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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My experience with the American Brass & Bronze industry has not impressed me with their quality and ethics. The bearing manufacturing I work for purchases a lot of Centrifugally Cast Bronze for bearing cages. This is preferred because centrifugally cast alloys claim "wrought" strength properties that are higher than simply cast versions. When a rash of cage structural failures occurred, we had the remains analyzed for chemistry and structure, both of which turned out "acceptable." We decided to order a few large blanks from which "dog bone" tensile specimans could be machined and tested. Ultimate (tensile) strength came back somewhat lower than claimed, so we filed a complaint with the supplier. Their response was that their product was not deficient because the industry specification that they follow allows them to keep trying new test speicmens until they obtain a group that passes. (The industry writes its own specs.) In the end we resolved ourselves to an addage frequently heard in old Western movies, "Yes, we know the game is crooked, but it's the only game in town."
This whole thread makes me feel good that I can only afford a Fish & Ski rather than such a fancy rig as this. My main corrosion problem is rusty fish hooks on favorite plugs and they are easy to replace.
Interesting deductions, but this failure was in 1974 after owning the boat for only 6 years.
Through-hull fittings, seacocks, propellers, and shafts are all commodity products, whereas the forestay fittings were made at the factory.
Stray current corrosion can occur on any metal. This is what occurs when your shore-power cord is plugged in at all times. ESPECIALLY if some joker connects the white neutral (current-carrying conductor) to the green safety ground.
GALVANIC corrosion is due to dissimilar metals immersed in an electrolyte. This is where the "de-zincification" can eat you alive.
Many alloys contain a little zinc (<5%), but when the content approaches 25-35% the de-zincification can weaken the material to the consistency of a sponge.
I owned a 1967 Broadwater power boat from 1992 to 1996. I always swam around the boat once after dropping the anchor to see if there were any loose screws. They were built with brass screws (mostly).
In fact, they had one barrel of bronze screws at the factory in Mayo, MD, for when the Marine Surveyor came by (they hid the brass screws and rolled out the barrel of bronze ones when he was there).
I can somewhat sympatize, since the bronze screws were much softer and it was easier to strip out the phillips screw head.
When we owned a 1966 Pacemaker in 1977, I noticed that MANY of the screw heads were stripped when they tried to drive screws into the oak frames.
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