Canal boat construction & maintenance

Metal-hulled boats

Modern narrowboats and widebeam ‘barges’ are almost entirely constructed of mild steel. Given that this stuff rusts when exposed to water and air, this might seem like a peculiar choice of material but there are good reasons for using it.

First of all, it’s relatively cheap. It’s very easy to weld and there are many, many people who have suitable welding skills. That means it’s easy to fabricate and easy to fix if you need anything mended or altered.

Secondly, it’s really tough. Narrowboating is not, contrary to some popular opinion “a contact sport” but inevitably given the width of the canals and the lack of any licence needed to helm a boat there, bangs, scrapes and other collisions do occur. By and large these are shaken off with no more than a gouge out of the paintwork and no damage to the hull itself, except in extreme cases.

Yes, steel rusts but you need a mix of water and oxygen for this to happen which explains why the worst area for rust on a narrowboat is not the underside (the baseplate of the hull, two feet underwater) but along the waterline where air and water meet.

To slow down the rust, narrowboats are typically painted every two-three years with a bitumen-based thick, sticky goo known as “blacking” which keeps the water from coming into contact with the steel. To do this, the boat either goes into a dry dock, is hauled out of the water on a crane (scary to watch) or brought out on a trailer. Any muck is jetwashed off then two or three coats of blacking are applied, left to dry, and the boat is popped back in the water. Typical prices for a marina to do the whole service are about £10-£15 per foot of boat length.

That said, a lot of these marinas are keen to get the job done as quickly as possible so the blacking can be slapped on with little care and not given the correct time to dry between coats or before the boat is put back in the water. If you’re worried about it, you can simply get the boat hauled out onto hardstanding, blasted clean  and then black it yourself.

An alternative to blacking is painting with two-pack epoxy. This typically lasts from seven to ten years between applications but is much more expensive to apply, especially if you had traditional blacking beforehand as all this will need to be shotblasted off first.

Some folks swear by two-pack, some swear by blacking; it’s just one of those personal choices you need to make. I reckon that since I’d always want to haul the boat out for an inspection every 2-3 years anyway, I might as well stick with the cheaper and easier-to-apply bitumen.

The other aspect of blacking you’ll find much contention about amongst narrowboat owners is whether you need to black the baseplate (the flat bottom of the hull).

You might think “of course! It’s underwater, it needs protection” and to a degree you’d be right. However, there’s a contrary school of opinion that says the worst rust happens on the water line and the lower in the water you go, the less oxygen there is so your baseplate is actually much less susceptible to rust than the sides of the hull.

Plus the baseplate is made of the thickest steel in the entire boat, typically 10mm thick compared to 6mm for the hull sides (and usually 5 for the cabin sides, and 4mm for the roof) so it will take years and years to even begin to rot.

Many narrowboats that have been in the water for twenty years without blacking on the baseplate have pre-purchase surveys reporting very little wear on it due to rust, maybe less than a millimetre.

Plus many marinas aren’t suitably set up to paint the baseplate since this requires the boat to be rather higher on its stands and for someone to be happy to lie underneath 15 tonnes of steel doing the painting… and they’ll charge extra for it.

Finally you also hear the argument that since the canals are so shallow in many places, particularly the edges where you moor, that any blacking on the baseplate will be scraped off by the silt in very short order so why go to the trouble and expense?

As far as I am aware, no one has ever done a controlled study of a boat with baseplate blacking versus one without and it would take years to do such a comparison, with each both being used identically so it’s not really surprising.

You simply have to make a decision for what you feel is right.

If you think slapping on a thick coat of bitumen every few years sounds a bit crude, then you’d be right but it works and is tried and tested which is why narrowboaters still do it. Would you be willing to trust your very expensive boat to a new, untried treatment even if it promised all manner of wonderous benefits? Most people wouldn’t.

I’ve had lots of people suggest truck liners such as “Rhino lining” could be used and that’s quite possible but it’s not cheap and it’s going to have to take a lot of bashing and stay stuck on while fully immersed in water. Personally I don’t have the funds or the willingness to risk my narrowboat just to test it. As soon as any coating gets a scratch in it, that’s a place for water to start seeping behind the coating and that means rust.

Rust isn’t the only type of corrosion to occur on metal though. Corrosion can also develop through electrical charges on the plating. Methods to stall this include sacrificial anodes and electrical isolation.

Sacrificial anodes are chunks of another metal that will corrode in preference to the hull material. These are welded to the hull usually on both side of the bow and stern at least but also along the sides if done comprehensively.

Electrical isolation is important because stray currents in the water can cause electrolytic corrosion. To prevent this, there are devices called galvanic isolators as well as more sophisticated (and expensive) gadgets called isolation transformers. These need to be wired into the boat’s electrical system between any shore power connection and the boat.

If all this sounds terribly inconvenient and worrisome, you may well ask why aren’t canal boats built of aluminium? It sounds on the face of it like an obvious alternative to steel. It won’t rust, is tough and light – so why the obsession with steel?

Well, aluminium is expensive. A company called Sea Otter made aluminium narrowboats and very nice they are too – you still see a lot of them around, unsurprisingly since they don’t rust – but even second hand they cost about double what a steel boat would, and that’s quite offputting for a lot of folk.

Plus the skills needed to weld aluminium are more rare than those for everyday mild steel; it’s not something that every wharf and marina can do in-house.

However, it is a fallacy to believe the aluminium is corrosion-proof; it is merely corrosion resistant, there being a distinction between oxidation (rust) which aluminium is not subject to, and corrosion which it is.

Specifically, aluminium is more likely to suffer the electrical galvanic corrosion which we mentioned above. So aluminium is not a panacea to corrosion.

A long, long time ago some narrowboats were made of wood. You still see the occasional wooden hull, preserved lovingly by someone who’s willing to put the time and money into it but most have rotted away over the years.

You also get a few metal-hulled boats with wood or fibreglass cabin tops though these are also rare. The issue here tends to be how to ensure a good and watertight fit between the cabin and the hull, especially when in hot weather the steelwork on a boat expands slightly and can detach itself from other materials. Keeping such craft water-tight can be a substantial problem involving a lot of sealant, renewed at regular intervals.

With the hull construction taken care of, the next things to consider inside are the engine and the insulation. The engine warrants its own pages here but insulation has again come a long way over the years.

These days most professionally-built boats are sprayed with a polyurethane foam to a depth of one or two inches. It’s not a huge amount but you haven’t got a lot of space to play with in a narrowboat.

The advantages of this foam are that is sticks directly to the metal and by doing so forms a barrier that’s impervious to moisture, which helps prevent the hull rusting from the inside out. It’s also one of the best available insulators in terms of how much heat it retains for a given depth, and it provides an even coverage with no gaps. Basically, this is the stuff you want.

Older boats, or some that are fitted out by people as a DIY project, can have a variety of other insulation types, typically slabs of solid foam block (“PIR” – Polyisocyanurate) such as Celotex, or just polystyrene.

Celotex and similar products are actually very good insulators which is why they’re much used in the housebuilding trade but they’re not so good on boats. Because they’re sold as large square or oblong slabs, you can end up with slight gaps between them (tiny but through which moisture can weep onto the metal) and you have to make sure they’re very well stuck to the metal otherwise over time, as the metalwork expands and shrinks in the summer and winter, the slabs can detach, again enabling moisture in the air to reach the metal behind.

Once that’s happened, moisture in the air of the boat can creep behind and condense onto the steel, forming rust. It does take a long, long time – years! – but it will gently be ruining the boat from the inside.

Polystyrene has the same problem and just isn’t nearly as good an insulator as the modern foam blocks to start with.

You also get the odd boat where thermal blankets are stuck to the walls or traditional loft style insulation stuck between the battens. The same concerns apply about condensation over time.

Therefore in all of the above cases except sprayed foam, it’s essential to have a waterproof vapour barrier taped over the insulation to prevent the moisture issues. Unfortunately, once the boat has been lined out with wood and all the interior fixtures and fittings, even getting to see what insulation was used is difficult if not impossible, and changing it would require gutting the boat.

My personal preference would be only to look for boats with spray foam, if possible.

Fibreglass boats

Though steel is the de-rigeur choice for most canal craft, there are plenty of river cruisers made of fibreglass, some of which are of suitable dimensions for the canals. Fibreglass has many benefits, chief of which are that it doesn’t rust and it’s relatively easy to patch up – no welding expertise needed.

However, fibreglass is more fragile than steel and in a canal collision between a traditional 15-tonne steel narrowboat and a fibreglass cruiser, it’s the latter that will come off worst.

Despite collisions not being uncommon on the canals – they are very narrow channels and you don’t need any qualifications to helm a canal boat – it’s nonetheless rare to hear of fibreglass cruisers being sunk by such damage. They are usually heavily-laden with plastic fenders by their owners, not that this would stop a narrowboat’s nose from landing squarely into the hull, and another boat would probably need to hit head-on to actually crack the fibreglass to any great extent (whereas in reality it’s more likely to be a glancing blow which the lighter boat could manage to deflect).

Thick winter ice is arguably a greater threat though I have read extensive discussion on the subject on canal forums and the consensus seemed to be that most fibreglass boats are tough enough to take it, though if said ice was pushed into the hull by a passing narrowboat then that could well be enough to cause damage.

Another aspect of fibreglass that is often claimed as a significant issue is ‘osmosis’, where water creeps into the fibreglass strands through any small areas of damage, causing blisters and weakness in the hull. Osmosis certainly isn’t great news but there are ways to treat it (largely by puncturing the blisters and drying out the water) but there’s a popular saying that ‘osmosis never sank a boat’ and you certainly do see some extremely ancient and frankly rough-looking fibreglass boats floating happily on the inland waterways so although you might not wish to put to sea in them, it’s probably fine for the canals.