When I removed the centerboard a few years ago, I discovered a set of nylon bushings had been installed for the pivot pin. I later found out that when Thomas Howe owned the boat, he had also removed it, and found an oblong wear pattern as you describe below....so he had drilled out the pivot pit hole to 1 inch (you would have to be careful to mark the center of the hole to the exact length...so the tang would continue to hit the stop pin in the same place). The nylon bushings (2 of them) are 1 inch OD, and 3/8" thick, with a 3/8" hole in the middle. You can buy them at local hardware stores, Lowes, Home Depot, etc for less than $1. They snuggly fit the hole, and separate the stainless pivot pin from the cast iron board. Presumably, this would stop the corrosion and provide an easily replaced sacrificial wear surface. A small stainless shackle is fitted to the lift end, and it has a 5/8" piece of Stay Set or some such line for the pendant. No wear was evident there. I do worry about the line being wet and rotting. The most severe wear comes at the point where the tang hits the stop pin. I think they should be two flat edges...one on the board that rests on another....perhaps a bushing made of rectangular nylon stock...with hole drilled out for the stop pin...fitted into the trunk...with the stop pin passing through it. The weight of the heavy board hitting the 3/8" stop pin is a concern. When I let the CB down....ever so slowly....there is still an audible "Clunk" when it bottoms out. If the pendant were to break and the board dropped hard....it would do some damage. A rectangular bushing, fitted inside the trunk....would give it more load bearing strength than just the stop pin spanning the inch or so of trunk space. For those new to the list who might be lurking out there, pictures and diagrams of all this are available on the MSOG: http://www.msog.org/models/m17/m-17-cb.cfm Howard M17, #278 Audasea On 9/10/03 11:54 PM, "John Fleming" <jfleming1231@earthlink.net> wrote:
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
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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.
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