Advice needed about aluminium hull in sea water

[Jaguar]

I have since reviewed a number of other resources and they all confirm what you say - an electrical connection is required between two boats for there to be any galvanic corrosion.

Even is there is a galvanic effect, I am sure that a suitably painted and protected hull would render the effect to be insignificant.

What the resources do point to is very rapid galvanic corrosion of aluminium boats and components if the two boats are electricaly connected, e.g. through an electrical earth or metal walkway on to a steel jetty - both of which can be easily avoided.

My own boat is aluminium with an epoxy painted hull, protected with zinc anodes. The paint has not shown any signs of blistering, so I guess the boat is well protected.

JJ



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[graham]

Iwork on an aluminium workboat that has been moored alongside a steel structure for most of its 15 year life.

It has a galvanic isolator ,a "leak to earth"monitor and alarm.
It also has anodes which are inspected/changed at least anually.

Also you must be carefull not to drop metal objects into the bilge as they can corrode the hull.

Our boat is usually professionally repainted but when we do any touching up it has to be very clean and dry and etch primer used first followed by two pack paints recommended for ally. Alot of ally boats are not painted at all but with a rivetted boat perhaps it will help to stop corrosion around the rivets.

Any seacock/skinfittings should be isolated from the hull with plastic inserts washers gaskets etc.Then test that there is no electrical path with a multimeter.

Best of luck with the project ,Where theres a will theres a way.

PS you mentioned the cost of replacing anodes.To save having to have the boat out of the water you can attatch anodes to wires fixed to the hull above the waterline and with the anode hanging in the water(obviously no good when underway )You would need some expert advise on what wire to use etc.

 

[charles_reed]

Yes it is a maze of conflicting advice and there appears to be more of black art than science to electrogalvanic corrosion.

There are so many variables that you should beware any simplistic answers - type of alloy, fresh-water currents, contaminants in the water, other boats nearby, the presence of magnetic materials and any powerlines all affect the issue.

In view of the date of manufacture of the hull (probably war-surplus aircraft alloy) I suspect any informed answer you get would be disappointing.

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[charles_reed]

Possible sources of info.

Materials departments of any of the engineering faculties in a good university.

Ie Southampton (they absorbed the plymouth navy engineering)
Imperial College
Cambridge

They're the 3 most likely to be able to give accurate informed answers

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[Chris_Rapson]

Re: Possible sources of info.

I am about to start building my next boat. I like the Dudley Dix Vickers 45 which can be built in aluminium or steel.
The costs appear almost identical for each, with aluminium being cleaner, easier and faster to work.
Could anyone comment on whether aluminium has the problems in practical service that one could imagine?

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[MainlySteam]

Re: Possible sources of info.

The anecdotal problems one hears for aluminium are much over-rated and much of what is stated regarding galvanic corrosion is misinformed gobbledygook. One just has to ensure good clean aluminium fabrication practices are used (eg cutting/grinding tools that have never seen any other metal, no debris - especially cable clippings - left in hidden parts of the hull, isolation of dissimilar metals), take care with the electrical installation and do not bond the negative of the DC to the hull, and ensure that you use an isolating transformer for the shore power connection. Most of those care things apply to steel too.

Aluminium construction has just about killed the use of frp for construction for small commercial vessels, and one has to assume that the commercial operators know what is the best construction solution.

My own boat is a custom built steel vessel, mainly because yards out here (New Zealand) which build smaller aluminium vessels mainly did commercial boats and so the finish quality we wanted was not available at that time. Otherwise we would have gone for aluminium.

John

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