Zinc Metallisation for Narrowboat Hulls – Is It Worth the Investment?

For most narrowboat owners, hull maintenance has traditionally meant a familiar cycle: lift the boat, pressure wash the hull, inspect it, repair any corrosion and apply another coating of blacking. A few years later, the process starts again.

There is, however, a considerably more comprehensive alternative: abrasive blasting followed by thermal-sprayed zinc metallisation and a two-pack epoxy coating.

Debdale Wharf Marina in Leicestershire has specialised in this approach for a number of years and currently offers a ten-year warranty on the adhesion of its zinc coating, excluding physical damage. The warranty is also transferable to a subsequent owner.

But zinc treatment represents a substantial initial expenditure. Is it really worth spending several thousand pounds protecting a narrowboat hull?

What exactly is zinc metallisation?

It is important to distinguish metallisation from zinc-rich paint.

After the hull has been grit blasted back to properly prepared steel, zinc is thermally sprayed onto the surface. The zinc forms a metallic coating that is electrically connected to the underlying steel.

This provides an important second line of defence.

A conventional paint coating primarily works as a barrier between the steel and its environment. Damage that barrier and corrosion can establish itself underneath.

Zinc provides sacrificial or cathodic protection. Zinc is more electrochemically active than steel and preferentially corrodes, helping protect exposed steel around local coating damage.

Thermal-sprayed zinc is certainly not peculiar to narrowboats. Metallised zinc, aluminium and their alloys are established methods for protecting steel structures, including bridges, marine vessels, offshore structures and other demanding applications. There are recognised industry standards covering their application.

Debdale then covers the zinc with a two-pack epoxy coating. The result is effectively a duplex protection system: metallic sacrificial protection underneath and a tough barrier coating above it.

Preparation is everything

There is an important qualification.

Metallisation is not magic.

Independent corrosion-industry guidance stresses that abrasive-blast preparation, cleanliness, surface profile, humidity, steel temperature and application technique are all crucial to successful thermal spraying.

In practical terms, therefore, the attraction of having the complete operation carried out in a controlled specialist facility should not be underestimated.

Blasting also has another valuable benefit for an older narrowboat: it exposes what you actually own.

Years of paint, blacking, corrosion products and previous repairs can conceal pitting, weld defects and local wastage. Once taken back to clean steel, repairs can be assessed before the protective system is applied.

That is particularly useful on older hulls that have already suffered substantial pitting or undergone overplating.

What does it cost?

Debdale prices the process according to hull area. Its currently published guidance suggests that, for an ordinary narrowboat, the vessel's length in feet provides a reasonable first approximation of hull area in square metres.

Current published prices are approximately:

• grit blasting — £39.90/m²

• zinc metal spraying — £48.30/m²

• two-pack epoxy finish — £23.10/m²

• transfer into and out of the blasting bay — approximately £315

Additional steel repairs are charged separately.

Using a 58 ft narrowboat as an illustration and assuming roughly 58m² of treatment area, those published figures produce a basic figure of around £6,770, before additional preparation, welding, anodes, bow-thruster treatment or other work.

The final measurement and quotation from the yard should always take precedence.

That is undeniably a substantial bill.

The correct comparison, however, is not £6,700 against the price of one tin of blacking. It is against perhaps ten or more years of hull maintenance.

Does it make financial sense?

Conventional blacking remains perfectly viable and is significantly cheaper initially.

Debdale itself currently lists conventional staffed hull blacking at around £22 per foot for sides and base, while other yards show broadly comparable costs depending upon coating and preparation.

A boat conventionally blacked every two or three years could therefore undergo three, four or possibly five dockings during a ten-year period.

Each visit involves more than paint. There can be lifting or docking, pressure washing, travel, fuel, accommodation, lost cruising time and frequently additional steelwork once corrosion is discovered.

Consequently, zinc metallisation probably should not be sold simply on the proposition that it is inevitably cheaper.

It may or may not be.

Its stronger economic argument is risk reduction.

If superior hull protection prevents significant pitting, welding or eventual overplating, the avoided repair cost could considerably outweigh the initial premium.

There may also be an effect on resale value. A buyer presented with documentary evidence of professional blasting, zinc metallisation and epoxy coating — particularly with a transferable warranty — may reasonably regard it as an asset.

As a surveyor, however, I would still inspect and ultrasonically measure the hull. No coating should be treated as a substitute for assessing the underlying structure.

Are there disadvantages?

There are several.

The first is obvious: initial cost.

For somebody intending to sell the boat within two or three years, recovering the whole expenditure through increased resale value cannot be guaranteed.

Secondly, metallisation depends upon excellent preparation and application. A poorly executed thermal-spray coating does not acquire extraordinary properties simply because it contains zinc.

Thirdly, mechanical damage remains possible. Narrowboats live a fairly hard life. Locks, submerged objects, piling and grounding can all damage coatings.

The advantage is that zinc can continue providing some sacrificial protection where a local scrape reaches steel, but damage should still be inspected and repaired.

It is also worth remembering that not every corrosion problem starts outside the boat. Internal corrosion from bilges, leaking plumbing or trapped moisture is completely unaffected by external zinc metallisation.

There is another useful note of caution. Long-term exposure research confirms the effectiveness of thermal-sprayed metals but also demonstrates that performance varies according to metal composition and exposure conditions. Zinc should therefore be regarded as a very good engineered corrosion-control system rather than an indestructible coating.

Getting your narrowboat to Debdale

There is one final consideration that tends to be overlooked: geography.

Debdale Wharf is at Kibworth in Leicestershire, on the Leicester Line of the Grand Union Canal, immediately north of the Foxton area. From Debdale it is only around 20 minutes' cruising south to the bottom of the famous Foxton Locks.

For owners already cruising the Midlands network this makes Debdale reasonably accessible, but somebody based several counties away needs to factor the delivery voyage into the project.

That can mean planning around locks, seasonal stoppages and available crew and allowing sufficient time to arrive before the booked date.

Debdale advises that the specialist process normally occupies approximately one week, and I would imagine works typically from Monday until the following Monday. Staying aboard while the work is undertaken is not possible. Therefore, costs are incurred staying locally.

For a distant boat, therefore, there are effectively three choices: cruise it there yourself and incorporate the journey into your boating season, employ somebody to deliver it, or investigate road transport where economically practical.

For many owners, turning the journey into a cruise is likely to make most sense.

The surveyor's view

For a sound steel narrowboat whose owner expects to retain it for another decade, I consider properly executed grit blasting, zinc metallisation and two-pack epoxy one of the strongest external hull-protection options presently available.

It costs considerably more at the outset than conventional blacking, and it doesn't eliminate the need to inspect the hull.

Nor should claims that a treated boat will "never require welding or overplating" be taken absolutely literally. Corrosion can arise internally and structural defects or physical damage can always occur.

What the system can do is fundamentally change the maintenance philosophy from repeatedly covering corrosion to properly preparing the steel, repairing what is exposed and providing both sacrificial and barrier protection afterwards.

On an older narrowboat worth perhaps £50,000–£100,000, spending £6,000–£8,000 protecting the single structural component upon which virtually the entire value of the boat depends can make considerably more sense than it first appears.

"Sometimes the cheapest coating is the one with the lowest invoice" (Anon).

The better question is:

Which system gives the hull the lowest risk and lowest total cost over the next ten or fifteen years?

For owners intending to keep their narrowboat, that is where zinc metallisation begins to make a particularly compelling argument.

Tom Crosby, Midlands Boat Surveys

This article reflects my professional opinion as a marine surveyor, based on my experience of steel narrowboat hulls, corrosion mechanisms and the information presently available regarding zinc metallisation systems. It should not be taken as an endorsement of any particular yard, contractor or proprietary process, nor as a guarantee of future hull condition. The effectiveness of any corrosion-protection system depends heavily upon the condition of the underlying steel, the quality of surface preparation, application standards, coating thickness, environmental conditions and subsequent use of the vessel. Individual boats should therefore be assessed on their own merits, with appropriate inspection and ultrasonic thickness measurement where necessary, and owners should obtain current written specifications, warranties and quotations directly from the treatment provider before committing to the work