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 three-dimensional
(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 8125. 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.
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
nothern 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
One of the most impressive images came from the survey of HMS
Hood, the pre-dreadnought battleship that was sunk as a blockship
in 1914. 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. 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
www.dorset-opc.com/Portland-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 1860. 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
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.
Enecuri (Spaniard)
Further north and lying against the southern breakwater is the
Enecuri (Spaniard), which sank in 1900. 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 1996.
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 1996. This was surveyed in more
detail during the Wreckmap Portland 2005 project.
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.
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 1805. 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 email should contact the NAS.
Broadband is recommended as each file exceeds 1 MB.
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