When I'm out diving, sketching and photographing wrecks, it's not unusual to be asked: 'How do you find your way round a wreck?'

To experienced divers this might seem a trivial question, but it's something that many up-and-coming divers would like to be better at.

The method that works for me breaks down into three parts: have an idea of what the ship originally looked like; build a mental picture of how it sank, settled and has subsequently broken; and be able to identify and name individual elements.

what the ship originally looked like
Original photographs or drawings are the obvious source of this information. It doesn't have to be fantastically detailed. Even a silhouette can be enormously useful.
Photographs and drawings for many popular wrecks can be found in diving guide books. A quick 10 minutes looking at a photograph while your hardboat chugs out to the wreck site will stick in your memory, so you can recall useful tit-bits during the dive.
Rather than limit yourself to reading dive guides, pick up a few general books with pictures of ships in them. Look through the pictures and pigeonhole each one into a generic class.
Ship design, layout and construction are reasonably standardised, driven by the purpose of the ship. For example, the classic merchant ship layout has a central superstructure and engine room, one to three holds forward and one or two holds aft. Wrecked ships of this layout are so common that we will use it for our detailed example.
The other common merchant ship layout is all holds forward with a superstructure aft. Most modern ships follow this pattern, as do older tankers and smaller coastal freighters.
As you get used to looking at ship pictures, other general designs will soon fall into place for trawlers, passenger liners, submarines and warships. When you dive, you can now make reasonable assumptions based on the class of ship.
When it comes to diving an unfamiliar wreck and there is no picture in the guide book, simple information such as '1500 ton steamship, sank 1925' will enable you to predict that the wreck follows the classic layout of central superstructure with holds forward and aft.

build a mental picture of how the ship sank, settled and broke
To simplify matters, a ship can have come to rest upright, on one side, or upside-down. In shallow water, a ship is much more likely to have sunk upright. There would just not be time for it to capsize or turn over between leaving the surface and coming to rest on the seabed.
There are always exceptions. Even in shallow water, a ship could capsize or turn turtle before sinking, and some types of vessel are much more likely to turn turtle than others.
For example, large warships are top-heavy because of all the armour and armament carried. In Scapa Flow all the cruisers rest on one side at about 30m, while the heavier battleships are upside-down in deeper water.
So the next question to ask before diving a wreck is: 'Which way up is it?' Even when unfamiliar with the site, on an intact wreck your boat skipper can make an educated guess from the echo-sounder trace.
It's never quite that simple. A ship could have jammed onto rocks, in which case it was most likely upright, but did it go straight in, hit a glancing blow, or get swept in sideways? Shipwreck books and local pubs often have photographs of stranded ships tight in against the rocks.
As a wreck decays, either with age or pounded by the waves when it is jammed against the rocks, some bits are stronger than others and are more likely to be intact.
The bow and stern are much stronger than the rest of the hull of a ship. Even when the rest of the ship has decayed flat to the seabed, there might still be significant structure at the bow and stern.
This month's Wreck Tour of the cable ship Behar is a classic example of this pattern of collapse, and last month's tour of the ammunition-carrying freighter Basil shows a less-developed stage of a similar pattern of collapse.
It is not unusual for a ship to sink bow- or stern-first, so that either end of the vessel takes the full force of hitting the seabed, and the wreck crumples or breaks at the first or last hold.
When a wreck comes to rest gently on a relatively flat seabed, most of the hull will be supported, except for the bow and stern, which are held up by the rest of the hull. Again, as the hull deteriorates the bow or stern can break off, typically falling away from the ship and to one side.
Later on, the rest of the ship might even collapse in the opposite direction!
The next strongest line on a wreck is the keel and lower part of the hull. The upper part of the hull and deck can collapse, while the lower hull and keel remain reasonably intact.
Finding the line of the keel will show you in which direction the wreck lies, but it is usually on the less-interesting side of a capsized and broken wreck. Head perpendicular to the line of the keel to the opposite side of the wreck to find most of the interesting parts of the ship.
The deck and upper works of older ships were often built of wood over a steel frame.
Deck-fittings that have to take a heavy load, such as winches, bollards and masts, would be mounted on a steel plate secured to the frame of the ship. The largely wooden superstructure will have disintegrated, even when the rest of the hull is in good condition.
Occasionally a ship will have broken in two before sinking, in which case you will have to consider the relationship between the two parts of the wreck and how each part came to rest.
Think about how the ship sank and the subsequent pattern of collapse during the first few minutes of a dive. The mental picture will help you navigate the wreck and locate all the key features.

settled and broken

The wreck of the Bretagne in Devon is upright and intact. Steel framework from the amidships superstructure partially survives, but all the wooden parts have decayed.

The Gascony in Devon is in a similar state of collapse, but originally came to rest on its starboard side and has now broken mostly upside-down. Again the bow is intact and fallen to port. The stern is completely upside-down, and two of the four boilers have fallen out of position, one on end.

The state of collapse of the Dakotian off Pembrokeshire has been helped along by the wreck being partly cleared using explosives. The bow is the only forward part of the wreck maintaining any sort of structure, having fallen onto its port side. The stern is beginning to split from the aft part of the wreck and is falling to starboard. I suspect the only reason it has not decayed further is its sheltered location in Dale harbour.

The Volnay has been substantially salvaged and cleared. Despite the destruction of the rest of the wreck, the bow and boilers are recognisable, as is the rudder post at the stern.

??other types of ship

Battleships are so top-heavy with turrets, guns and armour that they always turn upside-down. All the battleship wrecks in Scapa Flow are inverted, as are those at Bikini Atoll. Even the relatively shallow wreck of the Hood, scuttled in 18m across the entrance to Portland Harbour, managed to turn turtle as it sank, and that was when the intention had been to sink it upright!
Cruisers are smaller ships designed to be lighter and faster. They are less top-heavy, so are less predictable in the way they sink. Although the cruiser wrecks in Scapa Flow all rest on one side, I have dived other cruisers that have sunk upside-down.
 Battlecruisers are the size of battleships but more lightly armoured, so they have the speed of a cruiser. Like cruisers they are less predictable in the way they sink.
 Fortunately, even when upside-down the armoured strength of the superstructure and turrets causes a warship wreck to tilt one way or the other, leaving the superstructure and armaments exposed along one side between the deck and the seabed.

Modern freighter design is pretty much standardised, with holds forwards and a towering superstructure aft built entirely of steel, high enough to see over the rows of containers that would be stacked on deck. Unlike older ships with wooden superstructures, a steel superstructure will often remain intact, even after other parts of the ship have collapsed.
 The Aeolian Sky was only a year old when it sank following a collision in the English Channel in 1979. The wreck rests on its port side. In only 23 years the holds have collapsed towards the seabed. The stronger bow and stern are virtually intact, with the steel superstructure stretching across the seabed. Other notable features are the cargo masts and derricks laid neatly on the seabed, and particularly the huge central mast.

Submarines are characterised by their double hull: a very solid cylindrical inner pressure hull and a less robust outer hull. Inside, the pressure hull will be taken up mostly with diesel and electric motors, stacks of batteries and a very cramped space for the crew.
 Other machinery such as compressed air cylinders, steering and diving mechanisms, anchors, winches and torpedo tubes is located between the two hulls, with only control rods piercing the pressure hull to allow the crew to operate the external machinery.
The UC70 was bombed, depth-charged and sank in 1918. As is common with submarine wrecks, the outer hull has mostly decayed, leaving an intact pressure hull surrounded by a litter of machinery that would normally have been hidden between the two hulls.

  The term trawler covers a vast range of ship sizes and designs. We will focus on the classic steam trawler, as so many were converted for use in various wartime roles and subsequently came to grief.
 The features that characterise all such wrecks are an over-sized boiler and engine filling the aft part of the hull, providing an excess of power for towing the trawl nets, a hold occupying the forward part of the hull, and (unless removed during conversion for military use) a large trawl winch typically running across the back of the hold.
 In military use, lighter guns would be mounted at the bow and stern, with possibly a heavier main gun on a deck built over the hold.