IT MAY HAVE STARTED LIFE as the Firth Fisher, but it was not a fishing boat but a 443-ton coaster of the classical layout, with two holds forward and engine and boiler aft.
We begin at the back of the second hold, where a bulkhead (1) separates it from the bunker space. This is where skipper Martin Jones shot had caught when I last dived the Castlereagh and sketched the wreck.
Orientation is straightforward, with the bow just east of north. The dive usually begins while the tide is still just about running before truly slack water, to either the east on a flood or west on an ebb.
On a high-water slack, the bow is to the right as a diver reaches the bottom of the line, or to the left on a low-water slack. The depth will be 33m on a high-water slack, or a few metres shallower on low water.
A second indication of direction is that the port side of the aft hold has fallen outward (2).
What little remains of the deck has collapsed to the seabed, leaving the broken outline of the hatch coaming just out of the sand.
Continuing forward, between the holds is an intact area of steel deck, with pairs of bollards to either side and a cargo-winch (3) in the middle.
The position of the wheelhouse varied between two locations on coasters of this size - aft of the holds, above the bunker space and boiler; or, despite the machinery being located aft, between the two holds and therefore just forward of amidships.
The aft location was generally, but not always, favoured on later ships. A between-the-holds location was more traditional, and photographs show that the Castlereagh followed this pattern.
On our tour, all the structure of the wheelhouse has decayed, but the helm (4) still lies on the deck between the holds, just forward of the cargo-winch.
Crossing the forward hold, another cargo-winch (5) is solidly attached to the deck forward of the hold, just aft of the bulkhead separating it from the forepeak.
The deck at the bow has been ripped off to starboard, leaving a pile of anchor-chain (6) visible in the bottom of the bow, where the chain-locker would have been located.
Outside the hull on the port side, what looks like a domed water-tank (7) is trapped between the bow and the seabed. This is an unusual place to find equipment normally located close to the engine and boiler, and evidence of commercial salvage in the past.
On the other hand, this object may have nothing to do with the wreck, and simply have been trapped by the tide.
Rounding the bow, a scour in the soft rock of the seabed adds a metre or so to the depth. One of the Castlereaghs anchors (8) rests on the seabed just back on the starboard side of the bow.
I would describe the bow deck as having been ripped off to starboard, because thats the only way it could have been peeled and turned upside-down, with a pile of anchor-chain in the middle of it.
Normal decay of an initially intact wreck could not have resulted in the deck being turned over before falling. Perhaps this was collateral damage from an old smash and grab salvage of the coal cargo, or perhaps a sizeable ship once fouled its anchor.
The Castlereaghs anchor-winch (9) lies upside-down beside the inverted bow deck.
Returning aft along the starboard side of the wreck, the decay of the forward hold is opposite to that of the aft hold we have just crossed, with the port side more intact, and the starboard side fallen outward (10).
Back past the shotline and the bunker bulkhead, the bunker space (11) still contains a few scraps of coal. The footing for a mast has fallen to the bottom of this space, forward against the bulkhead.
Next we come to a single large boiler (12) that spans the width of the hull, separated from the engine-room by another bulkhead (13), which has fallen forward to rest against the boiler.
The engine (14) is a straightforward two-cylinder compound unit, often preferred on small coasters to the more efficient, but bigger, triple-expansion engine. Aft of the engine, a set of steps leads from the engine-room to the missing deck above.
Now at the stern, the deck still shows the outline and a few upright supports (15) from a cabin. The usual pairs of bollards are still firmly attached to either side.
Dropping over the side of the wreck again, the scour at the stern is considerably deeper, descending to 37m on a high-water slack.
Most of the propeller has been salvaged, with a single blade (16) remaining in the bottom of the scour. Next to it is what looks like part of a pump from the engine-room.
The rudder has been torn away, I would guess in the course of salvaging the rest of the propeller.
The steering was a simple tiller (17), pulled from side to side by chains or cables, now twisted by almost 180° to point out from the stern.
Which brings us to the end of our tour. Unless you stop to rummage or take photographs it wont be a long dive, so is ideal for those not wanting to get into too much decompression.
To ascend, local skippers generally prefer divers to use a delayed SMB.
After slack, the tide picks up fiercely, and a shotline with divers hanging on it usually gets pulled under.

Thanks to Martin and Bryan Jones.

LOST IN THE GALES
CASTLEREAGH, coaster. Built 1898, SUNK 1925

This 443-ton steel-screw coaster
started life as the Firth Fisher in the early days of 1898, when the keel was laid in the Paisley yard of Fullerton & Co.
She was completed that October, when the 88hp two-cylinder compound engine and single boiler was fitted aft, writes Kendall McDonald.
Her machinery was made by Ross & Duncan of Glasgow. She was small at 168ft long, with her breadth 25ft and her holds a mere 9ft deep.
The Firth Fisher was owned by John Fisher & Sons. Some years later she was sold to John Kelly & Co, which changed her name to Castlereagh.
She was described as a Belfast steamer of that name when she was reported lost mysteriously somewhere in the vicinity of the Needles during the fierce gales of Sunday and Monday, 22 and 23 of February, 1925.
On her way from Ayr to Shoreham in Sussex, her holds full of 482 tons of coal, she was last sighted off Prawle Point in South Devon.
Captain William Smythe and his crew of 10 disappeared with her. Two drowned bodies, of the cook and the second engineer wearing properly fastened life-jackets, were found three days later in the Western Solent, near Freshwater on the Isle of Wight.
The coroner expressed surprise that no wreckage had been washed up at the same time as the bodies. He took this to mean that the men had had time to don their life-jackets, even though their ship had been suddenly overwhelmed.
Ten days after the discovery of the bodies, life-belts and a timber hatchway were washed ashore in the Solent, but nothing else.
The mystery remained for 42 years, until diver Dave Weightman discovered the ship near Durlston Head, just a mile from Swanage.
He positively identified the wreck by bringing up the bell, which was engraved with her former name of Firth Fisher.
The ship was mostly complete, with its above-deck accommodation in place and the coal cargo in the holds. There was no sign of collision damage. The sinking was clearly due to the huge seas during the gales.

TOUR GUIDE
GETTING THERE: Follow the A351 past Corfe Castle to Swanage and the signs for the town centre and pier. Parking on the pier is limited, so be prepared to drop divers and kit and use the car park further up the hill.
HOW TO FIND IT: The GPS co-ordinates are 50 34.841N, 001 56.172W (degrees, minutes and decimals). The wreck lies with its bow just to the west of north.
TIDES: Slack water occurs one hour before and six hours after high-water Dover, at the same time as slack on the nearby Kyarra (Wreck Tour 47). Visibility is usually best at high-water slack.
DIVING: Sidewinder and Mary-Jo from Swanage Boat Charters, 01929 427064, www.kyarra.com
AIR: Air and nitrox are available on Swanage Pier from Divers Down, 01929 423565
ACCOMMODATION: The are many B&Bs, small hotels and camp sites. Contact Swanage Tourist Information, 01929 422885, www.swanage.com
QUALIFICATIONS: Just on the deep side for PADI Advanced Open Water or BSAC Sports Diver, especially at high water. The depth of the wreck is well suited to nitrox.
LAUNCHING: There is a slip at the Swanage boat park, by the lifeboat station.
FURTHER INFORMATION: Admiralty Chart 2615, Bill of Portland to the Needles. Ordnance Survey Map 195, Bournemouth, Purbeck and Surrounding Area. Dive Dorset, by John & Vicki Hinchcliffe. South Coast Shipwrecks of East Dorset & Wight, by Dave Wendes.
PROS: Convenient from Swanage, and worth a diversion from the Kyarra.
CONS: Frustratingly just a little bit deeper than 35m on a high-water slack.

DEPTH
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DIFFICULTY RATING
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