Copper-Nickel Boat Hulls: Performance and Corrosion

by Carol Powell; Consultant to CDA; 2002

Foreword

Cu-Ni possesses the ideal requirements for a boat hull. With both good resistance to corrosion and macrofouling, coatings are unnecessary, providing both savings in fuel and hull maintenance time and cost. Over the last 30 years, experience has been gained in constructing hulls using different product forms of Cu-Ni.

• Construction of the hull from Cu-Ni alloy plate onto steel or Cu-Ni frames
• Construction of the hull from Cu-Ni roll-bonded onto steel plate
• Cladding a fibreglass, wood or steel hull with Cu-Ni alloy sheet or foil

With tight restrictions on the use of organo-tin copolymer coatings and the quest for alternative antifoulants underway, Cu-Ni as a boat hull material can offer many practical benefits.

Contents

• Introduction.
• The Performance of Copper-Nickel Hulls
  • Solid Cu-Ni Hulls
  • Cu-Ni Clad Steel Hulls
  • Welded Cu-Ni Sheathing
  • Adhesive Backed Foil
• Corrosion Issues
  • Galvanic Corrosion
  • Velocity Effects
• Conclusions
• References

• Table 1 Vessels constructed with Solid Cu-Ni Hulls
• Table 2 Vessels constructed in Steel plate with Cu-Ni cladding
• Table 3 Results of Italian Fireboat Questionnaire 1992

INTRODUCTION

Coatings are applied to hulls for two reasons; to protect them from the corrosive effects of sea water and to minimise biofouling. Both corrosion and fouling can increase the frictional drag on the hull resulting in lower speeds or the need for more power and a higher fuel consumption to maintain operating speeds. Corrosion resisting paints and cathodic protection are required to avoid corrosion, whereas biofouling resistance requires antifouling paints. These methods are not permanent and require replacement which can vary from 30months to 5 years. Drydocking is required to renew painted surfaces of larger vessels.

In the 1970's, much attention was focussed on a shrimp trawler operating off the shores of Nicaragua called the Copper Mariner. The hull was very novel as it was made of 90-10 welded Cu-Ni plate and detailed assessments were made over a 4 year timespan. No coatings below the water line or cathodic protection were applied. The results were good; there was negligible corrosion and biofouling. Fuel savings were impressive and the concept of a hull with enough inherent corrosion and biofouling resistance to last a vessels lifetime became viable.

However, the 1970's also saw widespread use of a very effective coating system involving organo-tin copolymers which could reliably protect a vessel from biofouling for in excess of 5 years and initial costings were more competitive than Cu-Ni.

Today, environmental concerns about the effects of organotin copolymer systems have restricted and will soon prevent their use and other paint systems are being sought to provide similar efficiency. With up to 30 years of service experience of similar vessels to the Copper Mariner and other trials, a good understanding is available about the performance of Cu-Ni in this application. Indeed, different methods of attachment have been developed to add to its versatility.

THE PERFORMANCE OF COPPER-NICKEL HULLS

Solid Cu-Ni Hulls

Since 1968, the following boats have been recorded by the copper industry as having solid Cu-Ni hulls.

Of these information about the performance of Asperida II, Copper Mariner and Pretty Penny have been collected at times over the intervening years.

The Asperida II was the first Cu-Ni hulled vessel of modern times and was made of 70-30 Cu-Ni plate attached to a 70-30 Cu-Ni frame. It was built for a Professor at the University of Alabama after he had made an extensive study of a wide variety of possible hull materials. The 70-30 rather than the 90-10 alloy was stronger and better suited to his yacht design. Its strength and ductility were proved in early years when it with stood a night collision with a rock but did not fracture or hole the hull. When located 20 years on, the yacht was found to be still very much operational and moored in New York Harbour. The hull was found to be in excellent condition and had never required to be scraped to remove fouling ¹ .

As of 1997, the Asperida II had become the flag ship of the Earth Society Foundation which provided an environmental cadet programme around the Caribbean. The vessel was still giving excellent performance and fouling was negligible. If fouling did start to accumulate a light scrape would quickly remedy it.

The Asperida II led to a series of engineering studies by INCRA and INCO and the construction of the Copper Mariner in 1971. This was a commercial shrimp trawler with a 6mm thick 90-10 Cu-Ni hull over a steel frame. Standard paint coatings were successfully used to minimise galvanic corrosion of the steel framing. For 52 months the condition of the hull was closely monitored. The corrosion rate of the hull was found to be less than 1.3 m/yr reported for long term exposure. The hull plate showed no significant difference in metal loss between the heat affected zone areas adjacent to the welds where re-solution of any precipitated iron might have taken place and the bulk hot rolled plate containing precipitated iron. The hull of the Copper Mariner was found to be very resistant to fouling whereas in the first 4 years her conventionally coated, steel hulled sister ship had to be beached 6 times to remove fouling. Any fouling which accumulated from extended mooring became dislodged and removed at the 3-8 knot operating speeds to and from the fishing ground indicating a self cleaning property. After 20 years, the vessel was still said to be in service and the last word from the Ministry of Fisheries in Nicaragua was that the hull itself had still never required any maintenance. Inspections at dry dockings for propeller and shafting maintenance have shown no noteworthy signs of pitting and general corrosion.

The average fuel savings were up to 24.8%, calculated from a comparative measurement study take over 4 years from its steel hulled sister ship. The pay-back time was estimated at less than 7 years.

Details of design, economic analysis, fabrication, corrosion and biofouling performance after 4 years service are comprehensively recorded in Reference ² .

The Pretty Penny is a 10m yacht built in 1979 using 3mm thick 90-10 Cu-Ni 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, 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.

In October 1995, the yacht was fully removed from the water for the first time since her launch 16 years earlier and inspected. Prior to inspection the owner cleaned the hull for the first time that year removing a loosely attached grass layer with a piece of wood in a few minutes. Hardly any barnacles were noted. The boat had sailed very little during the previous 12 months. The surveyor concluded that the craft could be considered in almost new condition with the hull plating in the same condition as when it was originally built.

However, some thinning had occurred at the hull plate at the water line to a maximum depth of 0.3mm, apart from at the bow where a small hole had appeared. The cause for this was never identified as it was repaired before any assessment was made. This has not been identified in studies of other vessels and excessive grinding during fabrication cannot be ruled out. However, thicker section in this region on future vessels could be considered as a precautionary measure.

Boat designer and author, Bruce Roberts-Goodson, examined the Pretty Penny at this time and was so impressed he addressed building, maintenance and repair aspects of Cu-Ni along with steel and aluminium hulls in his book Metal Boats, Reference ³ .

Cu-Ni Clad Vessels

Cu-Ni can be metallurgically bonded onto steel to provide a corrosion and biofouling resistant outer surface on a cheaper, stronger one. The aim being to economically produce thicker plate sections but at a reduced cost compared with a solid Cu-Ni plate at the same thickness. The bimetallic plate can be formed by hot rolling steel and Cu-Ni together or by explosive bonding. Hulls of several vessels have been made using this techniques as shown in Table 2.

Copper Mariner II was the first vessel to have a hull constructed from Cu-Ni clad steel. 2mm of 90-10 Cu-Ni was roll bonded to a 6mm thickness of steel. The framing was in steel. The hull was found to be readily fabricated and welded and performance as of 1988 was reported in Copper Topics, Issue 63, to be good. Its initial success prompted the Italian Ministry of the Interior to commission 4 fire boats with Cu-Ni cladding for the submerged parts of the hulls in 1983. Fabrication and design details are provided in reference ⁴ .

The operators of the Italian 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, Table 3.

The Cupro was a small experimental ship operating in open waters near docks in Japan. The operation time was low to encourage fouling. A report on its performance after 2 years, Reference ⁵ , showed the vessel to have performed well with minimal corrosion. A 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.

Welded Cu-Ni Sheathing

Much test work has looked at developing a welded sheathing for steel hulls appropriate to large vessels. Detailed studies have examined costings and fabrication techniques and found the method to be technically and economically viable. Clearly weld integrity is critical as poor weld penetration can lead to detachment and galvanic corrosion of the underlying steel.

To assess viability, 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. Although the rudder was subject to turbulent flow and exposed to conditions where ice and silt were present in the sea water, the Cu-Ni was found to be very durable, Reference ⁶ .

A second hull panel trial was a 16 knot crude oil tanker, Arco Texas, assessing attachment methods and evaluating service performance. Twelve large Cu-Ni panes were divided into sets of three such that exposure covered fully submerged, alternate wet/dry and splash zone conditions.

After 2 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 scratches. The maximum corrosion rate was measured at 0.013mm/yr and no evidence of fouling was found on the Cu-Ni although it was found on the rest of the steel hull.

At the end of two years, the conventionally coated steel hull had a roughness of 250mm whereas the corresponding roughness of the Cu-Ni was only 53mm. Based on the assumption that a 10mm increase in hull roughness leads to a 1% increase in power penalty or corresponding increase in fuel consumption, it has been estimated that there would have been an improvement in efficiency of 19.7% had the whole hull been sheathed.

In comparison, the roughness of the Cu-Ni rudder after 14 months on the Great Land was consistently lower than 20mm compared to the painted hull which averaged 210mm.

The main concern expressed about this method is linked with the possibility of rapid corrosion of the steel hull if the sheathing is damaged and water leaks under the sheathing. The fear is this could lead to severe galvanic corrosion between the Cu-Ni and steel. However, the above trials indicate the toughness of the sheathing and resilience to tearing. Even so, it is now thought that if there is a tear, corrosion of the steel initially occurring will become stifled as the oxygen is used up in the restricted space between the Cu-Ni and steel, thus allowing ample time for repair.

Although technically viable, any welded system would have to be economically viable too. Detailed economic studies accompanied the technical evaluations. A particularly thorough assessment was undertaken in 1987 by Newport News Shipbuilding Co for CDA Inc under a contract from the US Maritime Administration The ship chosen was a fleet oiler and the results of the discounted cash flow economics were assessed for sheathing during new construction with a lifecycle of 20 years and also in a retrofit situation after 10 years service. Comparison made with a conventional copper bearing antifouling paint with primer and cathodic protection systems and an organo-tin co-polymer antifoulant. Fuel cost savings were particularly significant with savings of 27% being realised for the Cu-Ni compared to a conventional painted hull. The Arco Texas trials and 1986 cost comparisons are described in Reference ⁷ .

The next stage in the development of the welded sheathing concept would involve sheathing an entire hull and this is a challenge still to be carried out in practice.

Adhesive Backed Foil

A system of sheathing of a ship's hull with 90-10 Cu-Ni foil was developed by F.C.Mitchell and involved the application of adhesive-backed panels (approximately 210mm x 500mm) to prepared hulls, allowing about 15mm overlap. The Cu-Ni foil thickness is about 0.15mm and 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 corrosive action of seawater. The system has been applied to new hulls and as a retrofit.

An evaluation programme on the performance of the foil sheathing commenced in August 1993 with two commercial passenger ferries, the MV Koru and the MV Osprey; both of which are in-service around Auckland Harbour, New Zealand. MV Koru is a slow ferry (10 knots), constructed of fibreglass reinforced polymer, which was retro-fitted with Cu-Ni 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 40,000 nautical miles since construction.

Hull performance has been in line with documented experience on other vessels. The corrosion resistance of the Cu-Ni appears good. Initial teething problems involved galvanic corrosion of less noble fasteners, de-zincification of the manganese bronze propeller and unfavourable areas of a 70-30 keel shoe in contact with the 90-10 small panels. These were soon addressed and no further problems have ensued. Slime (microfouling) forms on the Cu-Ni but macrofouling is restricted. If colonisation does eventually occur, it can readily be removed, such that a light water blast will quickly remove any growth. The turnaround time for cleaning the MV Koru on the slip by this method is about 1.5 hours whereas removal of fouling from the equivalent painted vessels in the fleet can take up to one day.

Flow velocity and sunlight appear to influence biofouling behaviour. More algae formed on the Cu-Ni foil just below the water line on both vessels and was more prevalent on the side of the MV Koru hull facing the sun during out of service time. Typically, the stern and waterline tend to show earlier signs of fouling than other hull areas. Areas between the hulls of the MV Osprey which experience higher velocities remain almost entirely free.

The experience is described in more detail in References ⁸ and ⁹ (Reference 9 is available in full by clicking on the reference in the reference section of this paper.)

More recently, a sheathing product based on a very thin layer of 5mm square Cu-Ni interfacing tiles aligned together by a tough, high bond, insulating waterproof acrylic adhesive membrane has been developed. The surface to be exposed to the water has a protective cover that is only removed when the hull is completely sheathed. The matrix requires only a roller action at moderate temperatures to apply and can be used for new build or retrofit. A derivation of the product can also be integrated into the moulding of new hulls, Reference ¹⁰ .

CORROSION ISSUES

Galvanic Corrosion

As Cu-Ni alloys lie midway in the galvanic series being more noble than zinc, aluminium and steel and less noble than stainless steels, nickel alloys and titanium, due consideration needs to be given to galvanic effects in service to avoid unwanted corrosion of hull fitments and penetrations. Normally it is advisable that hull penetrations are of Cu-Ni to the first valve. Also, a more noble nickel aluminium bronze propeller was found to give better performance than manganese bronze on the Copper Mariner. This was because the less noble manganese bronze did not receive cathodic protection from the Cu-Ni as would be customary from a steel hull.

To obtain optimum biofouling resistance, the Cu-Ni needs to be freely exposed. Any galvanic coupling to less noble alloys such as steel or cathodic protection are known to restrict this property.

With a solid Cu-Ni hull, the inner surfaces as well as the outer hull have to be considered. Where the inner surfaces are likely to be wetted and in contact with less noble materials such as steel or aluminium e.g. in the bilges, the possibility of galvanic corrosion must be anticipated. In such instances, coatings should be applied. Also for situations where inspection is difficult and sufficient water allows satisfactory operation, anodes can be installed.

Velocity Effects

Erosion of Cu-Ni occurs when the sea water velocity exceeds the breakaway velocity such that the shear stress exerted on the surface film is sufficient to damage it. The breakaway velocity for condenser systems and pipework is well understood but the hydrodynamic conditions for a ship hull are entirely different and so the same velocity restrictions do not apply.

Fourteen month trials of the sheathed rudder on the ocean going vessel, the Great Land, indicated that even at 24knots (12m/s) the breakaway velocity is not exceeded. The highest recorded velocity is 38knots(19m/s) over 200hrs for a petrol boat showing no measurable thickness loss. The speed at which a Cu-Ni hull can comfortably achieve is still to be determined.

CONCLUSIONS

The experience gained over the past 30 years leads to the following conclusions:-

• The combination of seawater corrosion resistance and a high inherent level of biofouling resistance make Cu-Ni and ideal material for boat hull applications.
• Ease of fabrication allows the possibility of solid Cu-Ni or Cu-Ni clad steel hulls, welded sheathing or attachment by adhesive backed foil.
• No coatings and associated maintenance are required.
• No cathodic protection is required and its absence is necessary to maximise resistance to biofouling.
• Galvanic considerations need to be observed for hull attachments and fitments.
• Macrofouling is minimised by regular exposure to open waters.
• Any fouling colonisation occurring usually during extended mooring in quiet conditions, will be loosely attached and easily removed by a light scraping action or by getting underway.