Why Rena carried two grades of fuel oil

The Containership Vessel Rena hasn't broken in half yet, but the hull is seriously fractured, given the flexing of the ship. Here's a diagram from Maritime NZ showing how the ship sits half-on, and half-off, Astrolabe Reef. The bow is on the right:

Each technological disaster and close call offers a window into how systems work, and how they fail. The grounding of the Rena is an opportunity to learn about the modern combination of fuel and machinery that makes big ships get up and go.

By the early 1900s, the most common approach to powering a steamship was coal-> steam-> reciprocating engine. Ships carried thousands of tons of coal in storage compartments called bunkers, and men called coal trimmers shoveled it within reach of firemen, who heaved it into fireboxes, which heated a bank of boilers to raise steam. Later, shipbuilders turned from reciprocating engines to the more compact and powerful steam turbines. 

Loading and handling coal on board ship was a nuisance, dirty, and labor-intensive. Coal for bunkering ships began a long decline following the amazing discovery of Spindletop Field near Beaumont, Texas, in 1901. Ships began experimenting with crude oil, but the quantities required were large and oil had better uses. Steamship engineers turned to steam-generating systems that burned the cheapest grade of petroleum-derived fuel, a tarry residue from refining called Bunker C. 

Some power plants use similar stuff today; it's called No. 6 Fuel Oil, or a blend of No. 6 and No. 2.

Number 6 and Bunker C must be heated before pumps can force the stuff from storage tanks, through pipes, and into burner heads mounted under the boilers. Normally this is not a problem, since ships have a surplus of waste heat and use it to keep the tanks hot.

Later, shipbuilders began moving away from steam plants to marine diesels, but those gulped expensive diesel fuel. Filling an ocean-going tugboat with a load of diesel can cost upward of $30,000.

What to do? Big ships with diesel engines, and now some big tugboats, turned to a heavy fuel oil (called HFO) similar to Bunker C, heating it and injecting it at high pressure into the cylinders of a diesel engine. 

In a busy containership or tanker harbor, a crowd of heavy-fuel-oil-burning ships would contribute much to local haze. Many ports, such as Long Beach, California, prohibit ships from burning it in their waters. One legal solution is for a ship to carry a smaller quantity of gasoil or diesel fuel for harbor use, because these burn (relatively) more cleanly in marine diesel engines. (The biggest of shipboard diesel engines are very big, weighing 2,300 tons.)

Here's what a smoke plume from untreated heavy fuel oil looks like, when burned in an urban setting (photo from the Environmental Defense Fund):

So that's why the Rena approached Astrolabe Reef on October 5 with two kinds of fuel in its bunkers: 1,400 tons of cheap heavy fuel oil, and 300 tons of diesel to use where HFO is banned, like harbors and Antarctic waters.

Once the ship lost power, the heavy fuel oil started cooling and congealing. Here's what HFO looks like when washed up on a New Zealand beach:

There's about a thousand tons of such goo still on Rena, so we can only hope the salvors can get their augers and steam-generators working and force it from the tanks before the ship breaks in two.