HMS Colossus debris field survey 2005

After a very successful Geophysics trip earlier this year we returned to the Isles of Scilly in September 2005 to carry out the final expedition of this phase of our Colossus project.

Months spent carefully processing the magnetic, bathymetric and side scan data had produced a list of 291 magnetic anomalies and 59 side scan targets. The magnetic hits varied in predicted weight from 2Kg to 20 tonnes and many of them correlated closely with the side scan targets.

Magnetic anomalies located in the 2005 geophysical survey
Magnetic anomalies located in the 2005 geophysical survey

With so many targets and only two weeks to investigate them we had to prioritise, the hits were arranged into predicted weight order with a brief description of the shape of the magnetic trace. This would help us later when deciding which targets to investigate.

Equipment was collected together, volunteer divers were checked for medicals and qualifications and a boat was sought out. Kindly loaned to us by Peninsula Sub Aqua Club this 5.8 metre Rigid Hulled Inflatable would be slightly bigger and more seaworthy than the one used in 2004. This could prove useful should the weather turn against us once again.

Divers had to be accommodated and a base found to run the computers and evening planning sessions. So with some divers along for a week and others doing the whole fortnight half were to stay in a small flat and the others camping on the island.

With everything in place and fine weather forecast we set out with renewed enthusiasm in early September. The weather was kind to us and over the two weeks we managed to dive on 13 days. Over the two weeks fifteen divers were involved in carrying out searches.

The dive team head out to site
The dive team head out to site

Each evening a plan of targets for the following day was drawn up and searches assigned to pairs of divers. Weather forecasts, tides and charts were consulted. Morning dives were usually deeper searches for anchors and afternoon dives tended to be shallower searches nearer to the stern section itself.

We used the same methods for positioning a shot near to the anomaly as in 2004 and the same circular search technique. Once again both these systems proved very successful. Any finds were recorded, drawn and photographed in situ then precisely positioned by distance and bearing from the shot line. The usual difficulties of searching in thick seaweed restricted some searches to small areas whereas searches on bare sand often proved much easier and allowed for larger areas to be covered. Divers were constantly reminded to search a small area well rather than a large area and miss a find.

Divers conducting a circular search
Divers conducting a circular search

On return to the shore all record sheets would be carefully copied by the diver onto a neat paper copy and this was then archived with the original waterproof version in case of future discrepancies. All searches and finds were plotted onto an electronic plan each night and it soon became clear that some searches could be planned to cover more than one target. It also became obvious which areas were more fruitful with finds and so this influenced the concentration of searches in those areas. Good weather and an experienced team made for a very successful trip.

The table below summarises the searches undertaken in both years.

The table below shows the material found on the seabed over the two years:

A large amount of debris remains on the seabed and much of it is located outside of the existing designation. An obvious debris trail seems apparent running south west from the position of the stern site towards the reported bow position on the southward well reef. Although 11 anchors were found, none were of the correct size and style to be Colossus' main bower. However 3 of those found are of a similar size and pattern as Colossus' smaller kedge anchor.

Large iron anchor

Documentary evidence suggests that salvaged guns from Colossus were used to strengthen the Garrison on St. Mary's which was short of guns at the time. So during our time on the islands we carried out a survey of every gun on the Garrison. We discovered that 5 of them were of the same age and type as those on Colossus.

Another area of interest was a decorative carving in the Valhalla Museum on Tresco. Reputed to be the stern board of Colossus. After surveying and drawing the carving we now believe that this is too small and of the wrong type to be from Colossus and is possibly French.

Carved timber in Valhala, Tresco

A summary of our work was displayed in the museum on St. Marys throughout 2005. In the future the display will be positioned in the National Maritime Museum at Falmouth.

A complete report on our work was written by Kevin Camidge and is available to download from our website. We have also produced a booklet detailing the story of Colossus, her wrecking and the CISMAS project which is available for sale.

Brendon Rowe