keel stepped masts - corrosion at partners

[Combustion]

I fitted a second hand aluminium mast on my Tahitiana steel double ender, and for extra support made it keel stepped. Where the mast sprouted from the deck I made a 2 inch high steel collar with about 2 inches clearance around the mast. I made some tight fitting hardwood wedges that completely surrounded the mast then sealed it up with fibreglass. Not a drop of water ever came in, even in bad weather. Unfortunately after only 2.5 years I've had the mast out and found some bad pitting corrosion by the wedges.

Would be grateful to know if

A) this is inevitable with an aluminium keel stepped mast, or
B) it could have been avoided by using rubber wedges instead of wood.

Can anyone suggest a source of rubber for the job? I seem to remember some stuff you could pour in that made a flexible rubber seal that came out in one piece when unstepping the mast. Anyone ever used this?

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[30boat]

The name is Spartight.You can find it if you do a Google search.I used a home made version on my previous boat that worked very well at a fraction of the price.
I got some two part polyurethane rubber of the kind used for moulds from a resins merchant.For the mast hood I used the leg from an old wet suit .Instead of hose clips I used shock cord tightly wound around the mast and partners to keep the hood watertight.

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[richardandtracy]

I'm not sure if I'm the best person to try to answer this, but as no-one else has tried, here goes.

If no moisture has got in from above, then I would say it's due to one of two causes, these being:-
1) Corrosive constituants in the timber wedges.
2) Condensation.

I would suspect that it's more likely to be the former. Many hardwoods have acidic constituents. To give you an example, think of iron nails in oak - there is inevitably black staining due to the acids present in the oak. In one test of plywood at the place I work, the ply was found to contain found 5% by weight acetic acid from the gaboon wood used and 3% by weight un-cured uric acid from the adhesive. According to an ancient HMSO handbook of hardwoods I've got, some seasoned timbers contain up to 10% organic volatiles - almost all of which could cause corrosion problems.

So, what solution?
Prevent the acids from getting to the aluminium. This would suggest that you put a layer of epoxy paint over the area likely to be touched by the hardwood - epoxy is one of the best paint on barrier materials commonly available. This solution will also work if the problem is actually condensation too.

Would a rubber wedge be useful. Don't know - but there is a high probability some of the "rubbery" smell given off by the rubber will be an organic acid - may be a weak acid but in the conditions found at the partners it may be able to continue pitting the aluminium. Also, if there is any carbon black in the rubber & condensation present, the aluminium will corrode sacrificially to protect the carbon black in a traditional galvanic cell.

Finally, cast in situ rubber. I don't really like the idea, but if you really have to do this, I would suggest you put in a slight excess of catalyst if it's of an alkaline nature - will tend to neutralise any acids present. Not sure about peroxide catalysts though - may tend to exacerbate the oxidation of the aluminium.

Regards

Richard


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[tome]

You should be using only admiralty rubber wedges on an aluminium mast, and they should be fore and aft - you should never wedge the sides. The rubber needs to be compressed by hauling the mast backwards to fit the front 2 then other way too fit the rear ones (using washing-up liquid or whatever as a lubricant). Once in place, it's suggested that they are clamped with a large jubilee to prevent movement.

Suitable wedges are available from all the spar manufacturers and are generally 1.5x the width of the gap.

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[ean_p]

Whats the reason for never wedging the side's please Tom?

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[tome]

Ean

The mast is designed to move slightly from side to side so as to avoid stress reversal loading on the spar section when sailing.

Regards
Tom

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[ean_p]

is this just applicable to oval or rectangular sections or would it apply to fully round sections and /or the same in timber also?....

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[tome]

Ean

It applies to all aluminium keel stepped masts regardless of section as far as I'm aware.

I once had a gaff cutter with a wooden mast which was wedged all round at the partners. Whether this was correct or not I'm not sure but I never changed the arrangement and it's still going strong. Did quite a few channel crossings in dirty weather and never worried about the rig, though the lack of any guardrails or stanchions might be frowned upon these days.

Regards
Tom

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[Combustion]

Thanks for all the input, plenty to think about. Have put the wooden wedges back temporary, will probably go for Spartite eventually.

Thanks again.

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[Strathglass]

This seems to conflict with current thinking.

My new Sparcraft keel stepped mast (F305 section) is supplied with a one piece 'rubber' collar which is slid up the mast prior to stepping.
This is clamped into position between the deck plate fitting and the mast, then a plate is bolted into position which compresses the rubber collar.
I have used the term 'rubber' but it is probably some hi-tech synthetic material. The moulding is a complex shape it even has a bit which slides inside the sailtrack.
I supose technology moves on. I certainly used wooden wedges at the partners of my Alfred Mylne one rater a few years ago but will not hesitate in using the all round support system supplied by the manufacturers for my new spar.

I believe that other spar manufacturers also offer all round collars with their latest spars.

Regards

Iain


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