Before I do so, I have to disclose certain prejudices I carry around. Well, one in particular: a preference for free-standing rigs. Trophy Wife and I had a two-decade romance with Lasers. Actually, it was just short of two decades before we sold off the last of five boats. And the two of us directly lapsed into another menage a trois with a much larger and heavier free-standing rig.
The thing of it is, even before I realized that it has a free-standing mast, the Scandinavian Cruiser 20 had caught my eye.
And the builders of the Scandinavian Cruiser 20 state my case for un-stayed masts so much better than I can:
Or, as I explain my boat, the boom is what holds up the mast.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.
Here are some revealing shots of the Scandinavian Cruiser-20:
Res ipsa loquitur.