Sheeting (AMYA MY #177)

Sheeting for Drive or Efficiency?

by Lester Gilbert

A couple of years ago, I wondered what I might be doing when I sheeted the main and jib to close-hauled. Was I trying to maximize drive? Was I trying to minimize heel? Was I trying to optimize the efficiency of the boat? Jessica was an MSc student at the University and agreed to look into the question as a project for her dissertation.

Wind tunnel

The “7 x 5” closed-circuit wind tunnel has a low-speed 3.7 m by 4.75 m octagonal section with an operational velocity range between 1.5 m/s and 15 m/s. The dynamometer mounted underneath the low-speed section floor provides data for six components: drive, heel, and vertical forces, and pitch, heel, and yaw moments. It is mounted onto a turntable that can be rotated, allowing any wind angle. The turntable has a water bath to simulate the floating condition of a hull, sealing the hull to the tunnel floor.

My A Class Sword has been specially modified with strengthening plates and M4 strong points to allow it to bolt into a test cradle that is, in turn, bolted to the dynamometer. The cradle permits the hull to heel at up to around 40 degrees. Figure 1 shows the hull being set up, and Figure 2 shows a No.1 rig being fitted. The rig sheeting was controlled by one servo, commanded by a 2.4 GHz radio, using a transmitter in the tunnel’s control room.

Figure 1. Attaching the A Class Sword hull to the wind tunnel dynamometer.

Figure 2. Setting up the rig.

Experiment

The wind tunnel was set to a wind speed of 3 m/s, the hull heeled to 30 degrees, and the boat trimmed to close-hauled at apparent wind angles of 27, 30, 33, 36, 39, and 42 degrees, in turn. For each wind angle, trim was adjusted until maximum drive was shown on the computer data acquisition display in the control room, and force readings taken. Then, the sails were sheeted in by one “click” on the transmitter, and force readings taken a second time. Finally, the sails were sheeted out by two “clicks,” and a third set of force readings taken. A “click” on the transmitter had the effect of changing the sheeting angle of the jib and of the main by approximately 2 degrees.

Results

The force readings were analyzed, and for each test run, the ratio of the coefficient of lift, Cl, to the coefficient of drag, Cd, was calculated. The lift-to-drag ratio varied from around 5 when pinching at an apparent wind angle of 27 degrees, to around 3 when footing or close reaching at an apparent wind angle of 42 degrees. Figure 3 plots the lift-to-drag ratio for the three sheeting conditions at each of the six apparent wind angles.

Figure 3. Lift to drag ratio when sheeted for maximum drive, eased, or tightened.

Discussion

The graph shows that easing the sheets a fraction from a position of maximum drive sees efficiency—lift-to-drag ratio—increase even as drive is no longer maximized. And as might be expected, tightening the sheets a fraction from a position of maximum drive means that efficiency drops, as well as maximum drive. So what should I be trying to do—maximize drive, or maximize efficiency?

Ross Garrett’s The Symmetry of Sailing (1987, Fig. 3.35) tells us that optimal velocity made good (VMG) when beating to windward occurs in a curious and counter-intuitive position on the lift-to drag curve—when drive is a little higher than that seen at best efficiency, but not quite as much as when drive is maximized. Sadly, the experiment could not tell me about optimal VMG, and thus it could not tell me if easing the sheets by 2 degrees from a position of maximum drive would give me the optimal VMG I might be seeking. But it did confirm that old sailing adage—if in doubt, sheet out…

Acknowledgements

The project was undertaken by Jessica Ma under the supervision of Prof. Philip Wilson of the University of Southampton’s Department of Ship Science.