Boat Hull Experience

The Pretty Penny is a 10m yacht built in 1979 made of 3mm 90-10 copper-nickel plate and moored for the greater part of its life in the Thames Estuary on the east coast of the UK. The hull remained clean for the first 3 years, but did exhibit some fouling from mainly grasses and some barnacles in later years, particularly 150-300mm below the water line. This fouling was easily removed manually by a light scraping action while still in the water once or twice a year.

The Italian Ministry of the Interior commissioned 4 fire boats with 90-10 copper-nickel roll-bonded clad steel for the submerged parts of the hulls in 1983. The operators of the fire boats were contacted in 1992 by the Nickel Development Institute to find out their performance. No signs of corrosion were evident on any of the boats. Two, however, had shown some signs of fouling having spent a greater part of their lives in closed, stagnant moorings. A third was moored in polluted water showing no signs of biological activity. The fourth boat moored in freely flowing water showed excellent biofouling and corrosion resistance.

The biofouling properties have been confirmed by evaluating more recent boat hull experience. The benefits of copper-nickel alloys in maintaining a smooth hull surface have been obtained by a number of boat building methods .

A pilot boat was constructed from clad plate for the Board of Navigation, Finland in 1987. It was intended that the properties of 90-10 copper-nickel should be compared with austenitic stainless steel for use in the Baltic where ice is anticipated. Information in 1994 from Hanko Pilot Station where the pilot boat was located indicated that the vessel was giving good service and the copper-nickel had not been damaged by the ice. The hull had stayed free from fouling with the propeller and the steel front keel being the only places where some mussels had been observed. The welded seams of the copper-nickel sheets were in good condition too.

The Cupro, a small experimental ship with a 90-10 copper-nickel clad steel hull operated in open waters near docks in Japan; operation time was very low to encourage fouling. Over two years during the evaluation trial, the vessel performed well with minimal corrosion. A very low level of barnacle encrustation was found which was not enough to interfere with the operation of the ship and could be easily removed by hand.

To assess the viability of welded sheathing, sea trials were carried out initially by sheathing the complete rudder of a 24 knot roll-on/roll-off vessel called the Great Land operating between Washington and Alaska with 90-10 copper-nickel. Although the rudder was subject to turbulent flow and exposed to conditions where ice and silt were present in the seawater, the copper-nickel was found to be very durable.

A second hull panel trial was on a 16 knot crude oil tanker, the Arco Texasassessing attachment methods and evaluating service performance. Twelve large 90-10 copper-nickel panels were divided into sets of three such that exposure covered fully submerged, alternate wet/dry and splash zone conditions. After two years and seven trips through the Panama Canal, the panels were still intact even though they had experienced several forceful impacts on the sides of the water-way and several had severe scratches. The maximum corrosion rate was measured at 0.013mm/yr and no evidence of fouling was found on the copper-nickel even though it was present on the rest of the steel hull. At the end of the two years, the conventionally coated steel hull had a roughness of 250µm, whereas the corresponding roughness of the copper-nickel was only 53µm. In comparison, the roughness of the copper-nickel rudder after 14 months on the Great Land was consistently lower than 20m, compared with the painted steel hull which averaged 210µm.

The sheathing of a ship's hull with 90-10 copper-nickel foil has involved the application of adhesive-backed panels (approximately 210mm x 500mm) to the prepared hull, allowing about 15mm overlap. The copper-nickel foil thickness chosen is about 0.15mm thick. The panels are easily cut and manipulated even over the most difficult of contours.

The bonding system acts as an insulator, and as a barrier to seawater which further protects the hull from the detrimental actions of seawater. An advantage of the system has been that if impact occurs and some panels are damaged, it takes only a short time to repair the sheathing. The system can be applied to hulls on new vessels and as a retrofit.

An evaluation programme on the performance of the foil sheathing commenced in August1993 with two commercial passenger ferries, the MV Koru and the MV Osprey; both of which are in-service around Auckland Harbour, New Zealand. One vessel is a slow ferry (10 knots), constructed of fibreglass reinforced polymer, which was retro-fitted with copper-nickel sheathing in 1993. The other vessel is a fast catamaran ferry (22 knots) with a FRP hull, which was sheathed during construction in 1994. The older monohull vessel, MV Koru, was kept in reserve most of the time, whereas the catamaran, MV Osprey, has been in service for about 30,000 nautical miles since construction.

In addition, test programmes involving trials on immersed test panels, commenced over the same time (1993-1999) in Auckland Harbour, Singapore, and Langstone Harbour, UK. The overall trials have confirmed earlier observations about the biofouling properties of copper-nickel but added to the overall experience.

The biofouling resistance is in line with documented accounts such that slime (microfouling) does occur on copper-nickel but colonisiation of macrofoulers is restricted. If colonisation does eventually occur, it can readily be removed by a wipe or finger pressure, such that a light waterblast will quickly remove any growth. The turnaround time for cleaning the MV Koru on the slip by this method is about 1.5 hours. Removal of fouling from the equivalent painted vessels in the fleet can take up to one day of unproductive time per vessel.

The experience on MV Koru and the MV Osprey showed that green algae (slime) formed predominantly on the copper-nickel foil at, or just below the waterline on both vessels. In addition more algae were observed on one side of the MV Koru hull which was facing the sun during out-of-service time. Clearly, sunlight affects the rate of growth of the green fouling (photosynthesis), but the higher temperature of the surface seawater on sunny days may also be a factor.

The green algae were easily removed using rotary brushing underwater, but the growth became firmly attached and more difficult to remove if it dried when the vessel was on the slip-way. It was also observed that small, lightly attached barnacles, grew adjacent to the waterline on the foil when the MV Koru was left moored and unused in the harbour for longer periods of time.

Seawater velocity also had a substantial effect on the degree of fouling resistance of the copper-nickel foil. Areas of the MV Koru and MV Osprey hulls were almost entirely free of biofouling where the velocity of seawater experienced by the alloy exceeded some undetermined speed. Typically, the stern and waterline tended to show earlier signs of fouling than other hull areas.

The effect of water velocity can possibly be related to the sloughing of microfouling from the hull surface. Macrofouling on the hulls of both vessels only resulted after microfouling had been well established on the foil during quiet periods.

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