Wind tunnel (2)

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Rachel Nicholls-Lee, a student in Ship Science at Southampton University, has tested an IOM "A" rig in the Southampton low-speed wind tunnel for her final-year project.  More details on the lateral mast bend page, including photos of the model in the tunnel.

For those interested, this is a picture of previous work in the wind tunnel, and you can guess the sense of history and continuity I get from comparing this to the pictures shown in Marchaj's books and elsewhere.  The Southampton low-speed tunnel has a characteristic octagonal shape, a dead give-away.  The bright blue area on the right of the picture is a viewing window.

For this set of tests, the hull was set at an angle of 30 degrees to the wind (beta = 30), and the jib and main were sheeted at a number of different angles.  Specifically, the jib was sheeted at 10, 12, 14, and 16 degrees, and the main was sheeted at 0, 2, 4, 6, and 8 degrees.  The force coefficients are as calculated after windage adjustment and tunnel blockage adjustment.

The first row of graphs show the coefficient of lift recorded at each of three wind speeds.  You can see that Cl peaked when the main was sheeted at 0 degrees.  For the lowest wind speed, this peak was highest with the jib sheeted at 14 degrees, but as higher wind speeds were tested, the jib sheeting angle didn't seem to affect Cl much.

The second set of graphs show the coefficient of drag.  You can see that drag peaks when the main is sheeted at 0 degrees as well, but there are distinct secondary peaks when the main is also sheeted at 4 and 6 degrees at lower wind speeds and with a more tightly sheeted jib.  There is an interesting "crease" or valley in the response surface for the jib sheeted at 12 degrees, where the drag coefficient has a local minimum.  Could be an artifact.

The final set of graphs shows the drive to heel force ratio.  More or less identical graphs would result for lift versus drag ratios.  There are big changes in the ratio of drive to heel at the lowest wind speed, while the ratio flattens out for the highest wind speed.  Nevertheless, the best ratio of drive to heel is found with a freely sheeted main (8 degrees) and a freely sheeted jib (16 degrees) at low and high wind speeds, but something interesting has popped up for medium wind -- a second peak in the drive to heel ratio with the main at 2 degrees and the jib at 12 degrees.  Isn't that curious -- these are my "normal" sheeting angles!

What can we conclude?  The drive to heel ratio is most sensitive to sheeting angles at lower wind speed, and this sensitivity decreases greatly as the wind picks up.  I guess this is good news, since I'm not sure that the drive to heel ratio matters much at lower wind speeds -- the boat isn't heeled too much, and we want to maximise drive, not optimise it or balance it against heeling.

So at lower wind speeds, sheet the main tightly, and have the jib a little free to obtain maximum lift.  As the wind picks up, keep the main tight, and ease the jib just a little more.

Remember -- we're just looking at lift and drag here.  These conclusions may not be that helpful if you need to point, for example, or if you need to maintain drive and momentum through chop, different sheeting will undoubtedly be needed.  And remember that these values were obtained with "nominal" twist in jib and main.  In fact, for lower wind speeds, main and jib twist was quite low to my eye, and as higher wind speeds were tested, jib twist increased some, but main twist increased dramatically.


2011 Lester Gilbert