Valve Operation

In order for the Maple Leaf's double acting engine to operate, steam had to be supplied to the cylinder at one end while being exhausted at the other. This required a system of valves timed to open at the proper point in the piston stroke. Once steam pressure in the boilers became sufficient to move the piston, the engineer started the engine by manually opening and closing the valves that supplied steam to the cylinder. After reaching speed, the engine was made self-acting by using mechanical motion to automatically operate the valve gear. To accomplish this, the engineer engaged an eccentric that rotated with the paddle shaft. A rod linked the eccentric to a rocker shaft in the valve assembly on the engine. As the paddle shaft rotated, the offset center of the eccentric imparted a back and forth motion to the rod. This movement "rocked" the rocker shaft, opening and closing the engine cylinder inlet and exhaust valves. As long as the engine turned the paddle shaft the eccentric operated the valves. To stop the engine, the engineer simply disengaged the eccentric rod from the rocker shaft stopping the valve action. Valve operation is also integral to reversing the engine and backing the ship.

The only remaining components of the valve gear are two eccentric bands that hang loose on the starboard paddle shaft, one on each side of the coupling. They are badly bent and the eccentric arms are broken off. Their condition made it impossible to determine the throw, or horizontal distance, traveled by the eccentrics. Each band is a slightly different size, possibly due to corrosion or concretion formation. The starboard band is 1 3/4 inches wide and 1 1/4 inches thick. The port band is 2 inches wide and 1 inch thick.

The Stevens cut-off gear, invented in 1848, became the most commonly used valve operating system by the mid 19th-century. It incorporated the principle of steam expansion to operate the engine more efficiently. Supplying steam to the engine during the entire piston stroke did not take full advantage of the expanding steam. The Stevens cut-off, and other cut-off devices, stopped the steam supply to the cylinder at one half or less of the piston stroke. Using a smaller amount of steam to maximum advantage saved fuel and lowered operating costs (Holly 1987:225; Whittier 1987:17). The Stevens system employed two eccentrics, one to operate the steam valves and the other to operate the exhaust valves. It enabled the engineer to adjust the point where the steam valves closed in the piston stoke without affecting the exhaust valves (International Correspondence School [ICS] 1907: 57).

There is no way to tell for certain if the Maple Leaf employed this commonly used system but it is highly probable. A cut-off system could have been installed when the original engine, taken from the Sovereign, was rebuilt in 1850 and placed in the Maple Leaf. The new engine installed in 1854 certainly had a cut-off system to lower operating cost in the competitive steamboat market.

The Maple Leaf is unusual in that both eccentrics are on the same side of the paddle shaft. Normally, one eccentric is placed on the port and starboard shafts and connected to the rocker shaft on each side of the engine by eccentric arms (Figure 17A). The rocker shaft is actually a two piece bar that pivots on a common bearing in the center. The starboard rocker shaft controls the exhaust valves and the port rocker shaft controls the steam supply valves (ICS 1907:56-57).

The Maple Leaf may have had a cross link assembly between the shaft and engine similar to Figures 17B and 18. This configuration would transfer eccentric motion across the engine axis to the port side steam supply rocker shaft. This configuration is used in the Chesapeake and Delaware Canal Pump House Museum near Summit Bridge, Delaware (James Valle 1994, pers. comm.). Another possibility is that eccentrics on the port shaft have been removed. Later versions of the Stevens cut off used four eccentrics, two on each side, to increase efficiency. One set served for forward motion and the other for backing (ICS 1907:59). In either case, more field work is needed to answer this question.