The Colossus debris field survey

A personal view by Honor Thorley

As 2004 draws to a close let’s have a look back over our first year and the beginning of our first project:
The HMS Colossus debris field survey in the Isles of Scilly.

Having formed a committee and gained a small membership we successfully applied for a grant from the LHI and in June the first payment of the £25000 total went into our bank account. With this money we bought several essential items for the Colossus Debris Field Survey, such as:

  • Hand held GPS for position fixing.
  • Video camera and underwater housing.
  • Training in underwater film making.
  • Underwater surveying equipment.
  • Computer equipment, scanner and cables.
  • Accommodation on the Isles of Scilly.
  • Transport to IoS for people and the boat.

The aims of the survey were to identify and record the remains of HMS Colossus in the debris field between Southard Well Rock and the stern section of the vessel.

The expedition to the Isles of Scilly was carried out between September 4th and September 18th 2004. Over the course of the two weeks, eleven volunteer divers were involved with surveying the debris field. The boat used (a rigid hulled inflatable) was on loan from two of the divers involved, as was other equipment such rope, buoys and shot weights. The Scuba diving equipment was supplied by each volunteer for their own use.

A list was drawn up of the best magnetometer anomalies to investigate (there were 46 in total). Another list was made of the types of photographs and video clips that would be required. The museum at St Mary’s, Isles of Scilly was contacted to make an appointment to visit and begin plans for a museum display.

The magnetometer survey of the debris field had been conducted by the Archaeological Diving Unit in 2002. Each evening the magnetic traces were studied in great detail by the team and the ones that looked the most promising were put to one side for further, underwater, investigation. Each one was numbered for ease of identification and the number and chart position was entered into the GPS.

Besides having to plan the archaeological and recording aspects of the project, SCUBA diving obviously played a major role in the success of the project. It was essential that diving operations were carried out safely and accurately.

To this end, each evening, a dive plan for the following day was formulated. It involved matching divers into buddy pairs and listing the magnetometer targets to be investigated. Charts were consulted to check the depth of water around the targets so that the deepest dives could be first. No more than three dives would be done per person in a day.

Weather forecasts had to be consulted along with tide tables for tide times and heights. The chart positions of the magnetometer targets had to be entered into the GPS and double checked for accuracy. Most of this information was then written onto a waterproof slate so that it could be carried with the divers on the boat. The slate also had spaces left on it in order to enter each diver’s time of entry and exit into and out of the water and their maximum depth achieved. This slate would be kept on the boat during diving operations and the boat cox’n would complete the details.

An Example of the Dive Slate

Although everyone who went out on the boat was a qualified diver, not everyone was qualified in the art of boat handling, and so there were days when someone could not dive and had to handle the boat instead.

If the weather forecast and sea state proved suitable for diving operations, the boat was collected from the harbour mooring each morning and motored over to the beach to pick up the divers and all their SCUBA gear. Equipment was double checked, cylinders were checked for air content and, having been carefully stowed on board, and with everyone seated, the boat would set off for the Colossus site about 1 mile away from the harbour towards the island of Samson.

The chart positions of the magnetometer targets had already been entered into the GPS, and as the boat neared the required position one person would take the hand held GPS, turn it on and wait a few minutes for it to acquire enough satellites to know where it was. The boat was guided towards the position, moving very slowly, until it was within one or two metres of the target, (or less if sea conditions allowed!).

Meanwhile two other people on board would be preparing the buoy, a heavy weight, and enough rope to reach the seabed. The depth of water underneath the stern of the boat was measured by the echo sounder.

Once an accurate position had been attained the order was given to throw the weight over the side of the boat. The rope would follow smoothly, and then the buoy to mark the position on the surface. The boat would then come round again just to double check the position, and if it was accurate, the first divers would begin preparations to dive.

The divers would descend to the seabed down the shotline where they would attach a line from a reel to the base of the shotline. They would then swim out to a distance that was determined by the visibility and, spaced out equally along the line, swim slowly in a circle around the shot line carefully searching the seabed as they went. If something unusual was discovered, it would be carefully measured, sketched and recorded on a slate.

The seabed varied enormously from dive to dive. Sometimes searching was made easy by flat sand, with the occasional rock or lump of seaweed; these had to be investigated carefully to ensure that a rock was in fact a rock and that the seaweed wasn’t hiding something! However the next dive could be a major battle through very heavy kelp growth, and it was quickly discovered that it was very easy to swim straight over a huge pile of cannon balls without seeing them! With a current running most of the time the divers quickly became tired and used up their air, so great care and attention had to given to monitoring each other as well as the seabed.

Of 26 magnetometer anomalies investigated 16 revealed iron objects. The other 10 magnetometer anomalies that were dived upon revealed nothing but that may mean that whatever is there is buried in sand. This is a brief list of the items found:

  • Anchor and chain. Of a round crown type and so too late to be from Colossus.
  • Iron ‘bar’.
  • A pile of approximately 35 cannon balls.
  • Iron gun (gun 9).
  • Iron gun (gun 8).
  • Anchor. Again, of the round crown type and so too late for Colossus.
  • Anchor. Of the angle crown type so possibly from Colossus.
  • Iron shot.
  • Iron gun (gun 10).
  • Copper sheathing.
  • Iron gun (gun 7).
  • Several separate, unidentifiable iron objects and piles of iron shot.

This is a very brief description of the finds made. Anyone wishing for further details should download the Survey Report from the CISMAS web site. It is possible that more finds could have been made if five days had not been lost to bad weather. A most frustrating few days!!!

No firm conclusions have been reached at this stage in the project, although it is apparent that much of the debris lies outside of the protected wreck area. High on our list of priorities for Phase II of the project in 2005 are:

  1. Further magnetic survey of the area, (planned for early in the year).
  2. Two further weeks work surveying the debris field – including a search for any surviving Colossus bower anchors.

At almost six metres in length one would assume that locating this anchor would be an easy task. However experience now tells that this is not the case! Of Colossus’s three bower anchors we know that at least one was salvaged after she sank but we do not know the whereabouts of the remaining two, so the discovery of even one could offer invaluable information about the ship and her sinking. Part of the problem with this is that St Marys Roads has been used for centuries as a safe anchorage therefore the seabed is probably littered with lost anchors of varying ages and descriptions.

What’s next?
Because of the logistics of organising a trip to a popular tourist location like the Isles of Scilly, planning is already underway for both of next year’s expeditions. Anyone wishing to get involved in any way at all should contact Kevin Camidge as soon as possible.












Honor Thorley