Historical records and an 1856 ambrotype (Figure 1) indicate the Maple Leaf had two cylindrical boilers placed on either side of the A-frame. These boilers were new when installed at the Kingston Marine Railway Yard in 1851 (DBW March 26, 1851). Although only the starboard boiler was examined, the port boiler is considered to be identical in design and location. The starboard boiler begins at 86.5 feet on the baseline and has a diameter of approximately 7 feet. While uncovering the walking beam, the aft end of the port boiler was found at 114 feet on the baseline, indicating both boilers are 27.5 feet long and pass under the paddle shaft.

The starboard boiler is badly corroded and covered with debris making it very difficult to examine. Only portions of the top and face were accessible. It is a return, fire tube boiler with the firebox facing forward. The remains of the smoke stack are 2.5 feet in diameter and centered 6 feet aft of the boiler face. The stack is encircled by the steam dome on top of the boiler.

The steam dome is badly damaged suggesting it may have suffered an explosion. Shredded metal plating flares out away from the chamber's center indicating a strong outward blast. In addition, this blast also caused the boiler face to separate and lean forward, creating a 5 inch gap. The fire box doors on the boiler face were found in the open position, offering further indications of an explosion. Roderick Ross used explosives to clear debris from the wreck site in 1883. A charge placed down the stack or near the boiler could produce this damage. A second possibility is that cool water flooding the engine room made contact with the hot boiler causing an explosion as the vessel sank. Testimony taken at the Board of Survey, however, does not support this. Chief Engineer Samuel Johnson is adamant in his testimony that a boiler explosion did not cause the initial sinking and he was still on board when the vessel settled to the bottom. Neither Johnson, nor anyone else, mentions a second explosion as river water filled the engine spaces.

Large debris and timbers fill the open space in front of the boiler limiting examination to the upper port side of the boiler face. Figure 12 is a reconstruction of the face combining data gathered in the field with historical information on similar boilers. Dashed lines on the starboard side and lower portions are a hypothetical configuration.

A glass water gauge and four gauge cocks, or try cocks, positioned on the upper left side of the face indicated water level in the boiler. Water level had to be maintained above the fire tubes while the boiler operated. Otherwise, fresh feed water touching the hot tubes flashed into steam, dramatically increasing boiler pressure and possibly causing an explosion (Ward 1860:23).

The Maple Leaf's glass water gauge is a ball float design with a lower vertical pressure chamber and an upper sight glass (Figure 13). The pressure chamber is open to the boiler at the top and bottom through short brass fittings, allowing boiler water to maintain an equal height in both vessels. Inside the pressure chamber, a hollow brass ball floats on the water surface. A brass wire soldered to the ball passes through a leather gasket at the top of the pressure chamber into a thick sight glass tube. Water level in the boiler is indicated by sighting the wire against a scale mounted behind the sight glass. The thick glass tube is mounted in a brass holder and held in place by a tensioning screw. A scale mounted on the holder, behind the tube, is marked in one inch increments beginning with one on the bottom and nine on top. A cock on the bottom of the pressure chamber drained excess water.

Many 19th-century glass water gauges have a different configuration that omits the pressure chamber completely. The sight glass is directly connected to the boiler at the top and bottom by metal fittings. For safety, these fittings have stop cocks to turn off pressure to the sight glass if it breaks (Lardner 1848:116). The Maple Leaf's glass water gauge does not have stop cocks to isolate the pressure chamber. It is only open to the atmosphere at the top, through the sight glass. The pressure chamber is designed to seal itself if the glass breaks. The solder holding the wire to the brass ball float is shaped to fit into a conical depression at the top of the pressure chamber. If the glass broke, boiler pressure forced the brass ball against the top of the pressure chamber causing the solder to seal the opening.

While a glass water gauge gave an accurate measure of water level, they were prone to breakage. Gauge cocks provided a reliable method of checking water level with slightly less accuracy. Four brass gauge cocks were mounted diagonally to the port side of the glass water gauge. The top and bottom cocks are 8.5 inches apart vertically and align horizontally with the top and bottom openings on the water gauge's pressure chamber.

Placement of the cocks on the boiler face indicates the highest and lowest water levels allowable in the boiler to permit safe operation. "The highest cock is at a point above which the water cannot be permitted to rise without encroaching upon the room provided as a reservoir for steam; and the lowest cock is at a point below which the water cannot be permitted to fall, without endangering exposure of the flues" (Ward 1860:23). The two intermediate cocks show water between high and low levels. In operation, if one of these cocks is opened and emits steam, water level is below that point. If the cock emits water, water level is above that point (Ward 1860:23). The gauge cock shown in Figure 14 is in the closed position.

Large debris hindered investigation of the boiler face below the gauge cocks. Except for the port side fire door, much of the lower areas could only be examined by feeling around the debris. Openings for two fire doors are spaced 28 inches apart. The port door hinged outward away from the center of the boiler and was fully open when examined. A heat baffle on the inside of the port door indicates the opening measured 16 by 12 inches. Figure 12 shows the fire doors closed. Mud filled the fire box so the interior was not examined.

An opening detected 30 inches below the fire door is probably an open ash door. Obstacles made further examination impossible. The bottom of the fire box could not be found and the overall height of the boiler was not be measured.