We Live in Rapidly Changing Times

composite rudder

There was an age, not all that long ago in the grand scheme of things, when technological progress was a slow, measured affair. Revolutionary new developments in materials science, in hull design, in manufacturing processes and in many other engineering fields were few and far between. The “state of the art” progressed at a pace that allowed everyone to catch up before the next new development hit.

That age ended a century ago, and the pace of science and technology has accelerated rapidly in the intervening years. With new technology comes new decisions: Do we keep the old ways of building things, or can we do better now? It’s a question that needs to be asked periodically; even NASA and Rocketdyne are now taking another look at the F-1 engine from the Saturn V moon rocket. Their theory is that by re-evaluating a successful but imperfect design in the light of new knowledge, we’ll be better equipped to improve on it.

The marine sector may not have NASA’s resources, but we would nonetheless be wise to periodically revisit our old ideas. Quite often, “we’ve always done it this way” is code for “we don’t want to re-evaluate in the light of new knowledge”. The halyard reel winch, a.k.a. the Wrist-Breaker, is a prime example: it persisted for years after vastly superior drum winches were available, simply because that’s how things were done the last time the decision was considered.

A more modern example is the growing use of carbon fibre composites to replace metal parts. Carbon was, for many years, an expensive luxury item—the sort of thing you’d splurge on for a one-off racing boat where every piece was custom made, and kilograms were more critical than dollars. Such a wonderful exotic material was, of course, deemed far too expensive to consider using in more plebeian applications, such as on cruising sailboats.

Carbon—Not Just For Millionaires

Times have changed, and five minutes on Google and Alibaba will quickly reveal dozens (if not hundreds) of suppliers on each industrialized continent who are ready and willing to provide raw carbon and a huge variety of prefabricated carbon parts.

Consider a 62 mm (2.5 inch) rudder shaft, 2 m (6 feet) long. In regular 316L stainless, it’ll cost about $550; in the more corrosion-resistant Nitronic 50 grade, it’s roughly $1100. Depending on the exact specs needed, $800 to $1500 and a bit of time for tool-up and delivery should get you a similar shaft in pultruded or spiral-wrapped carbon. Pestering these suppliers for more details will net you a bewildering array of carbon fibre choices, some off-the-shelf and some semi-custom.

It seems that carbon’s problem isn’t that it’s rare or excessively costly, but rather that the multitude of possible ways to use it makes the designer’s job more complex. Specifying a stainless rudder shaft involves a few quick calculations and checking some standard reference tables; specifying a carbon one requires more careful analysis of how the part will be loaded and how the fibres should be oriented to handle the loads.

A Long Term Problem

Why, though, would we want to re-evaluate something as simple and well-proven as a rudder? The standard design—a foam/fibreglass blade moulded around a stainless steel armature welded to a stainless steel post—has, after all, been tested to death for decades on thousands of boats.

We re-evaluate it now because “tested to death” is an apt description. Stainless steel does not fare well in damp, anaerobic conditions like those found inside a water-saturated rudder blade. The difficulty of bonding fibreglass to metal, and the huge difference in thermal expansion coefficients, all but guarantees that water will eventually seep through the joint where the metal stock enters the composite blade. It works for a while, but the uneasy mix of metal and fibreglass eventually gets wet, and the salt water goes to town on the hidden stainless steel. The points where the armature is welded to the shaft are particularly vulnerable, and there’s no way to inspect them without destroying the entire assembly. These rudders can and do fail, sometimes catastrophically, and usually with repair costs that cause credit-card issuers to shiver with delight.

Until a few years ago, the stainless shaft and armature—despite its well documented flaws—was still the best solution we had. Fibreglass shafts aren’t stiff enough—they’ve been tried, but they flex like crazy. Bronze works, but its price is high and volatile, and it has the same thermal expansion issues as stainless when it’s combined with a composite blade. Titanium is even more costly, and the skills needed to machine and weld it are rare. All-aluminum construction can work, but is labour intensive—not to mention its vulnerability to corrosion, which calls for constant vigilance over the life of the boat.

A Better Sailboat Rudder

Now, though, we have a new material on the scene. Carbon composites are incredibly stiff, making them ideal for deflection limited applications such as rudder shafts, and they don’t corrode. Carbon’s price tag is falling rapidly, there’s a network of commodity suppliers, and a growing base of workers who can handle it properly.

Many still think of carbon as being so costly that it’s only appropriate for high-end, one-off parts; in fact, carbon parts are costly because they tend to be high-end one-offs. Now that carbon fibre is a commodity and can be deployed in mass production, it’s time to re-think some of our old ways of doing things. Either we’ll solve some long-standing problems, or we’ll gain a better understanding of why existing solutions work the way they do.

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[Do you have questions on the properties of the materials mentioned in this article? Now’s your chance to get the answer. Or do you have a first person account of a failed rudder, or of the use of carbon fiber in rudder shafts? Please leave a comment.]

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Matt, Engineering Correspondent, is a Professional Engineer and true renaissance man, with a wide range of expertise including photography and all things boat design. He has a unique ability to make complex subjects easy to understand and he keeps an eye on the rest of us to make sure that we don’t make any technical mistakes. Working as M. B. Marsh Marine Design, Matt designs innovative powerboats of all shapes and sizes.

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