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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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.
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.
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.
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."
@steveod21: Muntz metal is a particular kind of brass called a duplex brass. Like most other kinds of brass, it is susceptible to dezincification, but since sheathing was not structural, this may not have been considered to be very important. More important were its antifouling properties -- barnacles, seaweed, algae, and other marine life don't like to attach themselves to copper, or to copper alloys such as brass. The development of antifouling paints (most of which contain copper) brought an end to the era of metal sheathing.
Naval brass, which I described below, is very similar to Muntz metal, except for the addition of a small amount of tin (and sometimes some other elements) to prevent dezincification.
Although it is subject to dezincification, Muntz metal is resistant to many other kinds of corrosion, so it is still used in some applications.
I'm not an expert on metals, but I couldn't help recalling how an Englishman (with a fairly un-English name) made his fortune by developing an alloy of copper and zinc which came to be known as 'Yellow Metal' (or Muntz metal, after its creator). This was used extensively in ships, and clipper ships particularly as it happens, and the Cutty Sark and Thermopylae were full of it. Cheaper and stronger than copper alone, it was even used for bolts in some applications although its main use seems to have been for sheathing.
See http://en.wikipedia.org/wiki/Muntz_metal. It was the comments about battery electrodes that I found strange as Yellow Metal seems to have been in great demand for maritime applications in the 19th and early 20th centuries.
The Article doesn't say when this took place, but seems to imply a recent event.
Cheoy Lee's coming out of the Kao Shung yard in HK were known to have crappy metal fittings. Lifeline stanchions and pulpit/pushpit are likely out of 303 or 304 stainless, not marine grade 316. As a result, pitting and corrosion, and premature structural failure are known endemic problems with those vessels.
The author's uncle is lucky the fitting didn't fail when under dynamic load, or he would have lost the stick.
Yes, checking all underwater fittings each haul-out is important. The dolphin striker is a rod or cable that goes from the outward end of the bowsprit to a fitting, usually underwater, on the bow of the boat. There is no reason to disassemble this part except for the occasional inspection for corrosion. The part doesn't move and for the most part would be considered a permanent fixture - until it breaks. Most underwater hardware is designed to be taken-apart and cleaned, lubricated or inspected fairly easily and it is really unlikely that a turnbuckle on a dolphin striker would be made of an inferior alloy unless it was installed at some later date. I don't think I have ever seen a soft-brass turnbuckle or pin outside of a novelty shop.
@Rob: As one of the articles I linked to mentions, one telltale sign of dezincification is a loose, powdery white deposit. This is usually zinc oxide or zinc chloride, which are corrosion products of zinc. (Of course, in a marine environment, a white deposit might also be left by salt).
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