YO-65 class tankers in various stages of construction and fitting-out during World War II. Many examples of this class of small tanker are found along the coast as artificial reefs.
Note the all-welded construction
New Jersey Scuba Diving
Hull Construction - Iron & Steel
Iron and steel began to replace wood in ship construction in the middle to late 1800s. Timber-poor Europe ( especially England ) led in the development of iron ships, while America, with it's vast reserves of lumber, continued to build wooden ships for some time longer. However, as the economical size of ships grew to surpass what could be built of wood, America too began constructing iron ships. For a few years, composite ships were built with iron frames and wooden skins, as seen at right, but difficulties with corrosion between the wood and metal soon led to hulls built completely of metal, both sailing ships and steamers.
Ship design is a mass of compromises. There is a balance to be struck between the speed that is desired for the ship to make the fastest possible passage, and the fuel consumption, because getting this balance wrong at a time when oil prices are rising can make a ship rapidly uneconomic. There are the dimensions of the ship - length, breadth, depth and most important draught - to be considered, because there is no point designing a ship that will be too big or too deep to get into the ports she is scheduled to visit. The type of cargo will dictate the design of the increasingly specialist ships of today - some vessels are so specialized that they can carry but one commodity. Naval architects design a ship according to the needs of the ship-owner, who will supply certain broad criteria, such as the cargo requirements, the speed and fuel consumption, along with the dimensional "envelope" and any special operating conditions such the requirement to run in ice, or pass under low bridges.
Modern CAD drawing of steel ship construction
Unlike new designs of cars, or aeroplanes, there is no such thing as a prototype to assist a naval architect in refining his design: he has to get it right first time. However the modern naval architect is assisted by powerful computers, and may also use test tanks to try out models of the vessel, to check its sea keeping characteristics and performance in different weather conditions, before the shipbuilder starts to cut steel to build the ship as designed.
There is a great deal at stake, and naval architecture is a precise science, although experience of what has been designed and built before plays an important part. The ship has to float, but at exactly the draught that has been specified. Stability is critical, both from the point of ability of the ship to behave acceptably in rough seas, and so that the loading of cargo makes her neither top, nor bottom heavy. The underwater shape of the hull is a critical factor in the performance of the ship, in the resistance the hull offers to the passing water, which will greatly affect the power requirements and fuel consumption to meet the desired speed. The designer is looking to optimize the hull shape, so that the compromises which have been made between the various criteria result in the best possible solution and the most efficient hull design.
Because a ship has to operate in a hostile environment, much of the naval architect's work will be devoted to ensuring that the ship is strong enough for a long, hard life of 25 years and upwards in a corrosive, salt- encrusted atmosphere. There is no room for error and the designer has rules provided for him by the classification societies which prescribe strengths and thicknesses of the structure, which has to cope both with the stresses of shifting weights as cargo is loaded and discharged, or tanks pumped out, and the dynamic and often violent effects of the sea. A ship may be made of steel, but it must be assembled in such a way that there is built in elasticity to cope with the huge forces of the sea that will flex the whole ship, subject it to huge weights and pressures, and threaten, if it were not well designed, to tear the ship apart.
Mid-ship cross-section through an iron or steel ship
An iron ship is constructed similarly to a wooden ship. First a keel plate is laid, then the stem and sternpost are erected. U-shaped "frames", the equivalent of ribs in a wooden ship, are constructed and attached to the keel plate. The frames are capped along the centerline with a keelson plate for additional strength, and longitudinal stringers are installed between the ribs. Finally, inner and outer hull plating is attached, decks are constructed inside, and machinery and other finishing features installed.
The uppermost level that runs the full length of the hull is called the main deck. Below that are various lower decks, depending on the size of the vessel. The superstructure is built up above the main deck, and its levels are also referred to as decks, although this is technically incorrect. The breezy uppermost level of the superstructure is called the hurricane deck, and below it are variously the boat deck, cabin deck, promenade deck, and others, depending on the size of the vessel and the creativity of the people that make up names for decks.
Frames, hull plating, and fallen deck beams inside the Mohawk.
On old vessels, frames, keel, hull plates, and all other major components were attached using overlapping construction and rivets ( right ) or bolts. Rivets were superior, since they provided a near-watertight joint with no special sealing. Prior to World War II, welded ship construction was considered to be experimental, but during the war the technology was developed to a much greater degree, and replaced riveting entirely. However, most of the shipwrecks a diver will encounter predate the war in construction, and will therefore be of the older riveted construction. Notable exceptions include the Pinta and the Stolt Dagali.
Riveted ( left ) and welded ( right ) joints
Empty rivet holes on fallen hull plates on the Mohawk. The rivets seem to corrode much faster than the rest of the material, and have literally dissolved in the sea.
Iron ship construction displayed on the Dykes. The keelson runs across the bottom of the picture, with deep bottom frames extending out from beneath it. Behind that, the side of the hull is broken off and lying in the sand, with the relatively smaller upright parts of the frames evident. Inner hull plating has been carried away, leaving just rivet holes in the frames.
Liberty ships under construction during World War II
A modern shipyard ( in Poland? ) the enormous crane is typical - nowadays ships are built in modular sections which are then assembled and welded together. This method of construction was pioneered with World War II Liberty Ships.
Commercial shipbuilding has virtually ceased in the United States, undercut by competition from eastern Europe and the Far East. These are liquefied natural gas tankers under construction in Massachusetts in the 1970s.
The only major shipbuilding that remains in the US is military. Here, a nuclear super-carrier takes shape. Construction is completely modular, facilitated by the huge gantry crane.
Note the all-welded construction
Iron ships are heavy, and tend to settle into the bottom after sinking. The Algol has settled almost 20 ft into the mud since it was put down in 1991. Most iron ships are not left to the mercy of the sea, however. Because of their size, latter-day iron and steel shipwrecks usually pose a great threat to navigation, and are therefore blown up or wire dragged by the government. This generally results in a disorganized jumble that resembles a junkyard more than a ship.
Side-scan sonar image of a steel wreck - the Mohawk - that has been dynamited and wire-dragged to bits. Which end do you think is the bow ?
1956 by Ken Marschall
1991 by Ken Marschall
2003 by Ken Marschall
2016 side-scan sonar - UNH
If you could see the wreck through crystal-clear water, this is what the Andrea Doria might look like over the years. The paintings are slightly inaccurate, as they show the wreck as though it was resting on a hard surface, when in fact it has been slowly sinking into the bottom, as can be seen in the final image.
The deterioration of the wreck is typical for a steel ship. In deep water, the Andrea Doria was not regarded as a hazard to navigation, and never wire-dragged or demolished with explosives. Compare to the Mohawk below, which is an unrecognizable debris field after over 8 tons of dynamite were used to reduce the wreck. This is not actually extraordinary, 25 tons were used on the Fort Victoria, which sank in the shipping lane right outside New York Harbor. Practically nothing is left of her.
Side-scan sonar image of the Mohawk. The Mohawk sank upright in only 80 feet of water. The stern is at the left, and the wreck is buried 10-15 feet into the sand.
The little Pinta broke up in a very similar fashion. I can remember when the wreck was completely intact, like the sketch above. First, the superstructure collapsed. The the center of the upper wall of the hull collapsed, and the bow broke off. In fact, the Pinta now looks more like the side-scan of the Doria above than the drawing. Next the upright wall that was once the ship's bottom will fall over, and the wreck will lose any overall resemblance to a ship.
Knowing how a ship deteriorates after sinking can be a great aid to navigating the wreck. When you realize where you are and how you are oriented, it stops looking like a lot of random junk, and starts to make sense. "These are the boilers, and there is the engine, so the bow must be that way!" I seldom use a wreck reel, even on the Mohawk. Of course, when visibity is poor, it is foolish not to use every tool at your disposal.
I make no claim as to the accuracy, validity, or appropriateness of any information found in this website. I will not be responsible for the consequences of any action that is based upon information found here. Scuba diving is an adventure sport, and as always, you alone are responsible for your own safety and well being.
Copyright © 1996-2016 Rich Galiano
unless otherwise noted