The benefits of the Free-Standing, Rotating Wingmast

Beauty, Safety, Simplicity & Efficiency

Free-standing rigs are inherently more beautiful, safer, simpler, and more aerodynamically efficient than conventional rigs.

They are beautiful because of their sleek modern design and the absence of a myriad of standing rigging.

They are safer because stayed rigs are held up by multiple wires and spreaders, any one of which could fail or slip out and cause the rig to fall down. A free-standing mast is held up by just two parts—the deck and the heel fittings—so safety of the rig increases.

Free-standing rigs are more aerodynamically efficient because without wires, the sail-plan is no longer defined and confined by the triangular shape bounded by the backstay. The triangle is absolutely the worst possible plan-form shape that anyone could ever conceive of to be a lifting surface because of induced drag.

Induced drag is automatically created with lift. You can control it—make it bigger or smaller—but you can never get rid of it. Induced drag is a fact of life.

In any given aerofoil plan-form, the airflow on both sides of the surface are at different static pressures—high pressure to windward, low pressure to leeward—and they would really like to equalize. In a triangular plan-form, the airflow on the high pressure side gets a chance to equalize sooner, by virtue of the shape, than on a rectangular plan-form for example, by skewing up toward the tip and off the surface. This skewing of flow from the high pressure side, mixing with the flow on the low pressure side, creates a vortex off the tip. The bigger the skew, the bigger the vortex, and the greater the induced drag.

To reduce the vortex we can use a totally different shape for the plan-form, either elliptical or rectangular. The flow across an elliptical plan-form, as it turns out, has little tendency to twist off into a large vortex. In fact, the vortex is very small. A rectangular plan-form also has a pretty small tip vortex, and it can be made smaller, close to or better than that of an ellipse, if the tip of the sail is twisted to leeward. This is exactly how gaff rigs are shaped and why they are actually pretty efficient. It is also why we add roach to the leeches of mainsails—we are trying to approximate an elliptical or even a more rectangular, twisted, plan-form. You may have seen square-topped mainsails on modern multi-hulls and windsurfers. This is the reason—to reduce the tip vortex, and therefore the induced drag, to as small as possible. Less drag for the same amount of lift, or even greater lift, means more aerodynamic efficiency. More power is being devoted to making the craft move forward, not sideways.

The only reason we have triangular sail-plans is because we have wires that hold up the masts, and this necessarily makes sails triangular. And if you have wires in the way, you don’t want your sails to chafe on the wires, so we have triangular sails.

And the only reason we have wires in the rigs is because we are afflicted in modern sailboat design with arbitrary sailboat design rating rules that, for no good aerodynamic reasons, require the wires in the rigs. While many evolutionary changes have occurred in rig design over the years--most notably in new materials, first with metals, then with composites--standing rigging still remains steadfastly impacted inside the rating rules. And there is no relief in sight. Wires in the rig, and, therefore, triangularly shaped sails, are so inbred into our industry and our thinking that we blindly accept them without question.

It takes a bit of courage, I guess, to ask the question: “Why do we do this?” Well, sailors and designers are conservative people. There is no other explanation. The idea of a mast without wires is so foreign to most people that they just cannot fathom how a sailboat mast can stand up all by itself without something to hold it up.

Today, a Boeing 747 airliner at take-off weighs 875,000 pounds, carries 524 passengers, flies at 567 miles per hour more than 7 miles above the earth, and it does not have any wires holding the wings on!

Free-standing mast design and construction:

In engineering jargon, free-standing masts are called cantilevers. Stayed masts, on the other hand, are columns. Cantilevers bend, columns compress. The two behaviors are different, and so the structures are designed and built accordingly.

In a stayed rig, the boat heels due to wind pressure on the sails. Without wires holding up a normally skinny mast, the rig would fall over. But the wires hold the mast in place, pulling on the mast in tension and with their lower ends anchored into the deck and hull. This tension in the wires induces an equal and opposite compression load in the mast itself. The mast has to be big enough in cross-section with a thick enough wall so that it can handle the stress and not buckle.

In a free-standing rig, the wind pressure on the sails causes the mast to bend sideways and back. No wires support the mast, so the mast itself has to have a big enough cross-section and a thick enough wall to handle the load. This necessarily makes the mast bigger than its equivalent stayed counterpart.

And this is where carbon fiber plays such an important role. Carbon fiber laminates are about 60% of the weight of aluminum, the most common mast material, yet carbon is more than twice as strong. This makes carbon fiber a much more efficient material than aluminum when it comes to making sailboat masts (and other weight/strength sensitive structures, like airplanes).

The carbon fiber laminate is thicker at the base, where the load is greater, and tapers in thickness towards the top where the load goes to zero.

Wingmasts:

A wingmast is a mast shaped like a wing that is allowed to rotate. Wing shape and rotation further increase the efficiency of a free-standing rig. Unfortunately, mast rotation also falls victim to traditional rating rules—it is not allowed. This prohibition can be traced back to L. Francis Herreshoff, who had a patent on a rotating mast design, one of which he installed on an R class boat (Lwl = 20’) called Live Yankee in 1925. But when the regatta committee of the New York Yacht Club heard about this rig, it promptly passed a rule prohibiting “revolving masts, double luffed sails and similar contrivances.” This prohibition remains in current rating rules, and no changes to eliminate it are in sight. It really smacks of spite against a progressive designer and the yacht club’s desire to protect the status quo of the fleet at the time. But that was almost 80 years ago! It is truly amazing to me that such a prohibition has remained in place for so long.

However, in spite of the rules, rotating a stayed mast is difficult to say the least because the rigging wires simply get in the way. And actually, when sailing on the wind, stayed rigs are really very good. The airflow over a non-rotating mast attaches really well over the mast and mainsail. The power generated by a stayed rig on the wind and a free-standing wing-mast rig on the wind are pretty comparable. Two boats of the same type but with different rigs—stayed and un-stayed—do not have much advantage over each other—they sail about the same. The differences are really apparent when sailing off the wind. See the sketch below.

a. Stayed mast on the wind; b. Stayed mast off the wind; c. Mast off the wind and rotated.

When sailing on the wind (a), the airflow on the back side of the mast is well-attached. Off the wind (b), the mainsail on a stayed rig sets off the side of the mast because the mast can’t rotate. The leading edge shape--the most important part of the rig for generating power--is awful. The airflow quickly separates off the mast. To recover some lift, the sail-maker has to build enough camber into the sail to fool the airflow into reattaching. Then, when you go even further downwind, you lose lift altogether and drag is the only component left to make you go. Most people will say that the more downwind you go, the more pure drag you want anyway, to push you downwind. They obviously have not felt the adrenaline rush of acceleration and speed caused by pure lift from a properly designed and rotated wingmast rig. Get rid of the wires, and full mast rotation is possible. Rotate the mast (c), and all the aerodynamics change. Even downwind--especially downwind--pure lift is much more powerful than pure drag. The boat is considerably faster because so much more power is harnessed from the wind. This has been proven a number of times in actual sailing trials between stayed rig boats and boats with free-standing, rotating masts. The free-standing rig boats just run away from their stayed counterparts when sailing off the wind.

Another benefit of eliminating the wires is that a boat is much more directionally stable and more resistant to gybing. A boat with a stayed rig can sail perhaps ten degrees by the lee before the mainsail gybes. If not properly controlled, the boom will swing violently to the other side and crash against the lower shroud, perhaps breaking the shroud or itself. The boat may also broach if the seas are running fairly high. If something breaks or the boat broaches, the boat is instantaneously in danger of losing its rig and getting rolled over.

On a boat with a free-standing rig, the boom can set way forward of abeam wing-and-wing. You can sail ninety degrees or more by the lee without gybing. If the boat starts to round up because of a hit by a wave or gust, the sails will naturally pull the bow back downwind. And even if the boat does gybe, what happens if the boom gets away from the crew? Nothing, because there is nothing to hit—if it is not there, it cannot break! The sails will stop in a luff position all by themselves. It is very unlikely that the boat will broach. When the other boats crash, this one just keeps on going.

Wingmast design and construction:

My latest wingmast rig that is currently sailing is the one on Woebegone Daze, a Freedom 38 with a custom wingmast rig. The wingmast is in two parts: 1) A stub mast that fits into the boat and on which are mounted two large bearings; and 2) the wingmast which slips down over the stub mast and the bearings and rotates thereon. Both mast parts are made of carbon fiber.

The cross-sectional shape of the wing-mast is an ellipse. The leading edge of an ellipse is particularly friendly to airflow. Airflow stays attached to an elliptical shape much more easily than to a round shape.

The trailing edge part of the wing does not have to extend into a tail; rather it can be left as an ellipse. There is little or no drag in the transition between the mast and the sail. A little bubble of air may get trapped in the joint, but it stays there, the airflow skips right over it from mast to sail, and the transition is pretty fair on all articulation angles of the wing to the sail.

For more detailed information on the topic of free-standing masts and wingmasts, please visit my website.