Boats & Gear
- Category: Boats & Gear
- Created: Friday, 21 September 2012 16:20
- Written by Charles Doane
SOONER OR LATER owners of fiberglass sailboats become interested in how the rudders on their boats are constructed. Usually this happens after an owner notices there is water dribbling out of a boat's rudder long after it has been hauled out of the water. In the early days of fiberglass boatbuilding, when most sailboats had full keels and attached rudders, many rudders were still made of wood. These were constructed in the traditional fashion and consisted of a row of planks, often mahogany, joined end to end, usually with internal drift pins that were fastened to the rudderstock. You never had to worry about these rudders getting all full of water, but you did sometimes have to worry about the planks coming loose.
Since the late 1960s, almost all fiberglass boats have been built with fiberglass rudders. Not all glass rudders are created equal, but most are built on the same basic principle. Most commonly, the spine of the structure is a metal rudderstock (also sometimes called a rudderpost) off of which sprouts a lateral armature that supports the rudder blade. Traditionally, this armature is welded to the rudderstock and consists of a series of lateral rods or bars, or perhaps a simple flat plate. More recently, foil-shaped fins similar to those seen in the frames of airplane wings have become more common. This skeletal structure is embedded in a high-density closed-cell plastic foam core, which is sheathed in a thin fiberglass skin. This composite foam-core construction is relatively light with neutral buoyancy, which significantly improves the feel of a sailboat's helm while sailing.
Interior rudder structures
The key variable is the material from which the rudderstock and its armature are manufactured. If metal is used, the best choice is probably silicon bronze, but this is rarely seen anymore. Sometimes aluminum or even titanium are used to save weight, but the most common choice is stainless steel. We like to think of stainless steel as an "ideal" corrosion-proof metal, but this is really only true in limited circumstances. It does resist corrosion well when routinely exposed to oxygen, but is subject to pitting corrosion when trapped in a deoxygenated environment, which is just what you'll find inside a fiberglass-skinned rudder once its foam core is saturated with water.
Such saturation, unfortunately, is common in any rudder with a metal stock. The joint where the stock enters the rudder blade is apt to leak sooner or later, because the three different materials involved--fiberglass, metal, and plastic foam--all contract and expand at different rates as the ambient temperature changes. No matter how well the joint is sealed when the rudder is first constructed, small gaps through which water can intrude are inevitably created. Knowledgeable boatowners take this for granted. They assume their rudder cores are constantly absorbing water and so drill holes in the bottom of their rudder blades every time they haul their boats in order to let the moisture drain out. (A better alternative, obviously, would be for builders to install drain plugs in the first place.)
Another problem with stainless steel in rudders has to do with its welding characteristics. When stainless steel is welded, the carbon and chromium in it mix to form chromium carbide. This creates two sub-alloys--chromium carbide and chromium-depleted steel--that are different enough in their composition to form a corrosive galvanic couple within the weld. Insert this galvanically compromised weld inside a moist oxygen-depleted foam-cored rudder, and it is much more likely the rudder's stainless-steel armature will corrode and fail. A stainless-steel rudderstock is also apt to suffer from crevice corrosion inside the shaft seal in the bearing where it exits the hull, as this is another area where water is trapped and becomes stagnant and deoxygenated.
All these problems can be ameliorated if the stainless steel inside a rudder is high-quality 316-L alloy. This variant resists pitting corrosion much more readily than its lesser 302- and 304-alloy cousins. It also has a lower carbon content (thus the L designation) and is less compromised when welded. Unfortunately, there is no easy way to distinguish between alloys. Silicon bronze, by comparison, is virtually corrosion proof under the same circumstances, unless it is coupled directly to steel or aluminum.
Rudderstocks can also be fabricated from a composite laminate such as fiberglass or carbon fiber. The great advantage of a laminate stock is that the stock and the skin of the rudder blade can be the same material, which means the joint where the stock enters the blade can be permanently sealed. Also, the rudderstock can be bonded directly to the interior surface of the skin, thus eliminating the need for interior armature to resist twisting loads as the rudder turns back and forth.
Laminated rudderstocks generally must be wider than metal stocks in order to resist the transverse loads imposed on them. This means the rudder blade must also be wider, which tends to degrade the rudder's hydrodynamic form. One way around this is to flatten the sides of the stock into a trapezoid shape. This not only creates a narrower cross-section, but also presents a much larger surface area for bonding the stock to the skin of the rudder blade. Note, however, that a trapezoid stock needs bearing rounds installed where the stock passes through its rudder bearings in order for the rudder to turn properly.
Metal vs. laminate rudderstocks
In practice, unfortunately, fiberglass rudderstocks have not performed well. Some mass-production builders have embraced them, because they are cheaper and lighter than stainless-steel stocks, but there have been several incidents where fiberglass stocks have failed in moderate sailing conditions. Builders, as a result, are now more wary of them.
Carbon fiber is another story. Carbon rudderstocks have proven much more reliable, as carbon is much stiffer and stronger. It is also much lighter. An all-carbon rudder (i.e., a carbon stock bonded to carbon skins wrapped around high-density foam) weighs less than half as much as a conventional foam-filled glass rudder with a stainless-steel stock and armature, but also costs two to three times more. Carbon rudders therefore are normally seen only on race boats and high-quality cruising boats.
A carbon-fiber rudderstock
Another important thing to consider, of course, is the manner in which a rudder is attached to its hull. The more a rudder is supported by a hull or skeg, the greater its inherent strength. Unfortunately, the weakest structure, the high-performance spade rudder (see photo up top), is also the most popular. Here all the transverse load, which can be quite large, is carried by the rudderstock where it enters the hull. The hull itself should be reinforced at this point. The top of the stock should also be well supported. On some boats the deck does this job; on others some below-deck structure, such as a transverse beam or shelf, holds the top of the rudderstock in place. Any such structure should be bonded to the hull as strongly as possible.