Ziya and I played in the wind tunnel in July 2005.  Amongst other things, we wanted to see what the difference was between running goosewinged, and running with main and jib on the same gybe.

No.2 rig was used at a wind speed of around 6.3 m/sec, pretty much the middle of the rig's range.  We started with the wind on the aft quarter, and rotated the model in steps of 10 degrees to bring the wind fully aft and then over the other aft quarter.  For a given heading, the sails were trimmed to optimise forward drive.  With the wind somewhat on the beam, the jib gave significant lift as well as drag, while the main gave drag almost exclusively  The jib was trimmed somewhat independently of the main, so the results show configurations which might not be able to be achieved in practice with a racing IOM.  For both goosewinged and on the same gybe, the jib was tethered to the right attitude (around 120 to 150 for goosewinged, and around 150 to 200 for the same gybe) as necessary to prevent it swinging over as the tunnel was run up.

Note that the wind tunnel wind speed was the apparent wind, and that if the model had been sailing the true wind would have varied from about 6.9 m/sec at 120 to around 7.5 m/sec at beta=180.  This detail is not very important because the graph shows the coefficient of drag, which takes account of such variance in true wind.

I expected that goosewinged would be better with the wind more or less dead aft, but wasn't prepared to see that it remained better all the way forward as well, right up to a course of 120 degrees.

Maximum Cd is shown as around 2.2 or 2.3.  This is too high, probably due to an error in zeroing the dynamometer, but the trend is accurate and reliable -- goosewinged always gives better drive (ie drag) all the way around the clock, remarkably so in the 130 to 150 degrees range.

2005-12-18

©2024 Lester Gilbert