The term "shop brass" usually refers to a low-quality, copper-based alloy that can be made on-site in many production facilities. After doing some research on various copper alloys referred to as "bronze," I was disturbed to discover that few of the industry-standard alloys are marine-grade bronze. Marine-grade is made up of the classic alloy of copper and tin, which should be used in a marine environment.
The marine-grade bronze is intentionally devoid of zinc, since copper and zinc immersed in salt water makes a fairly good battery. The zinc can decompose in the marine environment, weakening the bronze bolt, screw, or plug. The bronze can also decompose, which is a sinking hazard. Since many yacht yards cast fittings that are particular to their specific boat models, the customer may be at the whim of the yacht yard as to what the bronze contains.
My Uncle Charlie purchased a brand-new Cheoy Lee, Luders-designed Clipper 36. This Ketch-rigged sailboat was a beauty: teak decks, black fiberglass hull, teak taffrail around the cockpit, wooden masts, a wooden bowsprit (complete with "Dolphin-striker"), and lots of chrome-plated bronze fittings that were made right at the yacht yard in Hong Kong. As you may guess from the description, these bronze fittings might be more properly described as "shop brass," since no one independently tested the alloy to ensure it was really fit for marine use.
After a day of sailing in late September, my uncle anchored and tucked in for the night. About 2:00 a.m., he awoke to a rather odd series of noises. First, there was a series of creaks, then a scream of tearing metal, followed by another loud noise. The rigging of the sailboat started reverberating after receiving a huge jolt.
My uncle turned on the spreader lights and went up on deck. He found that the bobstay that fastens the bottom of the bowsprit to the hull had detached from the hull. The entire 800 pounds of rigging was hanging from the eight-foot-long bowsprit, which was arched up about 12 inches at the end. Luckily, it was laminated into the decks and hull, or else the mast would have come crashing down, with potentially deadly consequences.
After motoring rather gingerly the next day, my uncle was able to get some assistance with repairs. Further analysis determined that the failed fitting was made of a bronze that was not sufficiently strong to hold the huge amount of stress that the wind on the sails would cause. It was not assayed to determine the exact alloy, but it was likely shop brass. Today, it is likely that this fitting would be made of stainless steel. Any experienced yachtsman knows that there are issues with stainless as well, but that is another story.
This entry was submitted by Dwight Bues and edited by Rob Spiegel.
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Brass and bronze are two different things. Brass is an alloy of copper and zinc. Bronze is usually an alloy of copper and tin -- although there are also aluminum bronzes, manganese bronzes, and silicon bronzes, which are alloys of copper with aluminum, manganese, or silicon, respectively. Each of these catagories (brass, bronze, aluminum bronze, etc.) includes many different alloys with different properties.
I'm not quite sure what "shop brass" is, but cheap screw machined parts are often made out of free-machining brass (UNS C36000). This is an alloy of copper, zinc, and lead. The lead helps to make it readily machinable.
As the article correctly points out, copper and zinc are very far apart in the galvanic series, so when a brass part is exposed to a good electrolyte (like seawater), the zinc acts as a sacrificial anode for the copper. Ultimately, all of the zinc dissolves out of the brass, leaving a spongy mass of copper with very little strength. I'm willing to bet that if your uncle looked at the failed part under a microscope, he would have seen this sponge-like structure. This process is called dezincification.
Naval brass is a type of brass (usually approximately 60% copper, 40% zinc) which also contains a small amount (0.5 - 0.8%) of tin. Small amounts of arsenic, phosphorus, or antimony might also be added. The presence of these elements help to inhibit dezinicification.
The absolute best copper alloys for saltwater service are copper-nickel alloys. These alloys have excellent corrosion resistance. However, they also tend to be fairly expensive.
This article is a good example of why proper materials selection is so important. It pays to do your homework -- or, better still, ask a metallurgist.
Bob from Maine, it sounds like you'[re suggesting this may not be a Made by Monkeys problem but rather a simple wear-and-tear-over-many-years problem. Are there ways to check this before it causes a serious accident?
Don't know how long you'd had this boat, but Cheoy Lee Clippers were made in the 50's and 60's, so this incident could be due to simple age. My experience with Cheoy Lee has been their hardware is of very high quality and their hulls of that era were quite over-built. Most "Bonze" shafts, propellors, seacocks, and general fittings have some zinc, though I can't remember the exact percentage. Boats that spend time at the dock, particularly where other boats have shore-power plugged-in have a tendancy to de-zinc many of their bronze underwater fittings due to stray eddy current in the water. Also the Dolphin Striker assembly is usually not disassembled during winter storage so is never checked and unfortunately is a fairly common point of failure for this reason.
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