Bronze casting

[petercrook]

We need some new bronze floors for our 1910 Itchen Ferry yacht (the original iron ones are wasting away). What sort of bronze should we specify and how much do we need to allow for shrinkage when making up the patterns?

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[Peterduck]

Specify gunmetal, usually abbreviated here in Australia to LG2. It contains a small amount of lead to help with the machinability. There are a bewildering number of bronzes, but two are definite no-go areas; Aluminium bronze is nearly impossible to machine [including drill] and manganese bronze is not a bronze at all, but a misnamed brass. It will dezincify quickly, leaving a weak spongy copper. Shrinkage only becomes a factor in largish patterns; I think from memory that the factor for gunmetal is about 2.5%. If you need to modify the shape of the floors once cast to fit the planking or frames, do it when they are cold. Heated bronze is extremely brittle.
Peter.

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[PBrooks]

Can't help you directly, but I know a man who can. Try Moray McPhail at Classic Marine. They cast bronze and seem to know a lot about it. tel: +44 1394 380390
(I've no connection with the company, except as a customer)

Good luck
Paul

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[paulrush]

contact i think it is south west castings at milbrook southampton. they know all about these things. very helpful and fair prices

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[DMGibson]

You need to be careful about changing metals under the water in a wooden boat, due to the effects of electrolysis.

If the original iron floors have lasted over 90 years, why not replace them with new iron ones? You can get genuine wrought oron, which the originals probably used, but even mild steel will last a long time if galvanised of dipped in hot tar.

The important question is what material is used for the rest of the underwater fittings, especially the fastenings. If they are all copper and bronze, then you can replace the floors with bronze ones. However, if the fastenings are iron, putting in bronze floors will cause the iron fastenings to rust away, and your boat will literally fall to pieces.

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[ccscott49]

Totallyagree.My brother has galvanised frames held with bronze bolts, in wood. A recipe for disaster! The wood around the bolts is rotting, due to the salts built up from corrosion around the bronze bolts. be ultra careful. i would replace like with like.

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[petercrook]

"You need to be careful about changing metals under the water in a wooden boat, due to the effects of electrolysis."

That's why I want to replace the steel floors with bronze. The keel is lead, the keelbolts are bronze, the fastenings (including those for the floors) are copper/bronze. The futtocks are showing signs of electrolytic degredation - things might have lasted for 90 years so far, but unless we do something different they won't last for the next 90!

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[ccscott49]

If what you say is true, then I dont doubt the floors are rusting away, you also need to check the bolt holes for degredation. This is exactly the formula for wood decay ue to electrolysis. Yes, wholeheartedly, replace the floors with bronze and she will be fine for many a year yet.

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[DMGibson]

If that's the case, you are planning to do the right thing.

The other responses indicate the problems you get with mixing metals, and you will be reducing the number of potential problems by going for bronze.

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[northele1]

I finished a restoration last spring and had rot on timber around mild steel fastinings.
The boat was built in 1949 with mild steel floors and mild steel bolts plus copper nails. I found problems close to the mast floors. It looked to me that steel and
copper in wood caused electolysis. I replaced the mild steel bolts with silicon bronze. Where this was not done I have salt build up... Its aflote for the winter
and will haul in the spring. I added two extra anodes and wired all four to each floor. I hope not to have any more surprizes next spring.
During the summer one of the bronze highfield leavers for running back stays broke in two... Is it possible to fix this or can I buy a new one. its 3 feet long.

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