Protective Treatment for Boats with Steel Hulls

Due to the lockdown there has been a backlog in getting boats out of the water to assess the condition of their hulls and renew their coating (often referred as Blacking). This procedure is best done every 3 to 4 years to prevent any deterioration of the hull. If rust is left unattended this can lead to long term damage to the hull. See photos.

Pitted surface, 4 years after blacking with poor surface preparation
Pitted bottom plate

The solution is to get the boat out of the water and dry, either by dry docking it, getting it on a slipway or craning it out, to look at all the submerged parts and deal with the maintenance of the hull. It is best practice to remove any traces of corrosion and then add a new protective coat of Bitumen paint as tar based paints are now forbidden.

Good preparation is essential and all traces of rust should be treated. If rust is left untreated, corrosion will continue creeping under the new paint and the results can be pretty dramatic. Holes can even appear (see photo) so it is very important that the best preparation is given to ensure a good job.

Pitted Hole
Repair on Pitted Hole

The best mechanical preparation is shot blasting but it is often not practical. It must be carried out in clean, dry conditions by skilled operatives and is an expensive operation that is only necessary on rare occasions, such as when an epoxy paint treatment is required. Epoxy is the best coating to fully seal a boat and completely prevent the hull from corroding from the outside but it is a very expensive process in the first place. However, the investment cost will be recovered over the years by the lower cost of subsequent blackings which will only consist of a lightweight pressure washing and a standard Blacking.

Epoxy and Bitumen Treatment after 6 Years

Usually, for the purpose of a standard Blacking, grinding of the hull is done instead of shot blasting. This is a dirty and dangerous job as handling powerful tools that are rotating at high speed and sometimes overhead is tiring and can lead to accidents.

Another method is to use very high pressure washing. There is a great deal of difference between using a “high” pressure washer (400 Bar) as opposed to a “standard” pressure washer (150-200 Bar). It is highly recommended to use the highest possible power to strip off flaking paint and patches of corrosion still adhering to the surface of the steel. This powerful preparation method will highlight any potential hull weaknesses and it sometimes bursts open holes where the hull has become paper thin. Without that powerful blast, those holes would appear only months after the blacking operation when the boat is back in the water.

Deep holes can often be filled with weld if the overall state of the hull is decent but if a hull is getting thin altogether (below 4mm), then the option of replating the boat may have to be considered. This is a very expensive process which is used as a last resort and it is very important to avoid coming to that stage so we strongly recommend that you take care of your hull when it comes to the blacking and that you use the best methods and materials and do so at regular intervals. Extra care should be given around the water line as this area is where the worst of the corrosion happens.

Following grinding or high pressure washing and carrying out any necessary repairs, any remaining surface corrosion can then treated using a decent rust converter, ultimately followed by the application of a Bitumen based paint. There are a number of proprietary products on the market and the cheapest may not always be the best in the long run. It is important to “read the tin” and follow the manufacturer’s instructions with regard to overcoating and immersion times.

One thing that is often an issue when dealing with blacking is the fact that the blocks (or bearers) on which the boat rests while being worked on means that some areas are often left untouched as they are impossible to reach. This leads to areas where the old paint is left on and any old corrosion is not treated. Very few places allow for the hulls of boats to be “fully” treated during their maintenance period (where 100% of the hull is blacked). If a boat is going regularly to the same place for hull maintenance, it is recommended that every effort is made so that it sits in a slightly different position on its bearers every time it gets supported to allow treating of those parts that were hidden and untouched at their last blacking.

If an independent contractor undertakes this process for you, then make sure that the preparation methods are adequate to ensure that any weaknesses are revealed and treated properly at the time of the Blacking. This is of upmost important for preventing any premature degradation of the hull. Given the time and expense involved in the process, it is fair to say that doing half a job is never a good idea in the log run

All boats should be fitted with sacrificial anodes which protect the steelwork from Galvanic corrosion. Whenever 2 dissimilar metals in contact are put into water, one will start to corrode and protect the other. When the anode has corroded completely, the previously protected metal of the hull itself is subject to all the corrosion and it degrades rapidly. The material used for the anodes depends on whether the boat is used in salty, fresh or brackish (mixture of fresh and salty water such as an estuary) water and is chosen so that it corrodes and protects the steel of your hull. When the boat is out of the water the existing anodes should be checked and replaced if they have corroded excessively.

A common rule is to provide 4 anodes per vessel but very often, extra anodes are necessary to protect longer boats. Average size anodes only have an effective protection of a few square meters and having too much distance between them lessens the protection at the furthest location from them (at the centre of a ship if the anodes are on either end). Boats over 50ft often have an extra pair of anodes amidship. For this particular item, the more, the better.

There are other areas that also require maintenance but we will look at those in future articles.