DIVING ON AN UNKNOWN WRECK, the pattern of wreckage can tell you many things. Even on a known wreck, it is well worth being aware of what it can tell you. After all, perhaps it will reveal that someone in the past got the name wrong.
Often the signs are so obvious that I am astonished that the discrepancy hasn’t been noticed before. Examples would be wrecks having too many boilers, or the wrong type of engine!
At other times, the clues are more subtle.
Of course, we would all like to find something like a maker’s plate or bell with the name on it, but these have mostly been recovered already, or else are well-hidden.
To the shame of divers as a community, many such discoveries from the last century have gone unreported, so the critical piece of evidence that would have tied down the identity of a wreck could now be unavailable.
Barring a convenient find, clues about a wreck’s construction and fittings can tell us plenty about the original ship and how to navigate the wreck. Let’s start with the propulsive machinery…

BOILERS
The vast majority of wrecks we dive around the coastline of the British Isles were powered by steam, sunk either during wartime or by accident. I like to start by counting the boilers. How many are there How were they arranged
If they are all still in place, this is easy. If they have shifted, either during the sinking or as the wreck decayed, this can be a bit more tricky.
If one or more have been displaced well away from the wreck site, the count could come up short. If one is obviously smaller than the others, it could be a donkey boiler, which often will not be listed or counted with the main boiler(s) in records such as The Shipwreck Index of the British Isles or www.wrecksite.eu
To double-check, on a broken wreck I look for boiler-mounts. These are curved or wedged cradles fixed to the keel on which the boilers would have rested. (1)
When boilers are in approximately the right place, I check on whether they have moved by looking at the orientation of the fireboxes or furnaces.
These should be in line at the bottom of the boiler. If it is still there, a boilermaker’s plate would usually be mounted above the fireboxes.
Most boilers are the cylindrical Scotch-type fire-tube boiler, where hot gases from the fireboxes are routed through wide tubes in a cylindrical tank of water. (2, 3)

ON OLDER BOILERS that couldn’t reach as high a pressure, a drum stacked on top, the steam dryer, would be used to apply a final blast of heat to the steam, removing any last droplets of water. (4)
So a big drum stacked on top suggests an older ship. But beware, the converse is not necessarily true.
Another clue to age is whether the boilers are riveted or welded, with welded boilers being the more recent construction. (5) Again, the converse is not always true. In fact the same can be said of hull construction.
While most boilers are fire-tube, the opposite configuration of a water-tube boiler, where tight clusters of narrow tubes of water are directly heated by the furnaces, is much rarer. It is generally specific to warships and some newer steamship designs. (6)
A warship is obvious from the rest of the wreckage, but finding water-tube boilers on a merchant ship could be a critical clue to identification.
The most common merchant shipwrecks we have with water-tube boilers are WW2 Liberty ships, although these boilers were by no means standard across all Liberty ships.
A boiler can also tell you the line of the ship, to help you get oriented at the start of a dive, especially as most skippers like to drop a shotline on the boilers. If it has not been displaced, the cylinder of a boiler generally points along the length of a ship.
Occasionally a boiler could be mounted transversely, and this is sufficiently rare to make it an important clue to identification. On the other hand, donkey boilers were often fitted transversely, so be careful which boiler you trust to point the way.(7)

ENGINES
A ship’s engine will usually be directly aft of the boilers. With nothing more than this, we now know which end is forwards and which is aft. Even if the boilers have moved from their original position, the engine and propeller-shaft will give the line of the wreck.
For the engine, I count the number of shafts and the number of cylinders. (8) Most steamships have only one shaft, but two or more shafts are not uncommon on bigger and faster ships.
Records may be simple and note “compound engine”, meaning just two cylinders. More detailed records could give as much as something like “eight cylinders and two shafts of triple-expansion engine”. This means two rows of four cylinders, with the low-pressure end of the three stages of expansion going into a pair of cylinders that work together, rather than a single larger low-pressure cylinder.
Alternatively, four cylinders in a row could also be quadruple expansion!

IF THE CYLINDERS ARE BROKEN and confused, looking below them at the crankshaft can also be used to figure out the number of cylinders by counting the number of crank sections and connecting rods. (9)
Divers’ measurements of piston diameters and stroke can be compared to builders’ records. Arthur Godfrey and Peter Lassey in the DIVER Guide Dive Yorkshire list these measurements for many of the Yorkshire wrecks, and used such measurements to confirm identification.
Steam engines work in multiple stages of expansion, with more stages being practical with higher steam pressures.
Single-cylinder engines are rare, because shipbuilders soon found that working with two stages of expansion, or compounding, enabled more efficient and powerful engines.
Engine design soon progressed to triple- and even quadruple-expansion, so in general two-cylinder compound engines are usually found on older ships.
However, long after triple-expansion had become the norm, compound engines continued to be fitted to some smaller ships such as coasters to save space.(10, 11)

MOST STEAM ENGINES ARE BUILT with all the ancillary bits on one side, leaving the other side clear for the engineering crew to access the engine.
If it is still there, a maker’s plate would usually be on this side, or on the end of an engine.
Behind the pistons of the engine, some larger and more modern engines could include a turbine, taking the last bit of energy from the steam of the low-pressure cylinders and adding it to the shaft. Such configurations are sufficiently rare to be good enough to tie down the identity of a wreck.
We don’t see many quadruple-expansion engines, because the engineering and economics of building and running them was unfavourable compared to doing the whole thing with steam turbines.
Turbine-only propulsion was generally used only on warships and passenger liners, though some freighters also had turbines.
In particular, WW1 Standard ships, the Great War equivalent of the WW2 Liberty ship, had turbine propulsion.
Turbines are smaller and more boxed-in than the towering machinery of a steam engine. If you aren’t looking for them, they could easily go unnoticed. Besides which, turbine blades would be made of non-ferrous metal, so turbines are often commercially salvaged. (12)
Easier clues to spot are the massive gears that would connect a fast-spinning turbine to a slower-rotating propeller-shaft. (13)
Nearly all modern ships are diesel-powered, so there are no boilers and the engine looks like a giant truck engine, with rockers and valves on top, injectors poking into the cylinders, and inlet and exhaust manifolds on either side.
Wrecks of large diesel or “motor” ships are rare before WW2 and are still in a minority after then, so identifying a wreck as a motor ship is another good clue to its age and identity. (14)
Diesel engines will also usually drive the propeller through a gearbox, although they don’t need as much gearing down as a fast-spinning turbine.
Other variations, though uncommon with wrecks in our home seas, are the diesel-electric or turbine-electric drives.
Here, rather than linking to the propeller-shaft through a gearbox, the diesel engine or steam turbine drives a generator, and the propeller is turned by an electric motor.
Things for a diver to spot are the electric generators and motors and the cables between them, though these would also be prime targets for commercial salvage because of all the copper wire, so are even rarer than the wrecks that contained them.

THRUST BEARING
The shaft leaves the engine via a thrust bearing. This would take the forward force of the propeller from the propeller-shaft and transfer it to the frame of the ship.
A thrust bearing is either boxed-in and complete or, because one surface would be made of non-ferrous metal, broken open for commercial salvage, leaving a series of rings on the propeller-shaft. (15)

PROPELLER-SHAFT
A propeller-shaft is either long enough for the boilers and engine to be roughly in the middle of a ship, or so short that the engine-room is aft and the shaft pops straight out of the stern.
With the engine-room in the middle of the ship, holds would be distributed forward and aft of the engine. With engine-room aft, everything else would be forward.
Smaller ships such as coasters, trawlers and tugs generally have the engine located aft with a short propeller shaft. So would most modern ships.
However, at the time most of our wrecks were sunk, the usual steamship configuration was for the engines to be in the middle. (16)
Half of the ship was pushed ahead of the thrust bearing, whilst the other half was pulled along behind it.
The exception for larger wrecks would be tankers and some purpose-built colliers, where the engine would be located aft. This was initially for safety, so that sparks from the funnel would be behind the flammable cargo.
On modern ships such a layout is more for convenience of construction and cargo-handling.
Tankers are usually easy enough to identify by the flat deck with pipework and valves. But if you dive a reasonable-sized ship with lots of hold space and steam engines aft, there is a fair chance that it was built as a collier.

PROPELLER
On a single-shaft ship, the propeller-shaft exits the hull where it narrows right at the stern. (17)
With two shafts, the lower part of the hull could split into one narrowing section for each shaft, or the shafts could leave the hull further forwards either side of the keel, to be supported on A-frames. (18)
These sections of the hull are very robust, and often survive where other parts of a ship have been smashed out of all recognition.
Multiple shafts are common on warships, passenger liners and ferries but relatively rare on other ships, so simply identifying them can greatly narrow down the identity of a wreck.
The speed at which a propeller is designed to operate has a big influence on its design. Slow, chugging merchant ships can get away with simple props with four long blades (19), sometimes bronze, sometimes iron or steel, though bronze units have usually been salvaged.
A faster-spinning propeller, for example on a warship, will typically have three blades that are much fatter, and a nicely tapered hub. Together, these enable the propeller to work at higher speeds without cavitating. (20)
More modern propellers often have much more sophisticated designs, with bolt-on blades, adjustable pitch, wider rounded blades and sometimes more than four blades.
Propellers often have manufacturers information stamped on them. This will be erased by concretion on an iron propeller, but on a bronze one this could provide a clue to identification.

HULL & FITTINGS

Having followed the propulsive machinery aft, in part 2 we will discuss the hull and fittings.
Like the propulsive machinery, variations in how a hull is built and fitted out can tell us plenty about the ship, and how to navigate the wreck.