Advantages of a cleanable hull coating (part 2)

As explained in Part 1, the right hull coating combined with correct cleaning, is the optimum solution to maintaining maximum ship hull and fuel efficiency while safeguarding the marine environment from chemical pollution and the transfer of invasive, non-indigenous aquatic species. In Part 2 we will examine in detail how this is achieved.

First requirement – a cleanable coating

There are two basic properties that make a hull coating cleanable in the water:

1. It must be a hard, durable coating which is not adversely affected in any way by cleaning, either in the water using brushes or other methods, or in drydock using high pressure water jets. This means that the coating must not be removed, damaged, made rougher, or experience any loss of thickness as a result of the cleaning.
2. The coating must not emit any harmful substances into the water when it is cleaned. This includes heavy metals such as copper, zinc and others, organotins including TBT and DBT (dibutyltin, despite the ban on organotins in antifouling paint, often used as a catalyst in silicone-based foul-release coatings), biocides, PFAS, microplastics or any other harmful substance.

This immediately excludes from in-water cleaning all antifouling coatings, since it is impossible to clean them in the water without reducing the coating thickness and their effective life. If any hard fouling is present, it is impossible to clean it off without damaging the coating. These coatings are porous and the glue of the fouling organisms, barnacles in particular, penetrates the coating. When the fouling is removed, so is some of the coating.

The same is true of a foul-release coating, usually silicone. These coatings are notoriously fragile. If hard fouling attaches, it cannot be cleaned off without removing some of the coating, making it rougher and less effective.

In-water cleaning of these types of coatings will have a harmful effect on the marine environment. Depending on the coating, the cleaning will cause a pulse discharge of heavy metals, biocides, silicone oils, PFAS or microplastics or a combination of these.

Contamination from these coatings can be so severe that dead zones are created. The area becomes impossible to dredge without spreading the pollution further and increasing the damage.

The options for hull coatings that can be cleaned in the water without liability are relatively few. It requires a hard coating which will not suffer damage or loss of thickness with even aggressive cleaning, and which is chemically inert with no active ingredients which can leach out or be forced out with the cleaning operation.

One such coating is Ecospeed. It can be cleaned as aggressively and frequently as needed to keep the hull smooth and fuel efficient without any loss of thickness of the coating or the release of any noxious or harmful ingredients into the water.

When to clean

While hulls coated with Ecospeed can be allowed to become heavily fouled without risk of damage to the coating, they can be cleaned whenever needed.

If optimal fuel performance is the goal, the hull should be cleaned when it has accumulated no more fouling than slime and light weed. This degree of fouling already carries a fuel penalty of between 9 and 31% according to tests carried out by Michael Schultz at the US Naval Academy in Annapolis, Maryland, USA. The idea is to keep fouling to a minimum.

If a ship has been idle for a while and has therefore accumulated medium to heavy fouling and is now preparing to sail to a different environmental zone, it should be cleaned before it sails. This obtains optimum fuel efficiency for the voyage and eliminates the risk of transferring hull-borne invasive aquatic species. In many ports, the ship would be required to clean the hull before being allowed entry. Sailing with a clean hull takes care of this requirement in advance.

Slime and weed are not considered invasive species as they are found everywhere.

The frequency of cleaning required will vary with the type of ship and its requirements, the sailing pattern and the climate in which the ship is operating.

The two requirements of most ports for in-water cleaning are that the cleaning not contaminate the water column with toxic chemicals, heavy metals or other substances and that any macrofouling cleaned off has been acquired locally and therefore does not pose a threat of spreading invasive species.

We have been successful in gaining permission to clean microfouling off Ecospeed coated hulls in many ports without the need to collect the material that is cleaned off.

Note on reclaim systems

Much publicity has been circulated about reclaim systems that are alleged to contain and collect unrealistically high percentages of material cleaned off the ship’s hull. Unfortunately this is false optimism. To really collect all the effluent particles, chemicals and organisms that come off a fouled hull during the cleaning of an antifouling or foul release coating would require prohibitively large and expensive equipment and take a completely unviable amount of time to carry out. Due to the shape of the ship hull, the behavior of the water, chemicals and particles involved, the force required to clean a fouled hull and the effects this has on the surrounding water, the reclaim claims are not valid.

The answer is to use a completely non-toxic, hard hull coating such as Ecospeed, to clean before macrofouling has accumulated and/or to clean off macrofouling that has accumulated locally and has not been imported from another environmental zone. and to clean before sailing.

How to clean

Until an industrial level in-water cleaning system is developed, the most successful cleaning system we have found is the use of powerful, hydraulic, diver-operated cleaning units with rotating brushes. Different types of brushes are used for different levels of fouling. With Ecospeed on the hull, even stiff steel wire brushes can be used to remove heavy fouling without any harm to the coating. Lighter fouling can be successfully removed using softer brushes.

A competent diver using a Subsea Industries three-brush, all-wheel drive, hydraulic machine can clean as much as 2,000 square meters in an hour. On a large ship, two or more teams can be cleaning simultaneously, resulting in a full hull cleaning in an acceptable period of time.

Where this system was used correctly, the shipowner claimed fuel savings of 10% compared to previous use of antifouling coatings.

Conclusion

There is a workable system which allows ships to sail with clean hulls, greatly reducing fuel consumption, eliminating marine pollution from heavy metals and other toxic substances, and preventing the transfer of invasive aquatic species.

It involves the use of a hard, non-toxic coating such as Ecospeed and regular cleaning to keep the fouling to no more than slime and light weed.

This is the proven, effective approach.