A reconstruction of Denbigh's engines, based on archaeological and historical research.
The following description of Denbigh's engines is condensed (pun intended) from a longer essay by Denbigh Project volunteer Gene Shimko.
Denbigh’s two steam engines are categorized as "diagonal" engines, based on the way they are mounted in the hull. The axis of each engine’s cylinder is aligned fore and aft. The two engines are aft of and below the paddlewheel shafts, so that the cylinders’ axes lie at a shallow angle, about 15 degrees from horizontal. The engines, which were built at Lairds’, are a matched pair, with each being a mirror image of its twin across the ship’s centerline. The top of the valve chest on each engine is about level with the upper surface of the deck beam amidships, and the engines themselves sit relatively high up in the hull.
The specifications of Denbigh’s engines, taken from both historical documentation and examination of the site, are as follows:
| Nominal power: Indicated power: Cylinder internal diameter: Cylinder external diameter: Length of stroke: Steam pressure: Engine speed: |
100 horses (total for both engines) 500 horses (total for both engines) 40 inches (102cm) 48 inches (122cm) 3 feet 6 inches (107cm) 22 lbs (about 1.5 atmospheres above ambient) 38 to 40 rpm max. |
Each engine consists of a single cylinder. They are double-acting, that is, steam from the boiler is released alternately into each end of the cylinder, so that actual power is applied to the piston moving in both directions. At the end of each stroke, waste steam is vented directly into a condenser. The purpose of the condenser is to convert the spent steam back into liquid water that can be returned to the boiler for recirculation. Although the condensers on Denbigh have not yet been excavated, it was common at the time for marine condensers to use an injection of relatively cold seawater to quickly condense the spent steam. This was an important part of the engine’s operation, as the partial vacuum that resulted in the condenser helped to "pull" the piston in the cylinder, and produce substantially more force than the engine’s low operating pressure would suggest. Indeed, early steam engines were often referred to as "vaccuum" engines, because it was this effect that provided the majority of force working on the piston.
Normal operation would be to keep both engines and drive trains mechanically connected so that the paddlewheels turned at the same speed. This eliminated much needless minor adjustment of each engine in order to match paddlewheel speeds exactly. However, each engine could be run independently, in both forward and reverse, and turn the paddlewheel on that side of the ship independent of the engine and paddlewheel on the opposite side of the ship.
Click here for a large (600kb) animated GIF image of Denbigh's engines.
The injection of steam into the cylinder and the exhaust of spent steam into the condenser is controlled by a rectangular valve chest atop each cylinder. Although the valve chest on the port side engine, the only one excavated, appears to have been partially disassembled by salvors in the 19th century, it is possible to describe its functioning in general terms, based on contemporary engineering texts.
A concept model showing a possible arrangement of one of Denbigh's engine cylinders. The boxlike valve chest, at top, feeds steam alternately into one of two channels leading to either end of the cylinder. This steam pushes the piston back and forth, giving the engine its basic motion. At the same time steam is circulated through cavities in the cylinder walls. This "jacketing" improves engine efficiency by keeping the cylinder warm and reducing condensation within the cylinder itself.
There are two valves in the chest, a cutoff (or expansion) valve and the main slide valve. The main slide valve assembly sits directly on top of the cylinder and slides on a machined face. The cutoff valve consists of two blocks connected by a double reverse screw. The cutoff valve slides within the main slide valve assembly, sitting directly on the top of the main slide valve’s ports. In effect, the cutoff valve regulates live steam to the main valve ports, which in turn passes it into alternate ends of the engine cylinder, pushing the piston back and forth. At the same time, the spent steam from the previous stroke is exhausted out of the cylinder to the condenser, where it will be converted back to liquid water and returned to the boiler. The cutoff and main valves are themselves powered by the rotating paddlewheel shaft via eccentrics (see below).
An overhead view of Denbigh's engines and paddlewheel shafts. Red arrow indicates the connecting rod recovered at the end of the 2000 field season.
The motion of each engine’s piston rod was transferred to the paddlewheels via a connecting rod. This iron rod, which measured about eight feet (2.44m) in overall length, changed the engine’s fore-and-aft motion to a rotary motion at the crank on each paddlewheel shaft. The connecting rod of Denbigh’s port engine was recovered during the 2000 field season. As detailed drawings of the area were being prepared, it became clear that the ends of the connecting rod had been detached from the piston rod and paddlewheel crank, probably in an effort to salvage the engine's bronze fittings shortly after the ship grounded and burned. This 19th-century salvage attempt greatly simplified the recovery of the connecting rod 135 years later. The connecting rod, as recovered, was estimated to weigh 1,150 pounds (520kg).
What appears to be a single shaft running across the center of the ship with a paddlewheel on either end is actually three shafts – one for each wheel and a so-called "intermediate shaft," that was used to connect the two to allow them to operate in unison. A pair of eccentrics – disks mounted off-center on each paddlewheel shaft – were linked to the cutoff and main slide valves in the valve chest to synchronize the flow of steam in and out of the engine with the rotation of the main shaft.