The Rudimentaries of Rudders - Professional BoatBuilder Magazine (2024)

Like other systems and gear aboard cruising and commercial vessels, rudders have terms to identify their parts and functions. When measuring a rudder, the span and chord are the vertical height and fore-and-aft width, respectively, while the top of portion closest to the hull is referred to as the root, and the bottom is called the tip. Another term frequently used when discussing rudder design, particularly for sailing vessels, is aspect ratio—simply the square of the rudder’s span divided by the rudder’s area. As a rule of thumb, longer, narrower rudders are more efficient than short, wide rudders, and the aspect ratio describes precisely this relationship. Thus, rudders on high-performance sailing vessels are said to have a high-aspect ratio. Walking around a boatyard one day and measuring a few cruising sailboat rudders, I came up with aspect ratios of between 1.7 and 2.1, while one high-performance sailing vessel’s rudder came in at 3.5. The 20-knot semi-displacement lobster yacht’s rudder I measured yielded an even 2.0 aspect ratio, which is considered respectable for this application.

More identifiable rudder components include the stock; web or armature; rudderport or log; stuffing box or compression tube; bearing; gudgeon; and pintle. Not every rudder has all these components.

Rudderstocks

The rudderstock is essentially a shaft or tube that protrudes from the top and sometimes the bottom, depending upon type, of many rudder designs. Because this component provides the primary connection between the rudder’s blade (the flat section that imparts the steering force) and the vessel’s steering system, its design, construction, and material are consequential.

Most stocks are made of stainless steel, bronze, or aluminum, while some are carbon fiber, and they may be solid or hollow. Stainless steel is by far the most common, but it has a penchant for crevice corrosion when exposed to oxygen-depleted water. Insidiously, corrosion nearly always occurs in places where it cannot easily be seen—such as inside many composite (fiberglass and core material) rudder blades and beneath flax-type stuffing-box packing (the problem is exacerbated when the vessel is used infrequently).

Of the stainless steel alloys, some resist this corrosion better than others. Stainless-steel rudderstocks should be manufactured with strong, highly corrosion-resistant proprietary shafting alloys such as A22. The next best choice is 316L stainless steel, which also resists crevice corrosion well. Critically important is the L suffix, meaning “low carbon,” a requirement if it is to be welded, as nearly every rudderstock must be, to the support within composite rudders, or to all-metallic plate-steel rudders. Failure to source low-carbon stainless steel for the stock or the web leads to weld decay, sometimes referred to as carbide precipitation, where the region around the weld loses its resistance to corrosion and rusts when exposed to water.

Aluminum rudderstocks are nearly always tubular. Common on aluminum vessels to reduce the likelihood of galvanic corrosion, aluminum stocks are also relatively common on fiber reinforced plastic (FRP) vessels, particularly large ones. Rudder blades, particularly on aluminum vessels, are often fabricated from aluminum. Of the various aluminum alloys, only a few possess the necessary corrosion-resistance and strength necessary for use as rudderstocks. Of these, the 6000 series, and 6082 in particular—an alloy of aluminum, manganese, and silicon—are popular for this application.

Because aluminum, like stainless steel, suffers from corrosion, it should not be used as stock or web material in composite rudders. Referred to as poultice corrosion, it occurs when aluminum is exposed to oxygen-depleted water. Because oxygen is what allows aluminum to form its tough, corrosion-resistant oxide coating, the metal should never be allowed to remain wet and starved of air as it would be inside a composite rudder blade after water makes its way in around the stock and pintle.

Bronze, a once popular rudderstock material, is no longer common in today’s production vessels. Although strong and exceptionally corrosion resistant (immune to crevice corrosion), bronze is not easily welded to attach to a rudder’s internal structural webbing, and has thus been supplanted by stainless alloys. Bronze rudderstocks, particularly those that have seen many sea miles, are also known for wearing, or hourglassing, within stuffing boxes, where the flax rides against the stock. If a bronze stock rudder is chronically leaky, disassemble the stuffing box and check for excessive wear. The same is true for stainless and aluminum stocks: chronic leakage is often an indication of corrosion at the packing. Finally, because of their galvanic incompatibility, neither bronze nor copper alloys should be used aboard aluminum vessels for rudderstocks or any other rudder or stuffing box components.

The webbing, or internal metallic support system, in most composite rudders must be strong enough to carry the loads of service and be made of the appropriate material. At one time, many rudders were built using stainless-steel stocks and ordinary, rust-prone mild or carbon-steel webbing. Inadvisably, some still are. The union between a stainless stock and FRP rudder blade is tenuous at best (the two materials expand and contract at different rates) and stainless steel’s slippery surface makes adhesion to the laminate resin a short-lived affair. Once water enters the gap between these two materials, it will reach the webbing and associated welds. Thus, all the materials within this structure must be as corrosion- and water-resistant as possible, and the core material must be closed-cell—often foam—and nonhygroscopic.

Additionally, where possible, the stock should consist of a single section of solid or tubular material; i.e., it should not be sleeved, reduced, or otherwise modified or welded unless done so in an exceptionally robust manner. The webbing must be welded to the stock, but the structure of the stock should not rely on a weld that would experience cyclical, torsional loading.

The webbing in the form of a plate or grid should be welded to the stock with ample horizontal gussets (small wedges welded where the stock and webbing interface), which will reinforce welds 90° to the primary web attachment.

Rudder Log

Whether the rudder is spade (supported only at the top) or skeg hung (supported at the top and the bottom), the stock must pass through and be supported by the hull. This is usually accomplished by a component known as a rudder log, or port. In its simplest form it’s a tube or pipe through which the stock passes. Nearly all logs incorporate two other components—a bearing and a stuffing box. The bearing may be as simple as a bronze or nonmetallic bushing or tube inside of which the stock turns; or it may be as complex as a self-aligning roller-bearing carrier that absorbs rudder deflection and prevents binding.

The log transfers tremendous loads and must be exceptionally strong and well bonded to the hull. Fiberglass vessels should rely on a well-tabbed-in purpose-made tube (its filaments are wound and crisscrossed and thus quite strong) that is supported with a series of vertical gussets that distribute the load to the hull’s surrounding structure. On some spade rudder installations, particularly where the log is not, or could not, be long enough, an additional bearing is used at the top of the stock, above the quadrant, where it is supported by the vessel’s deck.

On metal boats the design is similar but with a metal tube welded in place, supported by substantial gussets. For vessels with skeg-hung rudders, the strength of the rudder log is still important. However, because the loads are not imparted by a cantilevered structure, logs used in these applications may be less substantially supported.

Stuffing Box

Unless the rudder log’s upper terminus is well above the waterline or on the weather deck, it is typically equipped with a stuffing box similar to those used for propeller shafts. But unlike a shaft stuffing box, the rudder’s stuffing box shouldn’t leak much, if any, seawater. Because the rudder turns slowly, friction and heat are not a problem. Packing (i.e., waxed-flax packing like that in traditional stuffing boxes) can typically be tight enough to stem all leakage, and lubricating it with heavy water-resistant grease will reduce friction and leakage.

Stuffing boxes that are above the waterline while the vessel is at rest, such as those on many sailboats, are often the most chronically leaky, because the packing tends to dry out and contract. To avoid this, liberally apply grease to the packing material itself; this requires partial disassembly of the stuffing box. Alternatively, a galvanically compatible (316 stainless or Monel for bronze stuffing boxes) grease fitting may be installed and periodically pumped with grease to keep the packing lubricated.

Rudder Bearings

Rudder bearings range from the basic rudderstock turning inside a bronze log, to the sophisticated aluminum, stainless, or nonmetallic roller bearings installed in a self-aligning carrier. For most cruising vessels, the choice of bearing is not as important as knowing which type of bearing is in use and its strengths, weaknesses, and maintenance needs. The simple shaft that turns inside a bronze log is durable and reliable but more friction-prone than roller bearings. If lubrication access or a grease fitting is available, it should be pumped with grease periodically, although most rudders rely solely on seawater for lubrication, which is perfectly acceptable.

Nonmetallic sleeve and roller bearings, often made of ultra high molecular weight polyethylene (UHMWPE), require no maintenance, are extremely slippery, and will not absorb water, an essential attribute for nonmetallic bearings. Delrin and nylon, for instance, will absorb water, expand, and lead to rudder binding. On several high-performance sailing vessels, I’ve had to replace nylon or similar bearings with UHMWPE to restore the steering to its proper specification and effort level.

Propeller Removal

Whether a rudder is a spade or skeg-hung design, it’s important to determine how it will affect the removal of the propeller or the propeller shaft. Is there enough clearance between the shaft’s trailing end and the leading edge of the rudder to allow the propeller to be removed or to use a propeller removal tool? Can the shaft be slid out without removing the rudder? Some rudders are equipped with shaft-removal holes, while others are installed slightly offset from the centerline; or the rudder’s leading edge has an indentation to allow the shaft to be removed. The propeller should be removable without having to unship the rudder. The dimensional rule of thumb calls for clearance of at least the prop’s hub length between the aft end of the shaft and the leading edge of the rudder.

Rudder Stops

The rudder’s movement should be unimpeded as it swings approximately 35° in either direction, making no contact with hull or propeller. Just as important as the rudder travel is how its movement is checked. Other than for the smallest runabouts with jacketed cables, all inboard rudders should rely on hydraulic cylinders to check rudder travel (provided they are designed to do so, and most are) or be equipped with robust stops. Stops must be integral to the hull, supported by substantial tabbing or a welded and through-bolted structure for fiberglass vessels, or by welded angle and shelves for metallic hulls.

Some steering systems, such as pull-pull and certain geared drag-link units, are available with integral stops, but for nonhydraulic systems, keep in mind that rudder stops must be capable of withstanding violent forces when the vessel is backing down hard and the wheel is let go. While this should be avoided, the possibility always exists (especially on sailing vessels with twin spade rudders), and it often places the greatest load on the rudder, stops, and whatever the stops contact—typically the quadrant or tiller arm. In the event of a rudder-linkage failure, the stops may have to absorb substantial shock loads while preventing the rudder from striking and jamming against the hull or the prop.

About the Author: For many years a full-service yard manager, Steve now works with boatbuilders and owners and others in the industry as Steve D’Antonio Marine Consulting. He is an ABYC-certified Master Technician, and sits on that organization’s Hull and Piping Project Technical Committee. He’s also the technical editor of Professional BoatBuilder.