Chisholm Engine Interpretation

The Chisholm Engine Interpretation

It is difficult in the short time allowed at the depth of the Chisholm Engine to study the engine as much as one would like. Hopefully this description will allow you to appreciate what you are seeing a little better. This background should also help in the interpretation of other Great Lakes steamer engines since most engine rooms were laid out very similarly.

The Chisholm Engine is a double expansion steam engine, meaning that it has two cylinders. In steam engines the steam travels through the cylinders in series and is allowed to expanded in each cylinder -- thus the name. The cylinders are supported on six large columns and the connecting rods and valve push rods are visible under the cylinders. Perhaps a quick explanation of a cycle of the engine will make the machinery a little more understandable.

The steam enters the engine through the throttle valve on top of the engine. The throttle, of course, regulates the flow of the steam to the engine, then through the intake valve of the high pressure cylinder. As the high pressure steam starts to push the high pressure piston down, the intake valve closes, and the steam inside the cylinder expands to push the cylinder down. Although it might seem strange that the valve wouldn't stay open until the piston reaches the bottom of the stroke, engineers figured out early in steam engine design, that the efficiency of a steam engine was much higher if the valve closed early in the cycle. The expansion allows more of the energy to be extracted from the steam. Once the piston reaches the bottom of the stroke, the exhaust valve opens and vents the steam to the low pressure piston, and the cycle is repeated for the low pressure cylinder.

The number of cylinders in the steam engine was determined by the pressure of the steam. In the case of the Chisholm engine, which was an early steam engine, the pressure was probably around 100 psi only high enough to justify two stages of expansion. All of the other intact steam engines on the Isle Royale shipwrecks are triple expansion engines. They were build later in history with more advanced boiler technology allowing the generation of higher pressure steam. Once the steam exited the low pressure cylinder it ran through a condenser, which condensed the steam back into water. Usually the water was then pre-heated to the correct temperature before a piston pump called a feed-water pump injected the water back into the boiler, where it could be boiled back into steam. Almost all of the Great Lakes freighters used condenser systems for return water for the boiler.

Now with this background the parts of the engine and the surrounding machinery can be interpreted. Starting at the top of the engine you see the two cylinders, the high pressure cylinder, which had a diameter of 30" and the low pressure cylinder, which had a diameter of 56". Notice the fine decorative painting still visible on the cylinder base. On the forward and aft sides of the engine cylinders are two rectangular boxes with grid faces. These boxes house the valves and are called steam chests. Coming out of the bottom of the steam chests you will see the upper portion of the valve push rod which rests on an arched member called the link or valve slide. The valve slide has a lower push rod section on each end continuing down to the cams or eccentrics on the crankshaft. Each cylinder needed two valve push rods - one for the forward direction and one for reverse. Reversing the propeller rotation was accomplished by reversing the engine rotation and not by "shifting gears." This was more efficient and is actually fairly simple to do on a steam engine. The direction of a steam engine is determined by the valve timing. To switch the engine direction the engineer would move the valve slide to port or starboard to align one of the lower pushrods on the ends of the valve slide with the upper push rod which connected to the valve. The other end of the slider just luffed off to the side.

To move the valve slide on a large steam engine took a lot of force, usually more than one man could muster, so the operation was done with the help of a "reverse engine" which was basically a steam assist. The reverse engine is a single piston usually mounted on the rear or center starboard column of the engine. To reverse the engine, the engineer moved the reverse lever, which sent steam to the reverse engine. Through a connecting arm, it rotated the "reverse shaft" on the starboard side of the engine, which in turn was linked to the valve links and actually performed the reversing motion. The engine controls are usually on the starboard side of the engine and several of the controls can still be seen on the Chisholm Engine.

Moving to the port side of the engine, below the cylinders a large rocker arm can be seen protruding from the side of the engine base with one end attached to the connecting rod of the high pressure cylinder. This rocker arm operated a reciprocating pump just below the arm on the deck of the engine room. This was the circulating pump, which was used to supply the cooling water for the condenser. Notice attached to the pump is a tall cylindrical tank. This was an accumulator, which smoothed the pulsating flow from the reciprocating pump.

The large diameter pipe coming out of the low pressure cylinder bends down toward the deck and widens just above the circulating pump accumulator. This larger diameter part of the pipe is the condenser, which of course cooled the exhaust steam into water for return to the boiler.

On the fore and aft ends of the engine, at its base are what appear to be piston pumps, which have linkages to rocker arms on the same shaft as the circulating pump rocker arm. The function of these cylinders are still a mystery to me. I suspect they might be the feed-water pumps or general use service pumps. It is difficult to trace pipes in the engine room of a ship and tell the function a pump from its connections. Since ships had elaborate back-up systems, which allowed each pump to be used for multiple tasks, you tend to discover everything is connected together in some way. This makes it difficult to determine the primary use of each. Pumps were used for pumping boiler feed-water, bilge water, ballast water (in steel vessels), emergency fire fighting water, and could be used to pump return-water from the condenser to the feed-water heater, and to wash down the cargo holds or any part of the ship.

Aft of the engine the propeller shaft rests on a pillow block on which is mounted the thrust bearing. The thrust bearing is what transmits the thrust of the propeller and shaft to the ship hull. In this case it is a single collar thrust bearing. In other wrecks at Isle Royale you can see multiple collar thrust bearings. Finally the depth lines are still visible on the sternpost just forward of the propeller.