Marchart 3 is the vessel that started Fenton Patches. The 32 m, 244-tonne steel ship began life as a research vessel, was later converted into a prawn trawler and ended her working life as an oil-rig tender before being scuttled in 1988 as the first major structure in the NT Government's artificial reef program at Fenton Patches. She was powered by twin Caterpillar diesels and was built to cruise at about 11.5 knots, which gives some idea of her solid, workboat build. More than three decades on the sea floor have turned her into one of the most productive fish-holding structures off Darwin.

The wreck sits upright on a flat sand and rubble bottom in about 25–28 m, with the superstructure rising to within about 18 m of the surface. The hull is intact enough to be read as a ship: a raked bow, open forward holds, the deck house and wheelhouse aft, and a deep counter stern with the rudder still in place. The holds and cabin can be looked into with a good torch, but they carry a heavy layer of fine silt and are not places to go without wreck training; the rewarding part of the dive is outside the hull. The area around the rudder and under the stern is consistently the best for fish – golden snapper and saddletail snapper stack up in the shade, black jewfish hang in the undercut, and big estuary cod and Queensland groper take up residence in the holds.

Growth on the hull is typical of turbid Top End water: sponges, soft corals, small gorgonians, hydroids and oysters rather than hard coral, with trevally and mackerel sweeping past the mast area. Because the wreck is compact, one dive around it at slack water is usually enough to see everything, and most boats pair it with the Amanda Lee, the Antares group or Bus Stop Reef nearby.

Visibility is the variable that makes or breaks Marchart 3. On a good dry-season neap it can reach 10–15 m and the whole silhouette of the ship is visible from the shot line; after wind or spring tides it can drop to a few metres. It suits advanced divers comfortable with a descent on a line in open water, limited no-decompression time and a short tidal window. For Darwin divers it is a benchmark dive and the reference point for the rest of the Fenton reef field.

Fenton Patches lies about 17 nautical miles (roughly 31 km) north-west of Darwin in open water beyond the harbour mouth, so it is a boat-only site. Club and charter boats usually launch from the Cullen Bay ramp and marina (Marina Boulevard, Larrakeyah – ample trailer parking) or from Dinah Beach / Sadgroves Creek ramps inside the harbour, then run out past East Point and across Beagle Gulf. In a fast 6–7 m trailer boat allow roughly an hour each way in trade-wind chop; slower in a southeasterly above 15 knots.

There are no public moorings: the site is found on GPS and sounder, and a shot line or grapnel is dropped on the structure. Private boats need a competent skipper who can work the tide and keep a boat cover on the surface while divers are down.

Fenton Patches is open water in Beagle Gulf with no shelter. Dive it only on neap tides and time the dive for slack water (typically a window of 30–60 minutes around the turn; Darwin's tidal range reaches 7–8 m on springs and currents at Fenton run hard outside slack). The dry season (May–October) brings the south-east trades: mornings are usually calm, but a 15–20 knot south-easterly by late morning makes the run back uncomfortable, so plan an early start.

The build-up (October–December) brings glassy days but storms in the afternoon; the wet season (December–April) brings north-westerly monsoon winds, heavy run-off that cuts visibility, cyclone risk (November–April) and box jellyfish in the water. Visibility is best after several days of neaps and no rain.

Drop the shot onto the hull just before slack, descend the line together and tie off or check the grapnel. Start at the stern – the rudder, propeller area and the undercut beneath the counter hold most of the fish – then work forward along one side at 22–25 m, look into the open holds with a torch without entering, cross the bow and return along the other side over the deck and wheelhouse at 18–20 m. Turn back to the line with plenty of gas as the tide starts to run, and do the safety stop on the shot line; if the current picks up, send up an SMB and drift the stop.

Nitrox (EAN28–32) extends bottom time usefully at 25–28 m. Penetration of holds and cabin requires wreck training, a line and redundant lights due to silt.

No daily operator. Darwin Sub Aqua Club (DSAC) runs member boat dives to Fenton Patches on suitable neap-tide weekends in the dry season; Dive Darwin (learntodivedarwin.com.au) and Western Blue Dive Charters offer outer-harbour/offshore wreck trips on demand when tides and weather line up. Otherwise private boat only.