Sorting Out Self-Steering Options
Part I: Modern servo-pendulum windvanes blend traditional wisdom, modern materials, and innovative design features
It seems unfair to regard the self-steering windvane—a device that we often view as another member of our crew—as merely a collection of metal, space-age plastics, and cloth or wood. But for anyone considering buying a windvane, such details merit a closer look.This month, we’ll compare the design and construction of some of the most popular servo-pendulum models on the market today. In a second installment next month, we’ll review alternative windvane designs, including trim-tab and auxiliary-rudder systems.
Servo-pendulum vanes can be broken down into three main components: the sensor vane, the pendulum rudder, and the linkages. The sensor vane is the brains of the self-steering system, but it needs apparent wind to work. Downwind sailing in light air—when apparent wind may be only a whisper—is one of the biggest challenges for a windvane steering system.
Sensor vanes are of two types: horizontal axis or vertical axis. The latter respond like the weathercock on a barn roof, rotating so that the leading edge of the vane always faces the wind. All of the self-steering devices we’ll look at have the more powerful horizontal-axis vanes. The helmsman turns the vane’s leading edge into the apparent wind once the sails are trimmed and the boat is balanced on the desired course. When the boat veers off course (or the apparent wind shifts), the wind strikes the flat side of the vane, tilting the vane on its horizontal axis and triggering the forces that steer your boat back on track.
Although called horizontal-axis vanes, the axes of the vanes we’ll look at are actually angled about 20 degrees from horizontal, away from the wind. This slight angle brings air flow to the lee side of the vane as it tilts, effectively damping the vane’s motion, reducing yaw, and permitting a more accurate steering impulse.
A sensor vane will have a counterweight to ensure proper balance, which becomes more critical in light airs, when the boat’s roll can cause the vane sensor to tilt on its own.
Another essential part of the sensor vane is the set of control lines, or knob, that allow you to adjust its position when you’re setting a new course relative to the wind. You should be able to do this quickly and easily from a safe position in your cockpit, preferably one protected from the weather.
A critical feature to look for in the vane is sensitivity. Any friction at the vane’s axis will inhibit performance in light airs. Many manufacturers supply smaller, heavy-weather vanes or larger, light-air vanes to improve performance in those conditions. You should be able to quickly change or replace the vane without using tools or worrying about losing a thumbscrew overboard. Vanes are generally lightweight and fragile under impact, so carry spares. On some models, you can make replacement vanes from plywood or cloth.
A final consideration is space. Radar arches, davits, or other accessories on your stern shouldn’t restrict the sensor vane’s movement or block airflow.
Beneath the water, servo-pendulum windvanes rely on pendulum rudders (also called servo-rudders) to provide the muscle for course corrections. When a change in apparent wind tilts the sensor vane, it tells the servo-rudder to turn. Because the servo-rudder is suspended like a pendulum above the water, when it turns, hydrodynamic forces cause it to swing outboard. The substantial force generated by the swinging pendulum rudder is then transmitted to the wheel or tiller. Slowly rotate the broad face of a dinghy oar toward a 4-knot flow of water and you’ll have an idea of the power a pendulum rudder can generate.
Just as the vane requires apparent wind, the pendulum rudder relies on the boat’s motion through the water to do its job. Most manufacturers say that you’ll need at least 2 to 2.5 knots to generate the power needed for steering, although point of sail, sea conditions, and boat’s characteristics will affect this. The faster your boat moves, the more powerful the force that the servo-rudder delivers.
A good servo-pendulum rudder will be semibalanced and have a hydrodynamic profile so that it’s sensitive to the slightest impulse from the sensor vane. Minimizing friction is essential, particularly any friction on the servo-rudder’s main shaft, which the relatively weak impulse from the sensor vane must turn. The servo-rudder should have overload protection—a device that gives way during a collision—so that the windvane, its mounts, and the stern of your boat don’t bear the full impact. Freeing seaweed or a snagged fish trap from a servo-rudder should be quick and easy. You should be able to lift or fold the rudder out of the water and put it back in action while at sea. The pendulum rudder and its shafts must be rugged and resist corrosion.
One final consideration concerning the underwater unit is its ability to serve as an emergency rudder. Most pendulum servo-rudders are too small to steer the boat alone when used as an emergency rudder. Some manufacturers provide larger rudders that can be slipped over the servo-rudder in an emergency. If you’re going to use a servo-pendulum rudder as an emergency rudder, make sure its hull-attachment points are well reinforced because they’ll be subjected to much higher loads.
If the vane sensor is the brain of the windvane, and the pendulum rudder provides the muscle, then the linkages are the nerves, the messengers that relay information between them. Two important linkages govern the windvanes we’ll look at: the one between the sensor vane and the pendulum rudder, and the one between the pendulum rudder and the boat’s rudder.
The linkage between the sensor vane and the pendulum rudder has two roles. The first is to efficiently relay every twitch in the sensor vane to the pendulum rudder. In a servo-pendulum vane, this means transforming the vertical motion at the vane sensor into the rotary motion at the servo-rudder. Because the vane’s signal may not be particularly strong, minimal friction here is important. Any play in the linkage will also interfere with an accurate steering impulse. Usually, this linkage is accomplished through bevel gears, levers, and/or rods, and manufacturers use plastic bushings, roller bearings, or ball bearings (often in plastic races) to reduce friction in the linkages. Bushing advocates tout resistance to dirt and corrosion; proponents of roller or ball bearings boast superior performance. In either case, this linkage should be durable, since its components can be hard to repair or replace at sea.
The second role of this linkage is to coordinate just how far the pendulum rudder will swing in relation to the vane’s movement. This helps determine how much helm will be applied to the ship’s rudder. For example, the 2-to-1 ratio of the bevel gears linking the horizontal vane to the pendulum rudder, common to many windvanes, doubles the force of the sensor vane’s impulse and halves the lateral motion of the pendulum rudder. One of the vanes we’ll look at allows you to adjust the ratio of this linkage to suit conditions.
The second linkage is between the pendulum rudder and your boat’s rudder. In most servo-pendulum vanes, this is usually accomplished by low-stretch lines led from the servo-rudder to the tiller or wheel. These lines attach to the servo-rudder either above or below its point of suspension (the horizontal axle or shaft on which the pendulum rudder swings). Manufacturers have long debated which is more effective; heavy-weather performance is an especially contentious issue. Makers that attach steering lines below the point of suspension, such as Aries and Monitor, lead the lines through struts that prevent the rudder from swinging out of the water and losing power in severe seas or a near broach, when lateral loads might be extreme. The opposite camp—including Sailomat, Windpilot, and Fleming, among others—contends that allowing the rudder greater swing room protects the gear from breaking under extreme lateral loads, makes it easier to swing the rudder up in port, permits more direct leads for steering-control lines, and allows for a more compact design. The compromises in either arrangement vary among boat designs. Look for a windvane that you can install in a way that maximizes the range of movement in the steering line—usually about 10 inches—and minimizes friction.
The linkage between the servo-rudder and the main rudder should be easy to disengage under load so that the helmsman can quickly take over. Again, reducing friction here is important. Large, lightweight, roller- or ball-bearing blocks with swivels—if needed—cut friction and ensure direct runs. Proper line tension, which may vary according to sea conditions, should be easy to adjust. Wheel-steered boats add more challenges: the added friction in the connection to the rudder and the "freewheel effect" of the wheel’s inertia. Attaching a windvane to a wheel-steered boat requires an adapter (usually with a clutch) mounted on the wheel. Most wheel-steered boats are compatible with servo-pendulum vanes, although they generally won’t respond as well as tiller-steered boats. Boats with hydraulic steering generally aren’t good candidates for servo-pendulum vane steering, although vanes have worked on hydraulic systems that have bypass valves. To accommodate those boats that are incompatible with traditional servo-pendulum designs, manufacturers are turning to auxiliary-rudder designs, which we’ll look at in the next installment.
The primary differences among today’s servo-pendulum rudders are in construction materials, the linkages, and the ways they dampen the motion of either the vane or the pendulum rudder (yaw damping) so that he boat’s rudder receives an accurate steering impulse from the vane. Here are some of the main distinguishing features of the vanes we looked at in this class.
Aries: Apart from minor alterations, the Aries’ basic design has remained unchanged in nearly 30 years, and it’s earned a reputation as being rugged and reliable. Originally built in England and now manufactured in Denmark, the first mass-produced servo-pendulum windvane serves as a good standard for measuring those that have followed. Its formidable construction—cast aluminum, extruded-aluminum tubing, silicon bronze, and stainless steel—makes it one of the heaviest windvanes in this group. Galvanic corrosion can make cleaning or servicing troublesome on older, neglected models, but 20-year-old vanes that function perfectly are not uncommon. By turning a single screw, you can easily replace the plywood vane or angle it backward to dampen motion in higher winds. The vane swings on nylon roller bearings. Two lines control a toothed wheel that clicks with each adjustment and positions the vane to the apparent wind at 6-degree intervals. The servo-rudder is a semibalanced rudder made of foam-filled fiberglass. On older models, a breakaway tube on the rudder shaft affords 360-degree overload and collision protection. Newer models are protected from forward impact with a hinged shaft that eases raising or complete removal of the rudder. The vane-to-pendulum rudder linkage is a relatively heavy forked-aluminum rod linked to beveled silicon bronze gears with a 2-to-1 ratio. Nylon bushings at swivel joints also reduce friction on this linkage. The bearings for the pendulum rudder shaft are solid-plastic composite sleeves. The transmission lines are attached below the point of suspension, through built-in nylon blocks.
Monitor: Featured in the BOC Around Alone and popular among cruising sailors, the Monitor is quite similar to the Aries. The main difference is its construction: 1.9-inch-diameter, .049-inch-thick 316L stainless-steel tubing that avoids the galvanic problems of the Aries. It also features additional Delrin roller and ball bearings (as opposed to bushings in the Aries) in its vane-to-rudder linkage and in the pendulum rudder shaft. About five years ago, Monitor upgraded from 304 to 316L stainless steel. To protect the gear from impact, later models feature strut guards that also reinforced its substantial frame. Its sensor vane (angled 20 degrees) is plywood, easily replaceable, and can be infinitely adjusted by a line leading to the cockpit. A light-air sensor vane is standard. The rudder shaft is protected 360 degrees by a replaceable safety tube that collapses under extreme loads or impacts. Scanmar, the California manufacturer and dealer for several vanes, is known for good customer support and maintains a record of every Monitor. The Monitor has an emergency-rudder system that can be mounted on the pendulum rudder.
Sailomat 601: Sailomat’s fourth design in its familiar series of servo-pendulum vanes differs from the basic Aries design. The steering-control lines are attached above the pendulum’s point of suspension so the lines don’t need to cross before reaching a tiller. Remote course control for the vane position is optional. Forward impact protection is with a plastic composite sheer pin, and lateral overload protection is built in by its design, which allows the pendulum rudder to swing 170 degrees side to side—also useful when you’re in port or motoring. Rather than rely on bevel gears for the linkage from the sensor vane to the pendulum rudder, the Sailomat uses a universal linkage and a ball-and-socket joint, with solid Delrin bearings to reduce friction. This variable linkage allows you to adjust the vane-to-rudder ratio to suit varying wind and sea conditions. Yaw damping is provided by the angled pendulum rudder blade, which neatly follows the lines of many reverse transom boats. Body construction is cast silumin (an aluminum alloy) that’s chromated, wash primed, and painted with two coats of polyurethane paint. The solid, 1.6-inch-diameter shaft and large-diameter tubing (2.4-inch diameter with .24-inch wall thickness) is made of 6061 T6 aluminum alloy. Mixed Delrin and Torlon/PEEK roller bearings reduce shaft friction. Thread inserts insulate the stainless-steel fasteners from the aluminum. You can detach the Sailomat from the boat’s transom by undoing two bolts, making it one of the easiest to remove completely. Its compact setup occupies less horizontal space on the stern, but because the first set of lead blocks for the tiller control lines is mounted on the boat, an optional block holder will be required for some double enders. The unit features an optional emergency rudder.
Windpilot Pacific: The Windpilot Pacific melds two distinct design features: The steering-control lines attach to a lever above the pendulum rudder’s point of suspension; a 2-to-1 bevel-gear linkage between the vane sensor and servo-blade provides yaw-damping. This combination is made possible by 360-degree bevel gears, which let the servo-rudder swing up under lateral loads and make it easy to raise the rudder when it’s not in use. Adjustable friction plates securing the hemlock-and-epoxy rudder allow the rudder to kick up under fore-and-aft impact. You can adjust power transmission to the wheel or tiller by raising or lowering the point where the steering-control lines attach to the unit. The wheel coupling works like a disc break, allowing infinite adjustment. The vane sensor can be easily tilted back to reduce sensitivity and dampen motion. Body construction is cast aluminum (AlMg5) and 2.4-inch-diameter corrosion-resistant aluminum (AlMgSi5 F22) tubing, with a wall thickness ranging from .20 (in the vane tower) to .24-inches (in the rudder-shaft tube). The manufacturer uses no ball or roller bearings, preferring instead sliding bushings and sleeve bearings. Five different variable flanges facilitate installation on virtually any transom. You can remove the unit by undoing two bolts.
Fleming Global Major Series: Fleming, like the Windpilot, has the reverse servo arrangement to keep the unit compact, and keeps the 2-to-1 beveled-gear linkage between vane sensor and servo blade. The Fleming comes with both standard and storm sensor vanes made of polycarbonate. Its most noteworthy feature is its stainless-steel construction: high-grade (2205) castings and thick-walled 16-gauge stainless-steel tubing, with no structural welds. The bevel gears are also cast stainless steel. Delrin races for the stainless-steel bearings (needle bearings at the vane-to-rudder linkage, ball bearings at the rudder shaft) allow for easy removal. A 360-degree worm drive controlling the sensor-vane position can be operated by an endless line. Lateral overload protection for the stainless-steel rudder is by design, which allows the blade to lift completely out of the water (like the Sailomat and Windpilot). Forward impact protection is through a sheer pin on the hinged servo-rudder
Fleming offers a variety of mounting options, including one to accommodate open-transom boats.
Norvane: A relative newcomer, Norvane uses investment cast stainless steel and 316 stainless-steel tubing. It employs a reverse-servo design similar to the Fleming. Features include a polycarbonate sensor vane (two sizes) and two adjustable counterweights, which can be raised or lowered to change sensitivity or accommodate different vane sizes. Norvane also incorporates glass ball bearings in Delrin races on the vane sensor and the gear linkages and stainless-steel ball bearings in Delrin races on the servo-rudder shaft. Overload protection is through a breakaway tube on the pendulum shaft. The Norvane features a square socket joint to attach an optional emergency tiller. An emergency rudder is available. Mounting options fit a wide range of transoms.
Navik: This small, mostly 304 stainless-steel unit has been used successfully on Mini-Transat boats and many medium-displacement boats up to about 33 feet LOA. It’s not recommended for larger, heavy-displacement boats. The vane sensor is linked to a trim tab on the pendulum rudder, so that the underwater portion requires only minimal impulse from the vane. Control lines that run through a quadrant have the same effect as the reverse-servo arrangement in other vanes: lines to the tiller do not need to be crossed. The linkages on the Navik are carried out through ball-and-socket joints and a series of linked levers. These small parts are not as robust as the linkages in other servo-pendulum rudders. The Navik’s rudder has a sensitive overload protection that releases the oar on impact. It’s wide-swinging arc protects against lateral overload.
Cape Horn: A compact, stainless-steel unit, the Cape Horn introduces some novel changes to the basic design. The vane-to-servo-pendulum linkage uses a horizontal stainless-steel "Z bar" that reduces the number of moving parts. This arrangement allows a mounting option that conceals belowdecks part of the system, including most of the control lines to the tiller or wheel, and does away with the wheel adapter. A stainless-steel tube (3.25 inches outside diameter) is glass-laminated through the transom. Pivoting inside that tube, another smaller tube links the servo-rudder to a quadrant inside the lazarette. As the servo-blade swings, the inboard windvane quadrant rotates. Control lines are led through blocks attached to the boat’s steering quadrant (or tiller), then to a pair of jam cleats in the cockpit for quick disconnect or trim adjustment. The Cape Horn vane is the only servo-pendulum we look at this month that is recommended for hydraulically steered boats.
Installing the Cape Horn system belowdecks may not be suited to every boat. For this reason, Cape Horn also offers three conventional outboard models, with the control lines above the deck.
Read Part 2 of this article here.
Darrell Nicholson is a Cruising World associate editor. Contributing editor Michel Savage helped research this article.