Jib foot gap to deck

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Hannah Kemlo, an MSc student in Ship Science at Southampton University, tested an IOM "A" rig in the Southampton low-speed wind tunnel for her project in July 2006.  One of the tests included varying the gap between the jib foot and the deck.

The photo shows Hannah sat at the data acquisition PC, recording the forces registered by the dynamometer in the wind tunnel.

Hannah at the data acquisition console

The jib foot gap was set at "zero", and then at 116 mm.  Well, it wasn't actually zero, simply as close to the deck as was possible.  In fact the foot of the jib was around 26 mm above the deck, because of the 6 mm high deck track and the 20 mm jib boom.  To get the whole of the jib foot as close to the deck as possible, the mast was raked back as far as possible -- around 4 degrees.  The following two photos show the two gaps.

Jib foot set with "zero" gap (actually around 26 mm)

Jib foot set with 116 mm gap

The model was set at 30 degrees to the apparent wind, (beta = 30), trimmed to a "standard" IOM tune for A rig, and the tunnel was run at four wind speeds (approximately 3.1, 3.8, 4.8, 5.8 m/s).  The resulting (raw) drive and heel forces are plotted in the first graph.  It is immediately clear that drive force is greatly improved with the "zero" gap, while heel force doesn't seem to change that much.

"Raw" drive and heel forces

The next three graphs show the lift, drag, and lift to drag coefficients plotted against wind speed.  It is clear that, while the gap between the jib foot and the deck doesn't affect the lift coefficient that much, it has quite an impact upon the drag coefficient.

Lift coefficient plot

Drag coefficient plot

Ratio of squared lift coefficient to drag coefficient plot

The Cl^2 to Cd plot gives the measure of the efficiency of the rig, and should result in straight lines connecting the wind speed points.  It is clear that the lower the jib is to deck, the better, especially at higher wind speeds.


2022 Lester Gilbert