A healthy hull keeps the bugs at bay

Protection of a ship’s underwater parts is now a big and important business

By Michael Grey

In the effort to reduce the negative impact ships have on the environment, quite a lot of the battle goes on under the waterline, through the prevention of fouling and any sort of marine growth. Nothing new about this, of course – the old sailing ships’ sailors used to drag coir mats under their hulls to scrub off the weed and barnacles, and even light fires under their ships, hauled over in dock, to burn off the encrustations, in the days before anti-fouling paint.

The need to protect the environment adds a new dimension to this battle against barnacles and all the other forms of marine life that welcome the chance to adhere to a hull. In the past it was always about speed; today, there is a need to reduce fuel consumption and thus emissions, but also to prevent the transmission of alien species from one part of the world to another.

For some forty years or so we have known about the need to do something about the living organisms in ballast water, but more recently it has become known that equally invasive species can be carried in sea chests, rudder trunks and various inlets under the surface, where they can happily live during a sea passage. And some countries which are inordinately proud of their pristine port and coastal waters, such as

Australia and New Zealand, are demanding that ships arriving on their coasts are certified for ‘bio-security’ as it applies to their underwater parts. They will carry out spot checks on arriving ships and if something nasty is found under the waterline, they will be told to go away and get the ship cleaned. That could be a very expensive business.

Dual purpose

So, hull health – a term which was recently coined by Gareth Prowse of Svitzer Hull Performance Services – is very important for both the immediate marine environment and the move to decarbonise shipping. It has become big business too, from the development of high-performance anti- fouling coatings, mechanical and electrical devices that protect a hull, to a whole network of underwater service providers that can employ small submersibles to keep the hull free from growth. From divers with brushes, the underwater toolkit now employs very sophisticated magnetic cleaning de- vices that will remotely crawl around the submerged hull, controlled by an operator who stays in the dry. And because many places don’t want the scrubbed marine growth floating in their waters, these underwater cleaners retrieve everything they scrub for safe disposal. It has come a long way since sailors dragged mats under their wooden hulls.

The modern technology, in the shape of long-life underwater coatings that prevent fouling and smooth the hull to minimise resistance also have an added bonus in that they can save the ship having to be drydocked, which is both expensive and time consuming. The more that can be done with the ship afloat, the longer the intervals between drydocking can be. Sixty years ago, it was not unusual for a cargo liner to be drydocked once a year, mainly for a ‘scrub’ and to repaint the antifouling. Today, five- year intervals are not unusual, with the best coatings contributing to this extension.

Of course, the amount of fouling does depend upon where the ship is trading, how it has been operating and whether it has been in layup, anchored or in port for extended periods. Even a few days inoperative in tropical waters can encourage marine growth, which sounds like good business for underwater service companies. A long wait for a berth – such as those off Chinese ports this year – will make a big difference to the ship’s performance, once they get under way again.

There is no doubt that a clean hull really does produce immediate performance improvements. Even keeping the propeller clean, it has been said, will improve efficiency by several percentage points. Healthy hulls matter.