State of the Art Mapping of Portland Harbour - August 2004

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1 State of the Art Mapping of Portland Harbour - August 2004 In 2003 and 2004, as part of the "Diving with a Purpose" initiative the Nautical Archaeology Society mapped some of the wrecks in and around Portland Harbour in Dorset. In 2004, the state of the art mapping project provided some of the most comprehensive images of the wrecks to date, highlighting their condition, the impact they have on the seabed and even discovering some features that archaeologists are getting very excited about. As part of the 2003 Wreckmap Portland Project we were able to undertake a side scan survey of several wrecks including HMS Hood, the Countess of Erne and the M2 submarine. The side scan sonar produces and receives acoustic pulses of sound with the sensors normally being towed astern of the survey vessel on an armoured cable. By "flying" close to the seabed the transducer mounted on the towfish can produce an acoustic shadow behind objects standing proud of the seafloor. Whilst the sidescan sonar can present a clear image of the overall nature of a target and its surrounding environment, the image is often distorted and contains little information about the true 3-D shape of the object. This was particularly true of HMS Hood, which was difficult to survey due to its location between the breakwaters at the southern entrance to Portland Harbour. To overcome these problems it had always been the hope to try and use the latest marine geophysical tools on the wrecks at Portland and finally, in August 2004 after months of sweet-talking, the NAS managed to undertake a multibeam sonar survey. Modern high-resolution multibeam sonar offers an opportunity to cover a relatively large area from a safe distance above the target, while resolving the true threedimensional (3-D) shape of the object with centimetre-level resolution. The survey used the first of a new generation of dynamically focused multibeam sonars, a Reson This system sends out 240 beams distributed over a 120 swath every second. The near-field beam-forming capability of the Reson 8125 combined with 3-D visualization techniques provided an unprecedented level of detail. The 8125 was deployed on a custom-built pole mounted onto the starboard side of the MV Divetime, and local charter boat shipper, Paul Pike expertly steered the boat over the wrecks. This enabled the sonar to scan the seabed as easily as if painting a wall with a roller. To interpret the scans, colour is used to indicate the depth. White represents the shallowest depth through to pink, yellow, green and then blue for the deeper areas. The scale of the colours can be adjusted for each plot to suit the relative depths. By using additional software the images can then be viewed from different angles and perspective as though actually diving the wrecks. The black sections represent those areas outside of the scan's path together with any structures that reach above the water level. The harbour wall, for instance, will be shown as black. Below is a summary of the findings. The full results of the survey were presented at the November 2004 NAS Annual Conference that took place in Portsmouth.

2 Portland Harbour Breakwater Image of Portland Harbour Breakwater. The above image shows the southern end of Portland Harbour's breakwater, the central black section being the breakwater. The 19th Century Chequer Fort is situated on the breakwater at the top of this picture. Starting at the bottom end of the wall you can see the upturned HMS Hood lying between the two sections of wall, with its bow facing towards the southern breakwater. Working along the northern side of this breakwater, at about half way along is the Bombardon Unit, and closely behind the Landing Craft. Again on the northern side, near the top of the wall is the Enecuri (Spaniard) and just after that is a large unknown wreck. The Countess of Erne is shown at the very top of the picture just to the left of the wall, on the other side of the harbour entrance. Along the southern side of the breakwater, you can see evidence of the stone blocks used in the wall construction. HMS Hood Image of HMS Hood. One of the most impressive images came from the survey of HMS Hood, the predreadnought battleship that was sunk as a blockship in It clearly shows the upturned hull of the wreck stretching between the breakwaters, including the propeller shaft. Interestingly, considering the recent ban on diving HMS Hood imposed by the Portland Harbour Authority (PHA), the survey also shows that the upper parts of the hull at midships have collapsed. The Dorset County Council archaeologist, Gordon LePard is also very excited about the results of the HMS Hood survey as it not only shows an unprecedented picture of the wreck, but it also shows a line of "lumps" on the seabed (foreground) that stretch across the entrance, on the outer harbour side of the wreck.

3 It is believed that these "lumps" are the remains of the pilings that supported a railway line that ran across the southern entrance during the construction of the breakwaters and Chequer Fort in the 19th century. Charles Dickens described the railway line in April 1858 in an article, which appeared in a weekly journal "Household Words": "Its firm unyielding timbers will bear, almost without vibration, the forty-eight tons of the four loaded wagons, and the weight of the engine, too." The full text of Dicken's description can be read at Dickensarticle1.htm (link no longer available in 2008). This is the first time that the remains of this railway line have been seen by archaeologists and Gordon LePard believes it might be possible to even witness the repairs of the timber pilings after being rammed by the Swedish vessel Irene on 26th December Gordon goes further to suggest that these remains could be worthy of protection under legislation to protect important archaeological sites. At the moment the remains of HMS Hood are not protected, although the breakwater defences are themselves a Scheduled Ancient Monument under the Ancient Monuments and Archaeological Areas Act. These remains could also be worthy of protection under this Act making them as important as other Scheduled Monuments such as Stonehenge and Hadrian's Wall. As with these monuments the protection would not limit access to the site, thereby not preventing divers from visiting the wreck. Bombardon Unit and Landing Craft Image of the Bombardon Unit and Landing Craft. This image views the breakwater from the north. It shows, lying practically next to each other in about 16m of water, a WWII Bombardon Unit (right) and Landing Craft (left). The Bombardon Unit was an experimental wave-breaking unit built for the D-Day landings. The image shows the cross-shape of the device, with possibly some additional items on the horizontal surfaces. Further investigation took place in the Wreckmap Portland 2005 project and is covered in its report. The shape just off to the side of the Bombardon Unit is a small barge with its rudder and propeller still in place. The Landing Craft is virtually intact, and has its front landing doors closed and engine room exposed.

4 Enecuri (Spaniard) Image of the Enecuri. Further north and lying against the southern breakwater is the Enecuri (Spaniard), which sank in The bow of the Enecuri stands proud (shown in the Wreckmap Portland 2004 report), but very quickly drops down to the seabed. The stern though is hardly visible probably having been damaged by an oil rig that drifted over the site during a storm in October Adjacent to the Enecuri and just to the north the scan shows a substantial unknown vessel facing the same direction as the Enecuri. This vessel has been broken in two, which may also have been due to the same storm in This was surveyed in more detail during the Wreckmap Portland 2005 project. Countess of Erne Image of the Countess of Erne. The Countess of Erne was an iron hulled paddle steamer built in Dublin in 1868, which after a short career working as a passenger ferry was converted into a coal hulk. She broke her moorings on the 16 September 1935 and sank next to the north-east breakwater. She is now one of the most highly dived sites on the south coast and many divers remember doing their first wreck dive on her. The scan of the Countess of Erne illustrates the intact condition of the wreck on the seabed with her open stern and three cargo holds. We believe that this type of 3-D image can provide the perfect map of the site for the many diver briefings that take place each year on the wreck. The detail of the scan has highlighted the three open cargo holds, the open stern (left) showing the hull frames and the structure on the upper deck towards the bow (right). At the stern the scan shows some raised areas that warrant further investigation, and again this may be included in the Wreckmap Portland 2006 project.

5 Earl of Abergavenny Image of the Earl of Abergavenny. The Earl of Abergavenny was an English East India Company ship on its way from London to the Far East with 400 passengers and crew on board. A pilot from Weymouth sailed her into the Shambles Bank and badly holed her bottom. She finally sank in Weymouth Bay in 15m of water on 5 February Despite the many hours before she finally sunk together with the presence of rescue craft, approximately 350 people died. The scan shows the visible parts of the remains of the Earl of Abergavenny on the left side of the picture. The colour scale has been compressed here to provide better definition for the relatively small changes in depth across the area. To the left of the picture the sharper blue area indicates a depression in the seabed. The reason for this is not yet known and may be included for investigation in the Wreckmap Portland 2006 project. The NAS would like to thank the many individuals and organisations involved in the work in Portland, but principally to thank Gordon LePard from Dorset Coast Forum, Duncan Malace of NetSurvey Limited, Matt Wilkins of CodaOctopus Limited, John Fraser of Reson Offshore Limited, and Ron Howse from the Weymouth Lunar Society for making the multibeam survey possible. Readers wishing to receive any of the above multi-beam survey results in a higher resolution by should contact the NAS. Broadband is recommended as each file exceeds 1 MB.