High Density Polyethylene plastic. I don't have a density of that particular plastic handy but it's probably very similar to UHMW (Ultra High Molecular Weight) plastic which weighs 60 pounds/cu ft....too light to stay down without some lead added to it...
Tod -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jerry Montgomery Sent: Sunday, September 14, 2003 11:02 AM To: For and about Montgomery Sailboats Subject: Re: M_Boats: Centerboard Corrosion Discussion What is HDPE? How heavy is it? Jerry ----- Original Message ----- From: "dik lang" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Saturday, September 13, 2003 8:04 AM Subject: Re: M_Boats: 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: Re: M_Boats: 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 > > _________________________________________________________________ > Use custom emotions -- try MSN Messenger 6.0! > http://www.msnmessenger-download.com/tracking/reach_emoticon > > > _______________________________________________ > http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
