Informative writeup by Nigel Warren, John. Thanks for passing it on. That spouting problem I was quering the group on a couple weeks ago is due to a busted tang and the cb positioned lower that it should have been. I dove on the boat a couple times and positioned the cb correctly and marked the pendant. Fixed the problem. Now we'll see about fixing the board over the winter. Any thoughts on fixing the tang but rather than lowering the cb to the stopper bolt, making the pendant carry the weight? t
Tom Smith & Jane Van Winkle Sandpoint, Idaho M15-345, Chukar -----Original Message----- From: John Fleming [mailto:[EMAIL PROTECTED] Sent: Wednesday, September 10, 2003 9:54 PM To: For and about Montgomery Sailboats Subject: M_Boats: Centerboard Corrosion Discussion I recently bought a book titled "Metal Corrosion in Boats", by Nigel Warren. Really a fascinating book, with all sorts of hints and discussion about how to best arrange all your metal components on a boat and how to manage the effects of corrosion above and below the water. Anyway, there's a section in the Chapter "Underwater Problem Areas", that should be of interest to all Montgomery owners, especially those with metal centerboards. I have typed this section in for you. There are other sections that deal with topics near and dear to Montgomery owners, but I'll start with this one. John Fleming M-17: "Star Cross'd ---- Centreboards (Nigel's a Brit) Most centreboards are of mild steel, sometimes galvanized sometimes not. That in itself does not produce special problems but the pivot bolt and the hoisting gear often enhance corrosion. The bearing areas in way of both the pivot bolt and the lifting tackle are especially prone to a vigorous corrosion. The actual surfaces in contact rub together and are immersed in salt water. Any galvanizing or other protective coating soon rubs off exposing the bare steel edge. The hole in the centreplate then enlarges quickly. The lifting tackle pin is necessarily close to the edge of the plate and eventually after a few seasons can pull through the edge releasing the centreplate. This process will happen even with compatible metals - in most cases a galvanized shackle pin in a steel centreboard - but if a stainless or bronze shackle is used the rate of corrosion of the hole is many times faster because of galvanic effects. With a mild steel centreboard there is no long-lasting solution to this problem except to use compatible materials, i.e. a galvanized shackle on the end of the lifting wire and a galvanized pivot bolt, and then to accept that steady wear will take place. Each year the items should be inspected and replaced when necessary. In the case of the lifting tackle at least the wear of the plate itself can easily be avoided by fitting a pair of mild steel straps and having a replaceable pin. With more difficulty, the hole for the pivot bolt can be fitted with a removable steel bush[ing]. Then at least all the wearing parts are easily replaced. Obviously the thicker the bolt and the pin the longer their life; many boats with centreboards are fitted with pathetically thin pivot bolts which while being quite strong enough when new can tolerate very little corrosion. [How true is this for Montgomery's? I wonder.] The lifting wire itself is a problem. Constantly wetted, neither galvanized nor stainless wire is likely to last long. Stainless wire will give a short life underwater because of pitting, and if a coppersleeved Talurit splice is fitted this will corrode preferentially. Polyester (Terylene or Dacron) rope is one solution, though with a chafe problem, but galvanized chain is better. The lowest link will take the wear but if an overlong piece of chain is fitted in the first place it can be cut off in due course and the next link up used instead. The galvanizing will last only a few years so replacement of the chain must be considered as a matter of course. [My Montgomery has a little wire rope pigtail attached to the board, and then a polyester line going to the cockpit. I guess he's saying I'll have to check it every so often.] A complete long-lasting solution to the wear problem at the pivot and lifting pin on a steel centreplate is elusive. Obviously a longer life would ensue if the wearing areas were of a hard and corrosion resistant metal: this suggests stainless steel. The pitting problem would be reduced by the cathodic protection given by the steel plate. So, at the pivot, one would end up with a stainless bush fixed to the plate and with a stainless steel bolt, making sure that Type 316 stainless steel was used. The bush could be rivetted, bolted or even welded in place. Similarly the lifting point could be bushed, or stainless steel tangs could be fitted. From there a stainless shackle would lead to a polyester lifting rope. [I believe that the Montgomery's more or less follow this advice, except for the stainless bushing at the centerboard pivot. He seems to say that the pivot bolt must be pulled and inspected on a regular basis, and if any significant pitting has occurred, it must be replaced. Also, it implies that even though the centerboard is painted to control rust, there must be metal-to metal contact between the cenboard and the pivot pin.] Of course a centreboard of zinc-free copper alloy would solve all the problems because the attachments could then be of the same corrosion-free alloys. Suitable metals that come to mind are copper, gunmetal [a bronze (88% copper plus tin)] and copper-nickel. Copper alloys are much the same density or a little heavier than steel. A bonus would be that no antifouling would be necessary. The lifting wire could be of Monel (which is available, though at a price). Stainless steel for the centreplate could also be considered (again Type 316) with sacrificial anodes. [The new M23 Bob Eeg is making has a centerboard of silicon bronze, though he doesn't say here whether the pivot bolt is still allowed to be stainless.] Usually cost rules out such materials and mild steel is the only practical choice; but to avoid disaster it is wise to consider carefully the pivot and lifting arrangements. ---------------- _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats This message was scanned for viruses. _______________________________________________ http://mailman.xmission.com/cgi-bin/mailman/listinfo/montgomery_boats
