Repair and Maintenance of a Drascombe Lugger

Re-galvanising the Centreplate

This page is about checking and if necessary re-galvanising the Lugger's metal centreplate. The technology of galvanising is explained and there is advice on how to get a moderately corroded Lugger centreplate shot blasted and re-galvanised.

What is the centreplate made of?

The centreplate is composed of mild steel 10 mm thick. To prevent corrosion (rusting) of the steel, the centreplate is galvanised (dipped and thereby coated in molten zinc). This coating is remarkably effective because, for reasons of chemistry, the zinc when exposed to the atmosphere forms a basic zinc carbonate surface film that protects the underlying metal from further corrosion. In addition zinc, when coating steel, will sacrificially protect that metal. In simple terms this means that the zinc will corrode in preference to the steel but the protective carbonate film which forms on the zinc makes this a very slow process.

Why does the centreplate have to be re-galvanised?

Eventually the zinc coating will corrode away exposing the underlying steel. As the amount of zinc decreases so the corrosion of the steel increases. Typically the rusting starts along the leading (front) edge of the plate which is the part, when the boat is afloat, that is continually in contact with the water even when the plate is raised. This pattern of corrosion can be very deceptive as the top part of the plate can look in good condition whilst the bottom is rusting away! This is illustrated in the photograph below.

Corroded centreplate, photo courtesy of Ian Cowie Typical corrosion of centreplate

This centreplate from a 1972 Lugger shows a typical pattern of corrosion that has obviously commenced along the lower edge of the plate and migrated upwards. In this instance the rusting was so bad that the plate had to be replaced. The plate was replaced in late 2001 so the original galvanising had lasted nearly 30 years!

I am grateful to Ian Cowie for permission to use his photograph.

(Click on image for larger view)

What is the best way to check for centreplate corrosion?

A seasonal check on the plate can be undertaken by careening the boat (turning it on its side), and pushing the plate down so it can be examined. Alternatively it can be checked by crawling underneath the boat when it is on the trailer and examining the plate for corrosion along the leading edge exposed in the centreplate slot. The following photograph shows what Sospiri's plate looked like when it was inspected from beneath the trailer.

Centreplate viewed from beneath the trailer Sospiri's centreplate viewed from beneath the trailer.

The centreplate is showing signs of surface corrosion which means that the whole plate will need extraction and re-galvanising. This is to be expected in a boat which is 20 years old.

(Click on image for larger view)

Photo of centreplate showing slight corrosion Sospiri's centreplate after extraction

Extraction of the centreplate shows a similar pattern of corrosion to that of the 1972 Lugger. The brown colouration along the bottom of the plate shows that the shield of zinc has been eaten away and the underlying steel is starting to rust. In this case the corrosion is superficial and can be stopped by re-galvanising the plate.

(Click on image for larger view)

What is the best method of getting a centreplate re-galvanised?

The process consists of two stages:

The shot blasting is essential to remove rust and partially oxidised zinc and expose fresh steel. If you can find a company that does both shot blasting and galvanising then you are lucky!. I had to get the work done at two different factories.

Shot blasting

In the UK Look in the Yellow Pages phone book under the heading Blast cleaning. Alternatively try the same search on the Yell.Com web site. This site offers a choice for UK or USA users. Using this I was able to locate a suitable company about 15 miles away from home. The plate was transported by my wife in the back of the family car and it was blasted whilst she waited at a cost of 5.00 UK pounds.

Re-galvanising

Again a search on the Yell.Com web site under the heading Galvanisers located a local company. A phone call revealed that they had a minimum charge of 78 UK pounds but this cost could be reduced if the job was submitted as part of a bigger lot. It was then that I remembered that my local chandlers manufactured trailers and a phone call confirmed that they used the same company for galvanising their trailer components and that they would be quite happy to include my centreplate in their next batch. This they did and the cost was 23 UK pounds.

Photo of Re-galvanised centreplate Sospiri's re-galvanised centreplate

The re-galvanised centreplate. Total cost of shotblasting and re-galvanising was 28 UK pounds. This should last for at least ten to fifteen years before further treatment is required.

Note the additional piece welded on to the arm which was done before re-galvanising.

(Click on image for larger view)

What if the centreplate is too corroded and has to be replaced?

There are two options. Either purchase a new plate from a Lugger supplier. Ian Cowie got a quote of 233 UK pounds (excluding delivery) for a replacement plate for his Lugger.

The other alternative is to get a local steel fabricator to manufacture a new plate using the old one as a pattern. This is what Ian did at a cost of 105 UK pounds representing a considerable saving.

Project started March 2002 and completed, April 2002