2 sail. The average gybing time was 9.9 +/- 2.0 seconds when using the 5 m2 sail and significantly higher (p = 0.035) 12.2 +/- 1.3 seconds when using the 6.5 m2 sail. The results from this paper indicate that downsizing sail area to cope with the increased wind strength maintains harness force within a manageable range. For the subject in question the average harness line force was approximately 4.9 N/kg body weight.