HMS Colossus - overview

The ship

HMS Colossus was a Courageaux Class 74-gun, ship of the line, built at Gravesend by William Cleverley and launched in 1787. The Courageaux class ships were copied from the lines of the French vessel Courageaux captured by HMS Bellona in 1761. Colossus was one of four ships of this class built (Carnatic 1783, Colossus 1787, Leviathan 1790 and Minotaur 1793).

In December 1798 Colossus was on her way home to England carrying wounded from the battle of the Nile, the body of Admiral Viscount Shuldham and cargo including part of Sir William Hamilton’s second collection of Greek pottery. She was sheltering from a gale in St Mary’s Roads when the anchor cable parted and she was driven aground to the south of Samson. All but one member of the crew were taken off safely before Colossus turned onto her beam ends and proceeded to break up.

The site

The stern section of the wreck of HMS Colossus lies at a depth of 15m in sheltered waters in the Isles of Scilly. To date two main areas of wreckage have been identified, the bow and the stern. The bow section of the wreck, part of which was excavated by Morris, lies outside the currently designated area.

The stern wreckage.

Previous work

Salvage work took place on Colossus from the time of her loss until the early part of last century. Work included Braithwaite and Tonkin 1803-1806, the Dean Brothers in the 1830s and possibly Western Marine Salvage in the early part of last century.

Roland Morris, a marine salver, began searching for the wreck of Colossus in 1967. In August 1974 they located material relating to Colossus (Birchall, 1994). A large quantity of pottery, remains of Hamilton’s second collection, was recovered and deposited in the British Museum – where at least one of these reconstructed pots is now on public display.

Local divers continued to recover material from Colossus throughout the 1990s. Some of this material is on display in the local museum on St Mary’s.

Rediscovery

Areas of exposed timber and iron guns were discovered by divers in 2001. This material was some distance to the east of the area worked by Morris and turned out to be part of the stern of Colossus. One of the most striking features of this part of the wreck is the row of 18lb Armstrong pattern guns standing upright with their muzzles buried in the sand, still within the gun ports of the hull. The discovery of these guns and a large carved human figure - part of one of the quarter pieces from the stern of the vessel - led to the redesignation of the site in 2001.

What also makes this site so different from the many others in Scilly is the extent and remarkable preservation of the timber. When first uncovered, the timber looks perfect with fine surface detail visible. This was particularly apparent on the stern carving where much intricate detail was preserved intact. It was clear from the start that this timber had not been exposed on the seabed for the last 200 years. Indeed by May 2002 it was apparent that timber which appeared perfect when first seen in 2001 was now decayed and gribbled. Furthermore, it was also clear that more of the wreck was emerging from the sand as time went on.

The inevitable conclusion was that the wreck had been preserved because it was buried in the sand. Natural forces unknown are now causing the sand to disappear from over the wreck. It is clear neither why this is occurring nor whether it is a cyclic phenomenon or a more long-term trend. Observation of the site since June 2001 has shown a steady diminution of the sand levels over the wreck.

Survey

The original pre-disturbance survey was carried out in 2001 and 2002. The seabed remains were drawn using planning frames at a scale of 1:10. This survey allowed the identification of the newly exposed timber as the stern port-side of the vessel from the transom to just forward of the main mast.

Plan of the stern wreckage.

The recovery of the stern carving

The stern carving or quarter piece was one of the first things to be discovered on the site in 2001. It consists of a carved wooded statue depicting a male figure in neo-classical dress holding aloft in his left hand what appears to be a laurel wreath.

The stern carving on the seabed, being prepared for recovery to the surface

The figure would originally have been on the stern of the ship on the port side of a round-headed window opening. The carving is over 3.30m tall and is carved from several pieces of elm. Part of the carving, consisting of the right hand side of the face and the right arm, is missing. This was obviously originally a separate piece of wood – the smooth face of the original joint is clearly visible.

Recovery of the carving was begun in 2001. This operation was the subject of a Time Team television programme. The carving was recovered in spring 2002. Following it's conservation, undertaken by the Mary Rose Trust in Portsmouth, the carving has recently been returned to Tresco. The carving will be on display alongside the Valhalla figurehead collection, in the Abbey Gardens, hopefully by the summer of 2010.

The stern carving shown superimposed onto the stern of a 74-gun ship.

The excavation

In 2002, a small excavation on the wreck of Colossus was undertaken. The main aim of this excavation was to determine exactly how much of the ship survived and to confirm the disposition of the surviving hull timbers. It was completed in a total of ten working days with a team of six divers. This small excavation yielded more information about the surviving structure of the wreck than all the other survey work put together and amply demonstrates the value of limited excavation in understanding a wreck site.

Excavation using a reaction dredge on the Colossus stern site

The excavation consisted of a small trench approximately one metre wide extending across the wreck between the orlop deck and the upper gun deck. The excavation was effected by hand-fanning and the use of a water-driven reaction dredge. Once the sand had been removed, the inside of the ship’s hull was exposed. The trench included sections of the orlop and main gun deck planking.

Finds included 32lb iron cannon balls, leather shoes, glass bottles, lead musket shot, rope and fragments of clothing. Finds were recorded and then reburied in a carefully recorded location on site in labelled bags containing quantities of the original sediments. These finds may form a valuable research tool in determining the efficacy of reburial at some future date.

The stabilisation trials

In 2003, a two year stabilisation trial on the site was commissioned by English Heritage. The broad aim of the trial was to determine suitable methods for stabilising the timbers of the wreck exposed on the seabed

Three different methods of stabilisation were employed in the trials. The stabilisation materials used were a Terram mat, a synthetic mesh mat and an artificial frond mat system. Each trial mat covered an area 5 x 2.5m and was left in place on the seabed for a period of two years

To determine the relative performance of the three mats, timber sample blocks were installed under each mat. These were retrieved at intervals of 3 to 24 months and analysed to determine the amount of deterioration caused by wood boring organisms, bacteria and fungi. Timber sample blocks were also installed directly on the seabed to act as a control. The results from the analysis of these timber blocks demonstrated that the blocks from the Terram mat showed no signs of decay even after two years. The blocks from all the other trial areas showed some degree of deterioration.

The conditions under each of the test mats were monitored using a sub-sea data logger, deployed consecutively under each mat for a three month period. The data logger recorded dissolved oxygen, redox potential, pH, temperature and depth at one hour intervals. Results from the Terram and Frond mats showed highly anoxic conditions (less than 0.02 mg/l of dissolved oxygen) within days. The results of the stabilisation trial demonstrated that, of the three stabilisation methods, the Terram 4000 mat was clearly the most efficient and cost effective of the systems tried on this site

Debris field survey

Since the designation of the stern section of Colossus in 2001, it has been clear that parts of the wreck were spread over a fairly wide area. Although material has been recovered from the area around the wreck of Colossus by a number of individuals, in most cases the precise location from which it was recovered is not known.

The CISMAS team setting out on the debris field survey (left) and conducting a circular search (right)

In 2004 The Cornwall and Isles of Scilly Maritime archaeology Society (CISMAS) undertook a two year survey of the debris around the designated remains. This work was made possible by a grant from the Lottery Heritage Initiative. The first stage of this project was a geophysical survey conducted over a wide area around the wreck. This mainly comprised a magnetic survey using a Geometrics 881 marine magnetometer, loaned to CISMAS by St Andrews University. At the same time bathometric and sidescan sonar surveys were also undertaken. The magnetic anomalies were investigated and recorded by CISMAS members in two weeks of diving in 2004 and 2005. 

Sediment levels

From 2002 to 2007 the sediment levels on the site have been recorded. To achieve this, fourteen sediment monitoring points were established on the site. Measurements are taken from the tops of these points to the seabed. It became clear that there was a degree of sediment mobility on the site, the net result of which was a small diminution of seabed levels around the wreck over the last six years.

Diver measuring sediment levels

Stabilisation & recording

In 2008 part of the wreck of HMS Colossus was protected with a geotextile covering of Terram 4000. A relatively small area (3.8 x 5.5m) at the stern of the vessel was covered by this mat which was held in place with sand bags. The method of protection used was determined by the results of the stabilisation trials.

Timber sample blocks were installed beneath the Terram mat so that direct comparison with the timber samples analysed in the original stabilisation trials will be possible. A small seabed sign explaining the function of the Terram mat was placed on the seabed next to the Terram.

Detail of survey drawn in 2003. The timbers shown in pink no longer survive by 2008

So that the long term effects of the stabilisation can be determined, the area to be covered was recorded in detail prior to installation of the Terram mat. An adjacent, unprotected area was also recorded to act as a control zone. This recording consisted of planning frame drawings at a scale of 1:5 as well as detailed photo mosaics. The principal advantage of drawing at a scale of 1:5 is that it is possible at this scale to record more detail. For example, at the more usual scale of 1:10, an average treenail (25mm diameter) is only 2.5mm on the drawing, making it very difficult to record details such as wedging of treenails. It has always been clear that exposed timbers on the site were degrading but this is the first time that this degradation has been quantified. It has also graphically illustrated the fact that a site survey is only a snap-shot of what the site was like at the time the survey was made. This survey was recorded using Site Recorder GIS software.

The diver trail

A diver information trail was installed on the site in 2009. A number of seabed observation stations were established around the wreck at points of particular interest, these along with waterproof underwater booklets guide divers around the site. There are three dive charter boats operating in Scilly, they arrange the necessary permission for divers to visit the site and have copies of the underwater booklet for divers to use on their site visit.

A diver on the Colossus dive trail

The future

All wrecks deteriorate with the passage of time. However, the work to date has demonstrated that in the case of Colossus, due to the falling sediment levels, this deterioration is marked and rapid. It is therefore important that we get the most from the site while it is still extant.

The sediment monitoring will continue for as long as possible. It will be interesting to see whether the sediment levels continue to fall, or if at some point they begin to rise again. The general condition of the site will also be monitored. To help in this it is hoped that a detailed survey and record of the small finds exposed on the seabed can be made next year. This will allow us to determine the extent of deterioration and to quantify losses due to natural forces and visitor intervention.

Sponsors

The sources of funding for this project have been diverse. The initial survey, recovery of the stern carving and trial excavation (2001-2002) were funded by the licensee at the time, Mr Mace of Bryher in the Isles of Scilly. The stabilisation trial and subsequent stabilisation and recording (2003-5 and 2008) as well as the dive trail were funded by English Heritage. The survey of the debris field was made possible by a grant from the Lottery Heritage Fund. Survey and GIS software was donated by 3H Consulting Limited. Otter Watersports helped with the provision of subsidised dive suits. Last but not least I would like to thank the members of CISMAS who gave so freely of their time, and without whom much of this work would not have been possible.

Previous reports

There are now a considerable number of reports concerning the work already undertaken on Colossus. These have been produced by various authors and for this reason they are listed below. Those by Kevin Camidge and CISMAS are available to download as PDF files from the Download Centre.