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We are just about to finalise the purchase of a 1999 Sea Otter, having sold our lovely old Springer. The main advantage to us was the fact that it would not need so much hull maintenance. Otters seem to be as popular as ever and it has taken a few attempts to get to see one before it has been sold.
The Otter will be coming out for a survey shortly and we were therefore surprised when we were advised to black it while it was in dry dock. We have owned small aluminium boats before, which had no paint cover, just bare aluminium. We have asked around, but get conflicting advice. It is a similar situation with trying to get information about the dreaded "electrolysis" which apparently can destroy an aluminium boat overnight!
Would a Waterways World expert be able to tell us whether blacking is necessary and also explain, in simple terms, how to spot and avoid electrolysis.
Any information or advice would be much appreciated.
As these boats seem extremely sought after, perhaps WW might consider producing an article on them.
Emailed question from Jeff Jones

Asked by: Robert Cowling   | 9.21am, Monday 24 April

WW says:

We have covered Sea Otters in various reviews over the years- they are very well built boats with high standards of construction.
However, as with all inland boats, despite the heavy (rubber) fendering of the hull, abrasion damage will still occur. The advantage of having blacking is that this will be removed, rather than the outer thick aluminium oxide layer which protects the underlying metal.
A two-part epoxy would provide the best protection- but any hull paint (as long as it adheres to aluminium) would be good.
Aluminium itself is a very reactive metal (somewhere between zinc and magnesium in reactivity) but the oxide layer is stable and impervious, so protecting the underlying metal from reacting with the oxygen in air (or water). However, if the oxide layer is removed (either chemically or physically) such as by scraping against underwater protrusions. Once the oxide layer is disturbed, there is a risk of rapid oxidation which can cause localised damage to the metal in worse case.
So the blacking is both cosmetic (to keep the boat looking like other boats) but does also have protective aspects, giving a degree of protection to the oxide layer- so the advice to black is probably sensible. I believe that most Sea Otters were given two-part epoxy paint when they were built.
To spot electrolysis, you would tend to find blistering under the paint layer- if you find powdered aluminium oxide dust in places (such as the engine room) then there may be been distrubance to the oxide layer or accelerated oxidation (such as leak of battery acid- aluminium oxide is amphoteric - it reacts with both acids and bases (alkalis))
Ultrasound monitoring can check the thickness of the aluminium to see if there are any discrepancies within an area- the hull edge and near welds are the most vulnerable.
Electrolysis does occur if stray currents (such as stray earth currents) pass through the hull- and good earth isolation for 230V systems is important. A sound paint finish will help as well, as if the water is kept away from hull, electrolytic action will be minimised. Most damage occurs from abrasion allowing the aluminium to react, and boats moored in marinas where stray electrical currents from poor installations on other craft, as well as on the aluminium boat itself. A good check on the electrical system is useful.
Check also that the coolant is up-to-date in the engine- ideally it should be changed every two years, to avoid corrosion issues in the engine, engine room and especially the skin cooling tank. Inland boaters tend to be quite poor in changing coolant regularly, as they tend to top-up rather than drain down- and the anti-corrosion properties decay over time.
For any more specific advice, please contact me directly on m.langley@wwonline.co.uk

Mark Langley  | 11.01AM, Monday 24 April

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