How would you keep it down? ----- Original Message ----- From: "dik lang" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, September 13, 2003 11:04 AM Subject: Centerboard Corrosion Discussion
I have to ask the question. Has anyone considered using HDPE for a centerboard on the m-17? It is cheap compared to the alternatives, can be cut with a bandsaw, no corrosion, low friction, doesn't support maarine growth, needs no maint. Stainless bushings and stops could be inserted as needed. My only real concern is, will it be stiff enough laterally? If this is a real option would the lack of weight be of concern? could add'l ballast be placed elswhere? Jerry? Bob? any comments? From: John Fleming <[EMAIL PROTECTED]> Reply-To: For and about Montgomery Sailboats<[EMAIL PROTECTED]> To: For and about Montgomery Sailboats <[EMAIL PROTECTED]> Subject: Centerboard Corrosion Discussion Date: Fri, 12 Sep 2003 00:57:53 -0700 Jerry Montgomery wrote: Note that the M-17 centerboards were of cast iron- not steel, which is much more corrosion resistant. Does Mr. Warren talk about this? Also note that the stainless wire pennant is aftermarket; we used dacron. Jerry Hi Jerry, That's interesting, dacron rope. Is it stable when immersed in seawater? How often should it be checked and or replaced? Would I just tie a knot? Yes, Nigel Warren discusses cast iron, the complete discussion is below. To summarize, he says that iron and mild steel corrode at the same rate, but have different corrosion behavior. Cast irons with significant amounts of nickel, silicon, or chromium rust much more slowly than ordinary cast iron. Obviously, stainless corrodes more slowly, but it has problems with crevice corrosion, especially in areas of high flow or no flow. I'll scan that discussion in some other time. Regards, John Fleming M-17: "Star Cross'd" ---- While wrought iron is not much used today, cast iron is still popular. The various grades of cast iron are produced by varying the composition (and here carbon is important) and by varying the crystalline structure as seen under a microscope. The latter is done by various furnace procedures. Hence one sees names such as 'grey iron' and 'white iron', so called because of the appearance of a fracture. Grey iron is basic cast iron and under BS 1452 there are various grades with tensile strengths ranging from 10 T/sq. in. (150 N/mm^2) to 27 T/sq. in. (400 N/mm^2). Then there is nodular cast iron with greater strength (BS 2789), 'nodular' meaning that the carbon in the iron is held in compact graphite nodules rather than flakes as in cast grey iron. Nodular cast iron is also called SG iron, the SG standing for spheroidal graphite. Other names for it are ductile iron, nodular graphite iron, and spherultic iron. Nodular cast iron is a steel-like metal easily machined and giving a smooth finish. It is the most modern development of cast iron; previously the only reasonably ductile cast iron available was 'malleable iron'. Malleable iron is a white iron which has been heat-treated to reduce brittleness. There are three groups, whiteheart, blackheart and pearlite (BS 309, 3 10 and 333 respectively). These terms refer to the process by which they are made and their resulting crystal structure. More sophisticated irons include the nickel cast irons and the high-alloy cast irons, both having enhanced strength and ductility. The corrosion rate of iron in seawater is much the same as that of mild steel, but the form of corrosion is very different. Whereas mild steel gradually wastes away and becomes physically smaller, iron tends to retain its shape and outward size but rots away from inside. Superficially it may look sound, but a sharp prod can reveal massive corrosion underneath. This form of attack is called graphitization because a graphite residue is all that is left. The ordinary cast irons have a poor resistance to shock and fracture fairly easily; they are also readily attacked by sulphate-reducing bacteria. Ductile iron is the modern equivalent of cast iron. It does not fracture as easily but its corrosion resistance is much the same. Nickel cast iron with 1-3% of nickel is a finer-grained iron but again the corrosion resistance is barely enhanced. There are families of high-alloy cast iron: irons with large amounts of chromium, or nickel or silicon. The austenitic nickel cast irons have a low uniform rate of corrosion in seawater and do not suffer from graphitization; they rust but slowly on deck. These cast irons are often called Ni-Resist, but this name is actually a trademark. The high-silicon cast irons (about 14% Si) also have a much enhanced resistance to corrosion, as do the high-chromium irons (12-35% Cr). (See Tables 8 and 9.) Unless a high-alloy cast iron is used, with much increased cost, the various 'cheap and cheerful' cast irons corrode at much the same rate as carbon and low-alloy steels. To Conclude Common mild steel and cast iron are excellent materials in many ways; they are strong and economical - but they rust. But at least the rusting is obvious and gradual and the metal is unlikely to fail suddenly as can happen to stainless steel or high strength metals. Some low-alloy steels like Cor-Ten are 'slow rusting' in a marine atmosphere but rust much like mild steel when immersed in seawater. All steels should have the mill scale removed before painting. Hot-dip zinc galvanizing is generally far superior and more 'cost effective' than zinc or cadmium electroplating. To get the best galvanizing ask for the work to be done to a standard, e.g. BS 729. If the galvanized item is to be immersed it is essential to paint it, preferably by using a self-etch primer for the first coat. Galvanizing gives a tough self-healing coating and one which is quite inexpensive. Electroplating is a waste of time unless a standard is specified, e.g. BS 1706 Class A. Zinc electric plating is preferable to cadmium because the coating thickness is greater. Nevertheless hot-dip galvanizing will give a much longer life and yet its cost is only very slightly more than electroplating. Galvanized or plated steel makes a good base for paint which in turn gives a long life to the paint and the metal coating providing the paint is 'keyed' to the coat. _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
