This site holds two vessels on one patch of seabed at the mouth of Fremantle Harbour. The Lygnern was a Swedish steel steamship of about 122 m and 2,874 tons. In September 1928 she touched an uncharted shoal, now Beagle Shoal, while moving from Gage Roads, lost her rudder and engine function and went aground at the harbour mouth. She stayed upright and intact for years until she was demolished with explosives during the Second World War.

The Samuel Plimsoll was a 1,500-ton clipper launched in 1874 and famous for fast passages between Britain and Australia. She caught fire and was scuttled in the Thames in 1899, was salvaged, then was dismasted in a gale near New Zealand and ended her days as a coal hulk in Fremantle. In 1945 she sank in the harbour, by one account after a collision with the Dalgoma. The hulk was cut up, raised and dumped over the Lygnern.

The result is a flattened field of iron plates, ribs, frames and fittings in 8 m. It is a site for divers who enjoy history and working out wreckage rather than an intact ship. The steel supports sponges, ascidians and small fish, and the seabed around the debris holds rays and flatheads.

The challenges are practical: heavy commercial and recreational traffic in and out of the port and to Rottnest, a long run from the ramps and river water that clouds the surface layer. Choose a calm day at the end of the flood tide and keep the boat attended.

Launch at Leeuwin Boat Ramp in East Fremantle (Swan River) or Woodman Point ramp in Cockburn Sound. From Leeuwin, run down the river, out through the harbour entrance and south to the isolated-danger marker off the South Mole; the site is just south of the marker. From Woodman Point it is a longer run north past Robb Jetty.

Divingwawrecks describes it as quite a long trip from the nearest ramp. The site is next to the shipping channel, so keep a lookout and fly the dive flag.

Needs swell under 1.5 m and wind under 15 knots. The site is strongly affected by Swan River outflow: a layer of dirty river water often sits in the top 2 m with clearer seawater below. Dive towards the end of the flood tide for the cleanest water, and avoid the days after heavy winter rain.

Afternoon sea breezes in summer make the anchorage rough. Port traffic is heavy at all times.

Anchor or drop a shot just south of the navigation marker on sand beside the wreckage. Descend through the murky surface layer, explore the scattered iron plates, frames and machinery of the Lygnern and the dumped sections of the Samuel Plimsoll, then return to the shot. Ascend on the line and keep a close watch for vessels.

Keep the boat attended and monitor VHF for port movements.

No regular operator. Dived by private boats and clubs such as the UWA Underwater Club when conditions and port traffic allow.