Ship-shape combination - copper-nickel hulled vessels
The Asperida is an unusual sailing yacht. Her hull is fabricated from solid 70:30 copper-nickel and this year sees the 40th anniversary of her launch in Holland. The dream project of Dr Kenneth Coons, Professor of Chemical Engineering at the University of Alabama, USA in the late 1990s, the yacht represents the culmination of a decade of testing materials and planning. Her history has been varied – she has seen six owners, suffered damage during mishaps including a hurricane, acted as a flagship structure for an environmental foundation, and sailed great distances. More recently, after sitting for a long period in dirty water in North Carolina, USA, she was sailed to New Jersey by her present owner for refurbishment. This provided a rare opportunity for an inspection of the hull and evaluation of its long-term performance.
The right mix
The selection of 70:30 copper-nickel, also known as UNS C71500, for the main structure was based on its good resistance to seawater corrosion and biofouling. The latter is not always appreciated, but it is an inherent property of the material, if not cathodically protected, and avoids the need for antifouling coatings. The ratio of 70:30 was chosen rather than the lower nickel 90:10 alloy (now known to have even higher resistance to foulers), because it has a higher strength and was already well established for other marine applications of that time.
The builders, Trewes International of Holland, were working to a S M van de Meere design and were able to apply similar construction methods from steel yacht design to the 16m copper-nickel hull and subframe. The welders were given special training in the joining of the copper-nickel and in submerged arc welding using flux coated electrode W60715 to link the hull plate, deck beams and rib frames. The keel was made from 5mm thick strips and the hull plates were 4mm thick backed by a 6mm support frame, also in copper-nickel. The hull was painted above the waterline to achieve a sleek appearance.
In 1972, the Asperida was purchased by a new owner based in the USA. While cruising in Maine, she collided with a submerged rock. Instead of breaking a seam or splitting open the hull, the metal proved malleable, and only took a little reshaping. The owner never used bottom paint and reported that the hull stayed shiny and smooth when underway.
In some ports, however, a thin layer of dirt would settle on the hull and allow some growth to begin. In tropical waters, barnacles would grow on the dirt, but a plastic pot scraper readily restored the hull to a shine. Subsequent proprietors experienced similar biofouling behaviour. Even when barnacles occasionally attached, they could be removed by finger pressure.
Recorded material problems over the years include replacement of brass screws holding down the teak decks and the manganese bronze propeller, both of which were suffering dezincification by nickel aluminium bronze. There were other occasional collisions with rocks, and even a power line, but the hull overall remained in good condition.
Inspection for restoration
The current owner, Waldemar Cieniewicz, allowed the hull to be inspected in May 2004 during the early stages of the yacht’s restoration. This was carried out by Harold Michels of the Copper Development Association (CDA) in the USA, also a sailing enthusiast, and involved ultrasonic thickness measurements as well as a visual assessment of corrosion performance. The CDA concluded that:
- Weld defects were detected in the bow area and attributed to gas that was trapped during initial construction. They have not leaked and do not require repair.
- Both the C71500 hull plates and welds have exhibited excellent ductility and mechanical strength, which was apparent when the deformed hull regions were examined.
- The C71500 hull plates and associated welds show excellent general corrosion and pitting resistance. Neither have suffered from galvanic attack.
- Ultrasonic thickness measurements indicated insignificant metal loss from the original hull thickness just aft of the bow and amidships at the waterline. Those taken directly at the bow averaged 3.91mm, which equated to a corrosion rate of 0.003mm/a for the estimated time it had then been exposed to water.
- Thickness measurements taken on the keel averaged 3.48mm, equating to a corrosion rate of 0.045mm/a. Those on the stern averaged 3.22mm, which equated to a corrosion rate of 0.023mm/a. Grinding during construction may account for some of the metal loss at the stern and keel.
- The bilge and the interior of the hull had never been painted and required no maintenance.
- The wetted bottom of the hull has required little maintenance other than an occasional scraping, during which any biofouling was easily removed.
These results are particularly interesting as only rarely does the opportunity to assess the long-term performance of copper-nickel boat hulls in such detail arise. Data from laboratory exposure sites indicates that the corrosion rate for copper-nickel is generally in the range 0.025-0.002mm/a. This describes a decrease over the first few years to stabilise at around 0.002mm/a, as a result of a protective complex surface film forming on the alloy’s surface which matures over time.
Since the Asperida, around 40 other copper-nickel hulled vessels have been launched around the world. These have all involved the more economic 90:10 alloy, C70600, either using solid plate or copper-nickel roll bonded onto steel or applied as a coating on adhesively backed foil.
In the UK, the Pretty Penny vessel was located in 1995, 16 years after her launch, having been moored for the greater part of her life in the River Swale at Uplees off the Thames estuary. The 10m hull was formed from a 3mm plate attached to a copper-nickel frame and never painted inside or out. She was in excellent condition. Neither pitting of the hull plate or preferential corrosion of welds were observed. There was also no indication of impingement attack around the rudder or propeller. Occasional evidence of barnacle attachment was found at uneven laps and also at weld heat-affected zones where the frame was joined to the hull plates.
At some unspecified time prior to being hauled out, a small seepage hole had been found at the waterline near the stern. Ultrasonic thickness measurements indicated significant thinning around the hole. Additional measurements at the waterline went from 3.0mm down to 2.7mm and 2.0mm near the bow. The owner attributed some of this metal loss to grinding during fabrication. However, less than 0.3mm thinning over the rest of the hull at the waterline resulted in a maximum corrosion rate of 0.02mm/a. Thinning at the bow waterline has not been observed on other coppernickel boats but perhaps consideration should be given to providing extra thickness in these regions.
A third opportunity to obtain long-term information occurred in 2005 when contact was made with Chris Cowland, the owner of the Footloose, from Vancouver, Canada. In this instance, the glass reinforced plastic hull had been sheathed with adhesively backed 90:10 copper-nickel panels 20 years earlier. Cowland had owned the vessel for the last eight years and praised its biofouling resistance. If left on a mooring for three or four months, the boat would grow brown ‘organic whiskers about two or three inches long, which would drop off if taken for a good sail’. For haul-outs, a very light power wash would easily clean the hull.
The foil had become scuffed due to collision and abrasion with floating matter around the waterline but was very good elsewhere. Thickness of the foil from a sample taken above the waterline and another from an area submerged ahead of the prop shaft were 0.158 and 0.121mm thick respectively. A sample of unused foil measured from another project and therefore not from the same batch was 0.179mm thick. If this is taken to be close to the original thickness, then the submerged corrosion rate would be around 0.003mm/a.