September 2014 Issue
A Look at Silicon Bronze Versus Stainless Steel
As we embarked on our extensive test of halyard shackles, we were reminded of the longevity of silicon-bronze hardware. Silicon bronze should not be confused with brass. Brass is a pretty poor marine metal. An amalgam of copper and the much less costly and corrosion-prone zinc, brass’ reign afloat, at least in load bearing hardware, was short lived. But when copper is alloyed with less corrosion-prone metals, the result can be quite different. And although brass and bronze sound confusingly similar, their attributes tell very different stories.
Brass has held a decent shore-side reputation as a copper-based, yellow metal that’s reasonably strong, corrosion resistant, and able to be made into fasteners. However, when the potential for galvanic corrosion takes hold, and the presence of the chloride ion is added to the mix (all part-and-parcel of the saltwater environment), dezincification plagues brass. This is why marine metals experts early on turned to silicon, aluminum, tin, phosphorus, and nickel instead of zinc when it came to alloying copper and creating a stronger, more corrosion-resistant metal for marine use.
Among these zinc-free bronze alloys, silicon bronze represents the metallic sweet spot that combines strength, corrosion resistance, malleability, ductility, and a cost-effective price point. More esoteric alloys like high-nickel-content Monel are just too costly to be widely used in the recreational boating market. But silicon bronze was an alloy of choice for nearly a century.
One of its most redeeming characteristics is its tolerance of cycle loading and fatigue-inducing stress. Parts machined or cast from silicon bronze tend to physically deform before they finally break. This gives sailors a chance to visually observe signs (elongated holes, twists and bends) indicating that hardware needs replacing before complete failure mandates more complex repairs.
Looking over the running rigging aboard PS Technical Editor Ralph Naranjo’s Ericson 41, Wind Shadow—which has been revamped three times in almost 40 years of ownership—we realized that the silicon-bronze screw type mainsail halyard shackle was part of the boat’s original kit when launched. The reason it’s still in use today has nothing to do with nostalgia, and everything to do with design, engineering, and metallurgy. Silicon-bronze hardware is moderately strong and seriously corrosion resistant, making it an excellent choice for standing- and running-rigging hardware. Due to stainless steel’s initial strength-to-weight advantage, and probably its inherent shiny appeal, it has upstaged silicon bronze as the metal favored for sailboat hardware. Ironically, the aesthetic appeal of the bright, highly polished stainless tipped the scale, and the practicality of the green, self-protected surface of oxidized bronze lost out.
But it’s interesting to note that, over the years, most of Wind Shadow’s stainless snap shackles, bow, and D-tack shackles have crevice corroded, cracked, and either failed or were replaced in the nick of time. The longevity award for mainstream marine metals favors silicon bronze.
For the first 10 years, Wind Shadow’s silicon bronze halyard shackle had a chrome-plated surface, but as time went on and weathering took its toll, the shiny finish was replaced with a green patina. This verdigris actually protects the metal from more active oxidation. Add to this the way threaded silicon bronze self-lubricates and will not gall, and you can see why it was a great choice for a double-threaded halyard shackle. Today, a close look at the shackle reveals considerable surface pitting, but its substantial dimensions make up for this minor aesthetic shortfall.
A couple of years ago, we sailed aboard the L. Francis Herreshoff classic Ticonderoga and couldn’t help but note the deck hardware—a testimony to the utilitarian nature of silicon bronze. Built in 1936, the big ketch has gone through a handful of serious refits. Its deck and hull planking have been replaced, spars remade, and engine replaced, but much of its original, green-tinged silicon bronze deck hardware has stood the test of time. Sadly, this great material is disappearing from the waterfront, replaced by shiny stainless steel, a good metal but one with less longevity. The only place you’ll find silicon bronze shackles these days is used-gear chandleries.