Tuesday, 11 May 2010

Metal boats in sea water

The only way to solve this boats paint woes was to start again,grit blasting is highly controled now due to the mess and health risck,the option to used UHP (ultra high pressure) was the only route in this case,its was fast and when the water dried away,very clean,the Sigma etch prima was applied within a half hour of the steel plate drying.This is the only sure way to isolate your boat from the shore power as far as galvanic currents are concerned.
Note: UHP has the ability to remove the paints but importantly does not 'polish' the original grit blast surface off the steel.
The pictures show a steel box keel being UHP water blasted clean,thats totally clean of all paints applied over an eleven year period,from the original Intergard primer to its tie coat and antifouls over the period.Its easy to see the paint degrade on the side of the keel but why was this happening,it was fine when last hauled but I think the alarm bells were ringing then.

There was never any doubt about the manufacturing process of the steel work itself and having the etch primer applied after grit blasting by proffessionals,ensured that side was ok too,so what happened? My thoughts work around two possibles,both may be connected too,does Hout Bay Harbour have a high electric charge in its water and as the bottom of the keel could never be inspected,what was going on down there,basically we do not know,the problem with a slip on rails being you sit on blocks of wood,often quite close together,the boat had only ever been hauled on that type of slip,if rust had of started we would not know and if this is the case,that will be the start of the degrade in the paints as can be seen?

A boat that was steel built and kept on the HBYC Marina and left untouched for five years was eventually slipped,the owner and builder told me he was both happy and amazed that the annodes were still working and no degrade in the hull was to be found.He repainted,changed the annodes and went back to the marina,nine months later the boat,they were living on it,was found to be leaking so badly it was in danger of sinking and had to be craned out and taken to the hard,it needed a lot of new metal welding in.Why,what was going on? I have to assume some sort of galvanic process was taking place,the extension to the harbour wall was in progess then,lots of heavy welding was taking place in the ,did they have earth straps into the sea water? the other points are the possbles of being grounded to a shore supply,never ground the boat at all,the shore supply can be the only earth you need and when you do use a battery charger,which will ground the boats 12v supply and engine,use a Galvanic Isolator,your boat could become the main annode on the marina otherwise?

One last point is the pro diver I employ to check and service the prop shaft and its annode ,tells me its not uncommon for him when leaving the water and reaching up to the marina and its steel works,to recieve a shock,in this he is becoming a ground strap from the water to the marina,which has shore power and floats on plastic containers.

I was recently asked about the UHP metal keel blasting we did on a Dix 43 recently,the main question was why it was required? Why indeed,this is a wood/epoxy construction but the keel is a steel casing some 4.40mtrs long and 1.1mtrs tall,it has five tones of lead in it and weighs six tones in all.

The boat has been haulded three times before this time,the last time it had been in the water was a period of three years since the time before,at each hauling the keel annode was checked and changed as a matter of course,after three years the large pear shaped annode was hardly touched.

The reason for this should be simple to explain but some other outside element may have been creeping in and unseen,I had the keel made proffesionally by S Products,I then had Nautilus Marine collect the keel (empty) and take it to their premises to be grit blasted and etch primed coated with International Paints Intergard Red Primer.Tim of International Paints had recomended this as the correct process.Later a tie coat,also by International Paints was applied prior to the application of antifouls.

In the next three years the boat was hauled either at Grainger Bay or at the RCYC,both places haul you on a railway type slip,which gives good access  to all but the very bottom of the keel,this may later be an issue? at each removal from the water,anti fouls,annodes and the keel were checked closely,new antifoul being replaced on one haul.With new paints and annodes to both the keel and propellor shaft the boat was to be hauled next some three years later and at RCYC.

This was for antifouls,new annodes and as normal stripping and regrease of all bronze through hull fittings with water proof grease.This time it was noted that a small patch of paint had rusted to the steel just aft and above of the keel annode which is low down on the port side of the keel,it was also to be noted that some large patches of tie coats and antifouls were quite loose and could easily be scrapped off,leaving the Intergard primer looking like it was brand new.

This I think may have been noted more fully,why and after six years was there surface rust and patches of paint faling off down to the Intergard primer? The boat was fitted with a new keel annode and new antifouls,as before using International Paints products.It was to be some four years later after a diver told me my keel annode was none existant that we hauled again,I had always had the propellor annode serviced but with so little damage happening to the keel one,had not even considered that as a source of trouble.

To be continued:


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