How Much Coal?
Denbigh burned coal for fuel. When they could get
it, blockade runners preferred to use hard anthracite coal because it burned cleanly and
produced little smoke. In practice, though, anthracite coal was hard to come by, and most
blockade runners were forced to burn whatever they could find in sufficient quantity.
Denbigh Project volunteer Gene Shimko recently
made some estimates of the blockade runner's fuel consumption. Using contemporary
engineering texts, Shimko found that simple steam engines like Denbigh's
typically burned about five pounds (2.27kg) of coal per horsepower per hour. Superheaters, like that fitted to Denbigh's boiler,
increased fuel efficiency and may have reduced this consumption to 4.5 pounds per
horsepower per hour or less.
Using the record of Denbigh's first speed trial
in 1860, Shimko calculated the approximate power output of the ship at various speeds:
(nautical miles per hour)
As can be seen, a small reduction in speed will produce a
tremendous saving in the amount of power required, and at higher speeds even a small
additional increase will require a great deal more power.
Over a long voyage, a ship would generally
try to run at the most fuel-efficient speed, balancing fuel savings against the need to
get to port as quickly as possible. In Denbigh's case, several contemporary
sources suggest that by 1864 she was not capable of running at much more than eight or
eight-and-a-half knots. How efficient Denbigh's fuel consumption was is open to
question. The ship's boiler was said by one observer to be "in a bad condition,"
and this might have caused Denbigh to burn more coal than ordinary. On the other
hand, William Watson, an experienced blockade running master,
remembered Denbigh as being "light on coal."
Next, let us look at her route. When she was
new, running between Liverpool and Rhyl, carrying a large
amount of coal was not a problem, as her route was only about two hours in each direction.
But running the blockade was a different matter entirely. From August 1864 to the end of
the war she ran between Havana and Galveston, a direct distance
of 764 nautical miles. There was no significant amount of coal available in Galveston, so
blockade runners had to carry enough to complete a round voyage from Havana to Galveston
and back. Allowing a little extra as a reserve, it seems likely that Denbigh
sailed with a supply sufficient to carry her a minimum of 1,750 nautical miles.
How much coal would that be? At 8.7 knots, Denbigh
could cover 1,750 miles in just over 200 hours. Assuming the (attributed) poor condition
boiler negated any improved efficiency provided by the superheater, at five pounds of coal
per horsepower per hour, that comes to about 150,000 pounds (68,200kg), or 75 short tons.
How much space would that require? Different
types of coal have different densities; a given weight of hard, dense anthracite coal
takes up considerably less space than bulky bituminous coal does. We can figure, though,
that on average a cubic foot (0.03m3) of coal will weigh about 50 pounds
(22.7kg). Using these figures, Denbigh's coal bunkers must have had an absolute
minimum volume of about 3,000 cubic feet (85m3). Given the variable
density of coal, and that much of the time her fuel supply probably was poorer, light
bituminous coal, it seems likely that her actual bunker capacity was at least 10% higher,
or about 3,300 cubic feet (93.5m3).
Denbigh probably carried a minimum of about 3,300 cubic feet (93.5m3)
of coal on her later voyages. Such a volume would fill a box six feet high, ten feet wide
and fifty-five feet long.
Where, exactly, would that large volume of
coal have been stored on the ship? The obvious answer is that it had to be stored close by
where it would be needed, that is, immediately adjacent to the boiler furnaces. Beyond
that, however, it's all speculative. From an archaeological standpoint we don't know, as
currents have scattered pieces of coal all over the wreck. It is possible, however, to
make a few guesses. On either side of the ship, outboard of the boiler and engines, is a
narrow space that would have served well as a coal bunker. These spaces are small, though,
and even if they ran the full 39-foot (11.9m) length of the engine room, they still would
provide less than half of the volume necessary. It seems certain that additional space
forward or aft (or both) of the engine room was converted for use as a coal bunker.
Illustration showing four speculative arrangements of bunkering space aboard Denbigh.
Likely bunkerage along either side of the engine compartment is shown in dark gray; the
arrangement of other possible bunker space is shown in orange. In each case, the total
volume shown is the same, about 3,300 cubic feet (93.5m3). Click on the image
above for an enlarged version.