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Ship Sheathing: protecting the hull of a ship

Adjunct Associate Professor Mark StaniforthMaritime Archaeology ProgramDepartment of ArchaeologyFlinders University

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IntroductionWhat is sheathing?• Any protective layer of material over the outer surface of a ship’s hull below the waterline

Why is sheathing necessary?• To protect wooden hulls against attack by wood-boring animals such as Teredo navalis (the shipworm)

• To reduce or eliminate fouling (growth of marine organisms mainly algae)

• To help waterproof & protect the hull

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Shipworm: Teredo navalis

• A wood-boring bivalve mollusc

• Attacks and eats submerged wood leaving burrows lined with calcium carbonate

• Original range was tropical seas from Florida to Brazil and the African coast into the Mediterranean

• Voyages of Discovery = an early example of human activity expanding the range of occurence

• Teredo attack can literally eat out the wooden hull

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Limnoria (or Gribble)

Limnoria lignorum, L. tripunctata or L. terebranshas

A small marine isopod crustacean that attacks and consumes submerged wood

Like Teredo, Limnoria can cause enormous damage to wooden hulled vessels

Teredo and Gribble cannot survive out of water (dry) or in freshwater and are reduced/not present in very cold water

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Fouling Growth of seaweed (algae), coralline algae and barnacles on the hull

Slows the speed of a vessel and makes it difficult to handle

Solution = careening – pulling the ship over on its side in shallow water and scrubbing the hull clean

Anti-fouling compounds – chunam = a Tamil word (lime, sand and oil)

Graving – covering the hull with a mixture such as tallow and resin – can aid with waterproofing

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Waterproofing Colombini (et al 2003) provide evidence of the use of pine (Pinaceae sp) pitch sometimes mixed with beeswax for waterproofing Roman ships from the 2nd century Bc to the 5th century AD

The use of tar, pitch and resin appears to have been commonplace for waterproofing and graving wooden ships for more than two thousand years

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Lead sheathing One early attempt was lead sheathing – known from at least the 4th century BC (Kyrenia ship)

Lead sheathing has been found on Greek and Roman vessels – commonly attached with copper or brass tacks (nails)

Used by the Spanish (at the time of the Armada) and by the Royal Navy in the 17th and 18th centuries

No understanding of galvanic corrosion (two dissimilar metals in an electrolyte solution)

Result = one metal deteriorates

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Wooden sheathing Sacrificial planking – thin wooden boards (often pine) nailed over layers of pitch or tar and animal hair on the outer planking

Introduced in the 16th century, common through the 17th & 18th centuries & used into the 20th century

Requires stripping off & replacing from time to time (during careening or dry-docking)

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Filling nails Mailletage (in French) – hundreds of iron nails placed close together to form a barrier of rusted metal

Sometimes used as a supplement to wooden sheathing

Batavia (1629) has iron nails with large heads used like this

Invincible (1758) has copper nails (10mm diam heads) used like this on the false keel

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Copper sheathing (1) Copper sheathing on Chinese junks in the early 17th century has been reported

VOC experimented with copper sheathing in the 17th century

Nassau (1606) was partly copper and mainly lead sheathed

Royal Navy introduces copper sheathing in the late 18th century

Experiments with the Alarm (1761)

Iron bolts corrode so copper fastenings introduced

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Copper sheathing (2) Introduction of standard weights = 22, 28 and 32 ounces per square foot

Heavier grades used at the bow & lighter grades at the stern

Joints lap towards the stern (tip = tell bow from stern)

Royal Navy commonly used 14 inch x 48 inch sheets

120 gun ship (in France) required 4,700 copper sheets or 16 tons (1% of the total weight of the ship)

Use of Goring belts to allow for the shape of the hull

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Muntz metal sheathing

1832 George Frederick Muntz patents a 60/40 mix of copper and zinc (brass)

Muntz or yellow metal was cheaper and better than copper = widely taken up for merchant vessels from the 1830s

Zinc sheathinbg was also used – mainly in France

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A little knowledge …

The search for the Mahogany ship (a Portuguese caravel of the 16th century or a Chinese junk of the 14th century?)

Two chemists measuring metal ions from the sheathing in the water table = increased concentrations closer to the ship

Great idea but just one problem…

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Composite construction

Iron shipbuilding from the 1850s

Problem of fouling = requires regular (and expensive) dry-docking

Composite construction – wooden planking over an iron frame allows copper (or brass) sheathing to be applied over the hull

Cutty Sark had Muntz metal sheathing

Result = increased speed due to reduced fouling of the hull

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Antifouling paints Iron (and other material) hulls do not need sheathing to protect against Teredo but they do get fouled

Copper based anti-fouling paints

Tri-butyl tin oxide (TBT) – very effective anti-fouling paint now banned as too poisonous

Now using TBT free anti-fouling paints – some include cuprous oxide

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What This Means What can sheathing tell us?

Dating and identifying shipwrecks

Telling bow from stern Evidence of technological understanding (or lack of it) at particular times in the past