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1. Paulo Monteiro.
1. Paulo Monteiro.
Latest revision as of 01:30, 4 December 2010
(This article is incomplete)
The Angra A wreck - or Copper-fastened wreck - at Porto Novo, Angra Bay, Terceira Island, Azores, was briefly examined during the general survey of the inner harbor area in 1996. This wreck lies off the northern corner of Porto Novo, beneath approximately 5 to 7 meters of water. It is oriented along a northwest-southeast axis, but the stem and stern of the vessel have not been identified due to the lack of endposts, deadwood timbers, or diagnostic features on the keel. The site is characterized by an oval ballast mound measuring 35 meters in length by 11.5 meters in width; beneath the ballast mound is an articulated lower hull structure measuring 40 meters in length by approximately 11.5 meters in breadth. The northwestern end of the wreck is lodged between two large boulders.1
The wreck consists of a 40-meter length of structure from the bottom of a heavily-built wooden vessel. Ballast covers much of the hull, but exposed timbers along the southwestern side and southeastern end of the wreck provided much information about the construction of the ship. Identifiable timbers include the keel, the lower portions of frames (consisting of floor timbers and futtocks), a bilge stringer, and both external planking and internal (or 'ceiling') planking.
The condition of the visible timbers varied: the keel, frames and planking at the southeast end of the wreck have long been exposed to decay and the depredations of marine wood-boring organisms such as Teredo navalis (teredo worms) and as result are in a very poor condition; frame timbers amidships appear to have been protected by sand and ballast, and in some places retain their original surfaces, complete with tools marks. The wreck timbers still buried under the ballast should be in good condition. A 5.42 meter length of the ship's keel protrudes from the ballast pile at the southeastern end of the wreck. This timber was of substantial dimensions, 38 cm moulded by 29 cm sided. The keel's surfaces were considerably degrade, and we could not identify typical keel features such as the outer planking rabbets, a shoe (or 'false keel'), or timber scarfs.
The wreck's frames were, like the keel, of very substantial size, with the sided dimensions averaging between 23 and 28 cm and the moulded dimension averaging about 20 cm. The frames were very closely spaced amidships on 59 cm centers, with an average space of 5 cm between each set of frames. The frames exhibited very low dead rise - the angle of rise above the horizontal plane of the keel - suggesting that the vessel was nearly flat-bottomed with a pronounced turn of the bilge, giving the hull a U-shaped section rather than a V-shaped section. The minimal deadrise indicates that the hull was designed for maximum cargo capacity amidships.
The wreck's outer planking measured 9 cm in thickness; the badly deteriorated state of the planking at the southeast end of the wreck did not allow us to identify planking seams, and the average width of the planks is therefore unknown. The outer planking was attached to the frames with copper-alloy spikes. We did not see any evidence on the outer planking or keel of any type of metallic or wooden sheathing to protect the wood from marine growth or teredo worms.
Several strakes of ceiling planking were exposed on the southwestern side of the hull, and averaged 29 cm in width by 8 cm in thickness. A heavy timber extending along the southwest side of the wreck was identified as a bilge stringer. The stringer was quite large, 29 cm moulded by 23 cm sided, and was fastened to the frames with copper-alloy bolts.
There were no visible remains of the hull structure above the level of the bilge stringer, although it is possible that part of the vessel's keelson may still exist beneath the ballast. As noted above, the wreck contained copper-alloy fasteners in the form of spikes and bolts. We did not note any treenails in the construction of the frames or attachments of the planking, but the survey was limited in extent, and it is possible that such fasteners may have been overlooked. The presence of copper fasteners permits approximate dating of the vessel, since spikes and bolts of this material appear to have been uncommon in large wooden ships prior to the nineteenth century.
The use of copper for ship fasteners dates back to the classical era, and they appear on wrecks dating to the 5th and 4th centuries B.C. in the form of clenched nails. By the Roman era, however, iron replaced copper in shipbuilding , due to its greater strength and lesser cost.
Fasteners of iron or wood remained the standard through the medieval and early modern eras until the last quarter of the 18th century. The appearance of copper and copper-alloy fasteners was a result of the Royal Navy's experimentation with copper sheathing to protect the bottoms of wooden ships.
Copper sheathing was first used in 1761 and was a great success in terms of improving sailing speed, keeping ship's bottoms free of marine growth, and repelling teredo worms. There were drawbacks to copper: it was expensive, it wore out quickly, and perhaps most seriously, copper sheathing over iron-fastened hulls created a galvanic reaction that quickly destroyed iron.
By 1783 the Royal Navy learned to build its warships with copper spikes and bolts below the waterline, which greatly relieved the problem of galvanic corrosion.
From the 1780's to the 1830's copper-fastened and sheathed vessels became more common, particularly for naval vessels and larger merchant craft, but the great expense and short life span of pure copper plates were still a problem and kept this type of sheathing from being widely adapted. The problems were finally overcome in 1832, when George F. Muntz of England patented 'yellow metal' or 'Muntz metal', an alloy of 60% copper and 40% zinc, that was hot-rolled into thin sheathing plates. Muntz's alloy was flexible enough to adapt itself to a wooden hull, corroded at a much slower rate than copper, and because of the high percentage of inexpensive zinc cost substantially less than pure copper. Muntz aggressively marketed his new alloy during the 1830's, and it began to see widespread use by the 1840's.
The copper-fastened wreck in Porto Novo, Angra bay, represents the bottom of a relatively large wooden vessel. The ends of the vessel could not be determined during the survey due to the lack of a stem or sternpost or their related structures; the low angle of deadrise of the frames suggests that the vessel was quite boxy amidships, and it was most likely a cargo-carrying vessel. The presence of copper fasteners on the wreck in Porto Novo tells us that the hull was built sometime after 1783, very likely dates to the 19th century, and most probably was built after 1840. While no copper-alloy sheathing was noted on the wreck, we can assume that it was probably sheathed during its career.
Further investigation of the wreck's construction and of artifacts located in the ballast or between frames might permit closer dating of the wreck. The percentage of copper and other metals in the fasteners could also be tested to see if the fasteners conform to Muntz's 1832 patent specifications.
1. Report by Paulo Monteiro.