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RIGID WING SAIL UPDATE

OCT. 2003

Efficient wings, or sails, use an airfoil cross-section that generates a high ratio of lift to drag.  Traditional single sided cloth sails are attached behind a large diameter mast, which disturbs the airflow, decreasing lift and increasing drag.  Some windsurfer sails have been constructed with two sides along the luff, which fairs the airflow around the mast / sail interface.  Usually the two sides are wrapped around the mast and the shape controlled with mast bend, downhaul tension, and a powerful outhaul.  While an improvement, the small diameter, round mast is far from an optimal airfoil, which would be thicker (around 10% of the chord) and have a leading edge shaped more like a parabola.

My work in this area has focused on composite sails, where the first ¼ chord, which includes a spar, is rigid and carries all the bending loads.  The second and third quarters are two independent flexible surfaces, and the last quarter (leech) is a single flexible surface.

 The first wing sail I built was tested on a standard windsurfer.  It was heavy, undersize, and fragile, but the basic shape was good and the flexible surfaces deformed as expected. 

No battens were used, since the wishbone was expected to tension the skins.  In light air, however, the wishbone tension needed to be reduced to let more camber develop, and this made the wishbone loose and difficult to control.

Second generation wing sail at Lake Diaz.  Mount Whitney, the tallest mountain in the continental US is in the background.  Death Valley, the lowest spot in the country, is only 50 miles east.  The wing is mounted on a 15-foot Wave Piercing Catamaran.

The sail was very sensitive to changes in the Angle Of Attack (AOA, the angle between the sail and the wind. When the AOA = zero, the sail develops no lift and acts like a wind vane).  Changing the AOA just a few degrees resulted in a rapid drop of power.  All the testers noticed this effect and they agreed that it was an improvement over traditional sails.

          The second wing sail was also designed for a windsurfer.  The original concept was similar to the first design, except carbon fiber was used to reduce weight and increase stiffness.  Wishbone tension and battens controlled camber, and the wishbone was secured by bolting it to the leading edge and spar cap.  The plan was to build four five-foot long sections, which could be added or removed for various wind conditions.  After two sections had been built, it was clear that the weight was going to be excessive and the joints were not built with enough precision to allow them to be interchanged – especially in the field.  Rather than quitting, the first section of the wing sail was redesigned to fit a 15’ wave piercing catamaran.  A 2-foot long spindle was added to the bottom of the wing sail, and machined to fit a pair of bearings attached to the boat.  These changes resulted in the weight increasing to 85 pounds, a little on the high side for an 82 square foot sail.

The original plan was to step the mast by brute force.  Stand it vertical, center it over the bearings, and drop in into place.  The idea looked good on paper, but when the wing was assembled it became obvious that it was to large and heavy to move manually, and even the slightest wind pushed the wing around violently, making it impossible to center, much less step.  To help control the wing while raising or lowering, the upper bearing on the boat was hinged in the fore and aft direction, allowing the spindle to be inserted with the wing horizontal.  As the mast was lifted, the end of the spindle became captured by side braces and was gently guided into the lower bearing.  This was a major improvement, but stepping the mast was still only possible during calm conditions.  After one day at the lake, the general consensus was that it was dangerous to try and position something this large when wind was present. 

We did get a full day of sailing with the new wing, with winds up to 15 knots.  Performance was good.  Like the first wing, it was sensitive to AOA, and had a definite “sweet” spot.  One of the most interesting observations was that a main sheet control line was almost unnecessary.  The wing would adjust itself with little or no main sheet tension, apparently because of the balancing effect of the leading edge, which was about a foot ahead of the pivot axis.  As the wind increased a little more sheet tension was required, but not much.  Several times a gust prompted me to release the sheet in an attempt to dump the heeling force, but the wing just moved a little before stopping.  I was forced to grab the boom and push the sail into a weather vane position.  If I let go of the boom, it would immediately rotate perhaps 10 degrees and generate a substantial amount of force.

The WP 14 turned out to be a pretty good sailboat.  The balance was excellent, and the boat had no problem tacking and jibing.  Being able to rotate the wing a full 360 degrees makes a jibe easy and you can sail backwards or even sideways at will.  Fortunately the winds remained light.  With no way to reduce sail area, we would have been in a lot of trouble if the winds were stronger.

Later in the afternoon, I attempted to unstep the wing sail by myself.  I arranged the boat such that the sail was facing both into the wind and directly astern.  I slowly lifted the wing until the spindle came out of the lower bearing.  Without support from the lower gearing, the wing immediately started to fall sideways, out of control, eventually jamming against the lower bearing.  Fortunately help was available, and three of us managed to unjam the spindle and lower the wing.

Clearly a fixed area, heavy rigid sail is difficult to control, and it would be dangerous to attempt to step or unstep the mast unless conditions were dead calm.  The probability of an injury accident with this design is unacceptable, and everything aft of the spar scrapped.

Applying these lessons to our original windsurfer goals, it appears that you would need perhaps three wings, of various areas, molded in one piece, with provisions for rigidly attaching the wishbone to the spar area.  Using vacuum bags, graphite cloth, and foam core in the leading edge and spar web will minimize the weight, but I still expect the wing to be 5-10 pounds heavier then a standard windsurfer rig.  To accomplish this will requires an investment in full length tooling, solid enough to withstand the forces involved with vacuum bagging.  To shave the weight any further would require going to pre-preged, uni-directional graphite tape, which must be oven cured to around 200 degrees (F).

For a conventional sailboat, like the WP 14, the sail must be easy for one person to step and unstep, even with a wind, and, it must be reefable.  This pretty much rules out a one-piece wing sail, but perhaps its possible to use a rigid nose and spar, with a removable double surface cloth sail.  The nose, containing the spar, would be fitted with plastic sail track along both sides of the spar.  A double-sided cloth sail would fit into these tracks and be raised and lowered using a conventional halyard.  Luff, outhaul, and batten tension would be used to control the cloth’s shape, and the rigid leading edge would provide the desired airfoil shape.  With the cloth sail removed, the remaining structure would be light enough, and have low enough windage, to be lifted and stepped by one person.  I plan to save the nose and spar off the wing mast, just in case I decide to go this hybrid route.

It’s probably foolish to waste much time and money on these wing sails, since the efficiency payoff doesn’t really amount to much until the relative wind velocity reaches perhaps 40 knots.  Above 40 knots, a properly shaped, free standing airfoil would have a strong advantage over the best cloth designs.  Windsurfers fall into the lower region for these sails, but applying them to sand sailors or iceboats, where the apparent wind reaches 100 knots, could result in real benefit.