Steam, iron hulls and the load line

• Paddle wheels work in shallow water and are simple to build and repair, but as the ship rolls one wheel lifts clear and the other digs in. They suit rivers, lakes and sheltered crossings.
• The screw propeller stays under water at sea and is smaller for the same thrust. Brunel changed the Great Britain from paddles to a screw while it was being built; on its first crossing it averaged 9.4 knots (SS Great Britain Trust).
• The engine is the steam engine, built for a ship and set low in the hull. Sea water scales a boiler, so later marine engines condensed their steam and fed the same fresh water back.

Riveted plates of wrought iron, later steel, over iron frames: lighter than timber for a big hull, and no shipworm. Iron hulls allow watertight bulkheads: walls across the hull, so that one hole floods one compartment and not the ship.

• Carry every bulkhead above the waterline the ship would float at with that compartment flooded. Water that can pour over a bulkhead into the next compartment sinks the ship as surely as no bulkhead.
• Doors in bulkheads are shut at sea, and closable from above.
• Paint the iron: rust eats a plate from both sides.

In the eight months before the US Steamboat Act of 30 August 1852, seven boiler explosions killed more than 700 people. The Act required every boiler to be tested with water under pressure before use and every year after, fitted with a safety valve, held to a maximum working pressure, and run by a licensed engineer (the Act).

The Sultana, built for 376 passengers, was carrying almost 2,400 when its boilers exploded on the Mississippi on 27 April 1865; up to 1,800 died (Encyclopedia of Arkansas).

• Test every boiler with water — never air or steam — at a pressure above its working pressure, before use and every year.
• A safety valve that can pass all the steam the boiler can make, never tied down or weighted.
• Water above the lowest gauge at all times: a boiler run low overheats its plates, and when water reaches them it bursts.
• Licensed engineers, and a limit on the number of people aboard, enforced.

Shipowners insured for more than their ships were worth sent them out overloaded; Samuel Plimsoll called them coffin ships. The Merchant Shipping Act 1876 made a load line compulsory on British ships (Royal Museums Greenwich), and the International Convention on Load Lines (1966) sets one on every seagoing merchant ship (IMO):

• A ring 300 mm across and 25 mm wide, crossed by a 450 mm line whose upper edge passes through its centre, placed amidships at the ship's assigned summer freeboard below the deck line.
• Lines 230 mm long, 540 mm forward of the ring's centre: S summer, T tropical, W winter, WNA winter North Atlantic. Tropical and winter sit 1/48 of the summer draught above and below summer.
• F and TF for fresh water, aft of the vertical line: fresh water (1,000 kg/m³) floats a ship lower than the sea (1,025 kg/m³), so it may load deeper there and still rise to its marks at sea.
• The line is on the hull; loading to it is the master's duty, and checking it the port's.

The Titanic (1912) carried 20 lifeboats for 1,178 of the more than 2,200 aboard, which met the law of the day. The first SOLAS convention, in 1914, required lifeboats and lifejackets for every person aboard (IMO).

• Boat places for all, and davits that can launch them with the ship listing.
• A lifejacket for every person, in children's sizes for every child.
• Drills at sea, so every person knows their boat.