Tarting up your tinny
The reasons aren’t hard to find. Durable and long-lasting, an aluminium hull is immune to dents and scratches from sinkers and fishing gear. It’s easily cleaned by a simple hose-down and a scrub, is unaffected by UV radiation or anything the elements can throw at it.
It may not look as smart as a gleaming gel-coated fibreglass hull, and certainly can be noisier and less warm if not insulated properly, but for many owners the practicality wins out over appearance.
Another benefit of the tinnie is that it can be customised to suit a buyer’s preference. Unlike moulded fibreglass, where changes to layout require a whole new mould, an aluminium boat can have elements added or changed at any point during the initial manufacture or later on. Items welded on at a later date can be just as strong as the original construction.
In many respects an aluminium hull is pretty indestructible, and there are numerous examples of 30-year-old tinnies still going strong. But they do hide one dirty little secret which can turn an otherwise bulletproof work of art into a crumbling pile of white powder – corrosion. If left unchecked this will quietly eat away at your hull until it causes major damage. There are three types that afflict recreational boats.
The most common form is atmospheric corrosion, which is basically the metal reacting with oxygen in the air. Aluminium starts to oxidise almost as soon as it is made. That’s why a bright, shiny aluminium sheet turns a dull grey a few months after leaving the factory. Luckily, this usually causes few problems because the metal actually protects itself through this very process of oxidisation.
A thin layer of aluminium oxide (also called alumina) rapidly forms on the surface, just a few microns thick. This oxide layer is extremely tough, is bonded tightly to the metal and doesn’t flake off. The alumina itself is unreactive and acts as a natural barrier to salt, acids and chemicals, and will protect the metal from any further corrosion once it’s oxidised.
Note: Polishing the surface of your uncoated aluminium boat is a bad idea – it will corrode faster if you keep buffing the coating to keep that shiny ‘new’ look. And this natural coating is also why a 30-year-old alloy boat may be just as solid as a brand-new boat.
But two other types of corrosion do cause major problems, and these need to be guarded against. The first is crevice corrosion – where a gap or crack allows salt water to become trapped. This can occur along a seam or weld, in joints, where fittings are installed, coatings have been applied, and under carpets and decking.
Crevice corrosion is different from galvanic corrosion (see below) because only one metal type is involved – the aluminium itself. The structure trapping the water can be wood, plastic or another part of the same hull, but it is most commonly seen as bubbles forming under poorly-applied paint.
In this situation the initial oxygen in the trapped water is consumed as aluminium turns into alumina. More damaging though is that the remaining water then becomes more acidic, and without the presence of oxygen this reacts with the metal to form aluminium salts.
These form part of the white powder you see along such cracks and crevices. The best way to prevent this form of corrosion is to rigorously wash the boat with fresh water to ensure salt water isn’t trapped in corners or crevices.
By far the biggest cause of problems is galvanic corrosion, which occurs when two different metals are in contact with an electrolyte. And salt water is a very good electrolyte. The metals need not even be very different – even slightly different grades of aluminium are sufficient to set up a galvanic reaction.
Aluminium, remember, is usually an alloy containing traces of magnesium, chromium, copper, zinc, tin, manganese, iron and silicon.
Galvanic corrosion can occur anywhere and can be rapid even if the other metal is very stable. It is always the less ‘noble’ metal (on the galvanic table, which ranks the reactiveness of different metal alloys) that corrodes first.
There are documented cases where a dropped ball sinker made of lead has sat unseen in the bilge of an alloy boat until it eventually corroded a perfectly round hole right through the hull. Zinc is the only metal lower on the galvanic table (than aluminium) that’s commonly used around boats, which is why zinc is usually used for anodes on engine blocks and gear cases.
So what does this mean for maintenance or upgrading your boat? The first thing is to regularly check for places where corrosion could set in. Unfortunately, a poorlyprimed paint job on an aluminium hull can, through crevice corrosion, actually cause more problems than leaving it unpainted. If water is able creep in under the edge of the paint, you will see bubbles start to form and, after a disappointingly short period of time the paint can loosen and start to flake off.
Paint doesn’t stick very well to aluminium oxide, so painting an older boat involves sanding the alumina layer off down to fresh metal. Then, before it has a chance to oxidize again you need to apply a high-quality etch primer that is specially formulated for aluminium.
This is then followed by the appropriate number of surface coats of your chosen paint system. New boats generally get an acid wash before painting to achieve the same effect, but this is hard to do on an older boat without stripping it back to a bare hull before the acid bath.
When it comes to fitting accessories you obviously want to isolate the different metals, so they do not actually come into contact with each other. One way is using a suitable non-metallic waterproof layer like a piece of rubber or plastic, and most stainless rod holders come with just such an insulating pad for fitting under the flange. If not, a thick piece of rubber can be cut to the same shape as the fitting and located between it and the hull.
The bolts fixing any accessory in place are more problematic, because it is hard to get something to go around the thread and under the washer, bolt head
By far the biggest cause of problems is galvanic corrosion, which occurs when two different metals are in contact with an electrolyte.
and nut. Never use stainless steel screws with aluminium – everywhere the thread of the screw ‘bites’ into the aluminium will quickly corrode, and within months the screw will simply pull out.
The best option is a bolt right through the surface with a matching stainless washer and nut on the other side. This is also easier to insulate from touching the metal, and the solution here is a specialised insulating sealer. There are two products available from most chandlers:
Duralac is a hard-setting sealer developed by the US aerospace industry, where unseen corrosion could have catastrophic consequences. A bright yellow (and quite fluid) sealant, it is liberally applied everywhere that the two dissimilar materials come into contact, before they are fastened tight. The yellow comes from its active ingredient barium chromate, and it is best applied with a brush.
When left undisturbed it lasts for decades and is definitely the best option for a permanent solution. It is also best for areas which may be regularly splashed with salt water, such as on the deck or transom. Note the active ingredient is a hazardous compound so read the safety instructions when applying.
Tef-gel is the alternative product, but unlike Duralac it does not harden. Derived from a Teflon gel, it is a better solution for bolts that need to be undone regularly for maintenance. For example, the bolts holding a stainless anchor winch to an aluminium deck would best be protected with Tef-gel, as the winch may need to be removed every couple of years for cleaning and lubrication.
Since it is a gel, this product can be eroded by regular washing or dunking, so not suited for fittings on the boarding platform, for example. It pays also to remember that Telfon is an effective lubricant, so rigging bolts and structurally-critical connections should not have Tel-gel applied unless you are prepared to check regularly that they are not working themselves loose.
One final trick when fitting accessories is to try and attach
...you obviously want to isolate the different metals, so they don’t actually come into contact with each other.
them somewhere that is not going to be critical to the integrity of the hull. Some boat manufacturers, like Everyman Boats, weld additional plates to the transom for mounting transducers, water pickups, trim tabs and the like.
This means that the bolts holding these items are not going through the hull itself, making the accessories easier to attach and causing fewer issues in the long run. If you need to install a number of small metal items you might consider fitting a longer strip of wood or plastic, with suitable sealant on the bolts, and can then more easily screw the smaller items to the wood without worrying about corrosion.
A little bit of forethought and the proper preparation can make all the difference. It can be heartbreaking to spend thousands of dollars and considerable effort on upgrading your boat, then a few years later realise it is self-destructing because you neglected a small but critical preventative step.
BELOW Left to weather naturally, aluminium is covered by an oxidised layer – it provides protection.
ABOVE Notice the use of a compound to isolate different metals from one another.
OPPOSITE Another victim of poor surface preparation.
BELOW Two products that can be used to inhibit corrosion.
LEFT Some builders attach separate brackets for mounting transducers.ABOVE Typical blistering resulting from poor surface preparation; if you want to paint your vessel, make sure the surface is bare aluminium.