It continues to amaze me that there are still yards operating on the canal network who do not, as a matter of policy, routinely clean and paint the bottom plate of narrowboats during periodic blacking. Several reasons are often given such as “it will only rub off”, “there is no corrosion due to the lack of oxygen” or “owners never ask for it”. The most recent excuse given was the health and safety implications and inherent danger of working beneath a 30 ton boat. Which implies that any marine surveyor willing to crawl beneath a narrowboat to survey the bottom plate has failed to appreciate the dangers present. Having in the past seen narrowboats supported ashore on a handful of empty beer kegs there are clearly many dangers to consider…

Regardless of how shallow the cut is or how many submerged bicycles there are; steel is steel.  It wants to rust away and revert back to iron oxide and your mission as a boat owner is to reduce the rate at which that happens to as close to zero as you can manage.  That way you avoid having the bottom re-plated or overplated which is never cheap. The only way to preserve the mild steel used to make a hull is to de-rust and paint the inner and outer hull surfaces on as regular a basis as is needed.

It would be fair to say that in the absence of any electrolytic, galvanic or microbially induced corrosion, the most prevalent form of corrosion noted during narrowboat surveys is general or uniform corrosion. The most common cause of this corrosion getting underway is bare, unpainted steel plate below the waterline. The rate of corrosion observed varies greatly with the type of corrosion present and it is normal to expect a thickness loss of 0.1mm/ year for general corrosion. For a vessel built with a 5mm bottom plate thickness (Springers come to mind), it will not take long before the hull becomes too thin to insure.

The easy answer to this problem is to protect the steel using some form of coating and in the marine world paint is the most cost effective solution. As any tin of paint in a DIY store will tell you, ‘proper surface preparation is essential to achieving a good finish’. For a boat hull that will spend prolonged periods immersed in water, the paint finish needs to be excellent if the steel is going to remain protected until the next lift out. Such a finish is achieved through properly cleaning the hull of all surface contaminants, ensuring the surface is de-greased and smooth enough the achieve good paint adhesion, and then applying the paint in sufficient thickness to maintain the protection until the next planned maintenance period.

Aggressive bottom steel plate pitting on narrowboat

Photo 1: Narrowboat Bottom Plate (Blacked) With Pitting

Surveying any steel vessel in an unwashed state increases the degree of uncertainty regarding the condition of the steel plate and the rivets or welds securing the plates in place. When obscured by fouling the condition of the plate becomes an increasingly large ‘known unknown’[1] and makes any sort of reliable assessment much more difficult. Pits in the steel plate are harder to spot and measure, rust tubercles and other specific forms of corrosion remain hidden and the ability to obtain a representative sample of ultrasonic thickness readings reduces. It would be hard to give a fair insurance assessment of any narrowboat hull presented in an unwashed and fouled state.

The methods used to clean hulls varies greatly from yard to yard. At its most basic it will be a quick rinse off with a domestic grade pressure washer as seen on many driveways across the country at the weekend. The typical water pressure of 110 bar is good for shifting mud and slime and most marine growths but is not very effective at deep cleaning. The next step us is a heavy duty pressure washer commonly seen in haulage fleet workshops which uses a diesel powered pump to produce 200-250 bar water pressure. These will happily shift all but the most tenacious marine growths nestling in deep crevices.

Better still is to use a hot water pressure washer which is very effective at cleaning off all fouling and has a moderate de-greasing action on the steel plate. These will typically run at 200 bar, are usually mains powered but diesel powered versions are available. The best option is an Ultra High Pressure water blast cleaner which is effective at removing all historic paint coatings, loose corrosion and corrosion by-products such as tubercles and flakes. These machines typically have a working pressure of up to 3500 bar after which the steel is left in a bare state which makes surveying highly effective and also presents a ‘clean canvas’ on which to execute remedial welding work and then to prime and re-paint the hull.

Those yards which employ UHP blasting machines tend to produce the best remedial work and subsequent paint application, normally using two-pack epoxy primers and paints. The results achieved, which will cost more in the short term, are a sound investment in the long term and should be good for 5-10 years service before next lift out.

UHP Blasted Iron Hull of a 1915 Humber Barge

Photo 2: Clean Plate of Riveted Iron Hull After UHP Water Blast Cleaning

The other side of the external hull corrosion problem is internal hull corrosion. Almost all narrowboats and barges are in essence, a wooden box built inside a steel box. Access to the void between the two is usually minimal to non-existent within the accommodation area unlike within the engine space, lazarette and possibly forepeak where the wooden ‘box’ is absent. The inability to access, clean and re-paint parts of the inner hull surface is a detraction in any vessel. Depending on the type and quantity of insulation that is fitted there will inevitably be condensation present at various times of the year and this will collect inside the hull. Usually in small enough quantities to not activate any bilge pump or alarm, where this collects on unpainted steel, rust is inevitable.

‘A steel hull will happily rust through from the inside as well as outside’.

Leaking shower sumps, poorly fitted through-hull fittings and windows all have the potential to contribute to this water build up in the bilges, typically just forward of the engine space bulkhead, by frames with blocked limber holes and beneath fixed tanks. This is where rust can be most active and the steel found to be at its thinnest. With one small floor hatch to look into, there is very little ability to clean and preserve the inner hull surface. As a result, the quality of inner hull surface preparation at fit out is a critical determinant of the longevity of any hull.

So to summarise:

  • Cleaning your bottom (inside and out) is critical to preserving the thickness of your hull.
  • Not all cleaning techniques are fully effective at achieving a good enough surface finish to re-paint.
  • Not all yards are willing to clean and paint the bottom plate unless you ask for (and often pay more) for it.
  • Cheap paint is what it is; don’t expect it to last for more than 2 years.
  • Keeping bilges clean and dry is a critical element of boat maintenance.

© David Pestridge 2020

[1] https://en.wikipedia.org/wiki/There_are_known_knowns