Narrowboat hull protection ranges from traditional tar through the latest 2 component epoxies and beyond into the highly technical world of thermally sprayed metal alloy coatings.
Hull Protection… An Introduction
A fun subject and a fun task, narrowboat hull protection is necessary if we wish to protect our investment long term.
The reasons behind the need to protect the narrow boat hull are simple, but the corrosion mechanisms themselves are complex. Aside from galvanic corrosion, we talk a lot about the harsh day to day life of the narrow boat hull. For some of the year it sits immobile in often dirty or oily water. For the rest of the year it is bashed and battered about in locks and against mooring pilings.
Therefore, coatings are applied to prevent corrosion of the steel and there are two basic application methods:
1) Hot sprayed metal coatings have been used in the offshore marine industry for many years and now this technology is available to the inland waterways boat owner.
2) Conventional brush or roller cold applied systems that are considered integral with anode attachment to the overall protection system.
If you’ve read our Anodes section you will be aware that anode manufacturers advise on a 3 year scheme of protection. For most owners that coincides nicely with the re-coating of the hull, which also seems to need doing at around this interval.
Thermally Sprayed Metal Coating
This established technology has been around for over a century. These coating solutions can be used to provide wear resistance, electrical conductivity and thermal protection but it is their proven track record in providing a corrosion resistant barrier that makes the technology particularly suitable for the marine environment.
These systems are extensively used in the offshore industry to protect oil platforms, steel superstructures and both internal and external surfaces of shipping vessels.
Moving ahead to the 21st Century and with the increase in pleasure boating on our inland waterways; it is an obvious technology to apply for prolonging the life of our pride and joy.
It makes perfect sense when you consider that this process is simply a proven technology transfer of anti-corrosion coatings. We already accept the use of zinc anodes in the battle against galvanic corrosion, so why not coat the whole hull in a system that carries long term guarantees and offers real benefits that outweigh the cost of the process.
Rather than a cold paint or roller applied system, thermal spray technology uses a low power arc occurring between two electrically charged wires to atomise the wire and turn it into a spray stream. The wire is known as the feedstock and, in the case of inland fresh water systems, is made from zinc.
In effect, the process is very similar to that of arc welding where the welder briefly touches the electrode or welding rod against the work piece and then withdraws it to form a stable arc. In the case of thermal spraying, the arc is between the wire feedstock rather than the welding rod and work piece.
As a result of the non-contact arc this type of sprayed metal coating is very thermally efficient and little heat is transferred to the part being coated. This is of particular importance for coatings below the waterline where it is impossible to remove insulation and panelling from a fitted boat.
So, if you have ordered a new sailaway shell to DIY fit, you build shells commercially and want to add value for your customers, or you have an existing narrow boat that you want to give the ultimate hull protection to, this process is available to all owners and canal boat shell builders alike.
Traditional Hull Blacking
Traditional, standard blacking products widely available from most marine chandlers and on-line. Not so resistant to oil and grease but easy to apply, quick drying and with an attractive even finish.
These coatings are cost effective and with care and maintenance will last until the next blacking session. “Bodied” formulations lay on thicker than standard bitumen and are less cost effective per brush stroke.
Bitumen can be applied over well weathered tar based products. Typically lasts 2-4 years before re-application is necessary.
- Coal Tar
Better resistance to oil and diesel than standard bitumen. Fast drying and glossy. Easy to apply due to low viscosity. Cannot be applied over bitumen based coatings. Typically lasts 2-4 years before re-application is necessary.
- Vinyl Tar
Can give a higher gloss than bitumen based coatings but can be a lot harder to apply.
They are not affected by oils or diesel. Can be applied over any coal tar based product. Cannot be applied over bitumen based coatings. Typically lasts 2-4 years before re-application is necessary.
- 2 Pack Epoxy
A 2 part system which when fully cured is water impermeable and fully chemical resistant. Said to be abrasion and impact resistant in use. Gives a smooth, attractive finish. Cannot be applied over bitumen, coal tar or vinyl tar based products and as a consequence the hull should be blasted before application if it has previously been coated with an incompatible system. Typically lasts 5-8 years before re-application is necessary.
So How Do You Choose?
By price? The phrase “don’t spoil the ship (sheep) for a ha’p’orth of tar” could not be more appropriate.
The more technical the system, the more it is likely to cost. The flip side is that you will get more protection. For example, thermal spraying of zinc wire is guaranteed for 10 years but in practise is expected to last much, much longer than the 3 years typical life of a brush or roller applied coating.
If you decide for the conventional roller or brush applied coating from a tin firstly you have to find out what the hull has already been coated with.
If there are potential compatibility issues and you don’t want to have the hull sandblasted, then stick with what you’ve got.
There is nothing wrong with the standard bitumen based coatings so long as they are applied correctly and there is not too much heavy diesel in the water around where you moor (there shouldn’t be anyway but that’s for another discussion)!
Budget for a schedule of blacking every 3 years or so and accept the work involved.
If your option is for thermal spraying you have already accepted the extra cost of the process is vastly outweighed by the benefits of having it done once and the guarantee the process carries.
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