The revival of small steamboats began in the 1950's and was primarily among people who were interested in a creative mechanical hobby - similar perhaps to classic car restoration; but more practical and fun for the family than, say, model trains. The boat or hull was little more than a floating platform for the center of interest - the steam power plant. This is remains so today! Even in the best designed and most aesthetically pleasing steamboats, the focal point is the power plant. To this end, a rudimentary knowledge of what "makes the propeller go 'round" is necessary.
Since the early times steamboats have used a very wide variety of engine and boiler types. There is no such thing as a "typical" small steam boat. Not only the hull design but also the power plant are dictated by several factors: the location of operation (river, lake, or sound); availability of machinery (many early engines and boilers were crudely built by local foundaries); and, most important, the size of the owner's wallet. This is still true.
The oft spoken "simplicity" of a steam engine is a myth. Most of the steamboat engines around have as many moving parts as a two cycle gas engine. The advent of onboard computer chip controls in internal combustion engines, however, brings the steam power plant back to a time when you could "fix-it-yourself". This, to me, is one of its most endearing values.
You boil water in a boiler, thereby producing steam which is under pressure. The steam is transferred via a pipe to the engine where a valve admits steam to the engine's cylinder at appropriate, set intervals, pushing a piston up and down. Through a connecting rod this motion is transmitted to a crankshaft which revolves and turns a propeller. No gear box or clutch is needed. There are practical details of equipment and operation which can be learned over time. It is not necssary to know the more esoteric concepts in design and thermal efficiency to sucessfully, and pleasureably, operate a small steamboat.
There are three primary elements: 1. the fire; 2. the boiler; and 3. the engine. The fire is contained in a firebox ( or furnace ). If solid fuel is to be burned, there must be a grate to support the fuel so that air can reach it. The space under the grate is called the ashpit. Large quantities of air are needed to support combustion. This draft should enter through the ashpit door and flow UP THROUGH the fire. If liquid or gas fuel is used, there is no grate or ashpit and the air generally enters in a fashion similar to the oil or gas burner of a home furnace. Note: a 2HP steam engine suitable for an 18' steamboat will burn the equivalent of 1 gal. of furnace oil per hour - 135,000 BTUs. Three times the heat output of the average home furnace when running "flat out"!
The boiler holds a quantity of water in such a way that most of the heat from the fire can be transferred to the water. The water boils and gives off steam under pressure, so the boiler has the double purpose of transferring heat and containing pressure. The greater the surface area exposed to the fire, the more heat can be transferred to the water ( and the less wasted up the stack ), and the more steam produced. The best way to create this large surface area in a small space is to fill the space with pipes, or tubes. Therefore the heating surface of all steamboat boilers is made up primarily of tubes.
Boilers are described as firetube and watertube, depending on whether the hot gases from the fire pass through the tubes, or the water is contained in the tubes and the hot gases pass around them. A firetube boiler is basically a cylindrical shell, or drum, with a top and a bottom plate and the tubes passing between; under full pressure and usually holding a large quantity of water. Thus it holds a great energy reserve in the heated water, permitting steady steaming even with fluctuations in the fire. A firetube boiler is slow to raise steam, even in small sizes, and holds great potential, in the large mass of stored energy, for a lethal explosion if not properly maintained. It is, however, easier and cheaper to build and, if constructed to ASME code standards, fully as safe as a watertube boiler.
Watertube boilers consist of an open arrangement of tubes, connected to pressure drum(s). The tubes are arranged to encourage rapid circulation of the water, thereby distributing the fire's heat evenly and allowing greater heat transfer for a given surface area than is possible, or desireable, in a firetube boiler. A general rule of thumb is: for a given surface area a watertube boiler will produce TWICE as much steam as a firetube boiler. The danger of explosion is greatly reduced due to the decreased quantity of water held and the small size of the pressure drum(s). Watertube boilers must be fully enclosed in a casing strong enough to support the weight of the boiler, and well insulated to prevent the loss of heat. They are, as a general rule, more difficult and expensive to build.
When the steam reaches the engine, its admission to the cylinder to push the piston up or down must be controlled. A valve, usually moved by the engine crankshaft through an eccentric (much like the camshaft on an automobile), moves back and forth across a valve face to admit steam through ports to the cylinder and to exhaust the spent steam to the atmosphere or into a condenser. The valve is connected to the eccentric by an eccentric rod (similar to the "push rod" on an internal combustion engine). The valve may be a flat, rectangular affair - called a D or slide valve. Or it may look like a thread spool and move in a cylindrical chamber and be called a piston valve.
Most marine steam engines are double acting. The steam alternately pushes against each side of the piston, so there are two power strokes per revolution of the crankshaft. This is one advantage of steam over internal combustion - a single cylinder steam engine supplies the same turning effort as a 4 cylinder ( 4 cycle ) gas engine of equal dimensions and cylinder pressure. This does, however, result in some additional complications. A piston rod slides through a packing gland which prevents pressure from leaking through. The piston rod is then connected to a crosshead, to which the upper end of the connecting rod is also joined. The crosshead is prevented from moving from side to side by the crosshead guides. The pieces of the engine are mounted on a bedplate which is in turn secured to the hull.
Water is supplied to the boiler by a feedpump, usually driven by the engine so that the quantity of water provided is proportional to it use - as steam - by the engine. There should also be additional ways of getting water into the boiler, such as a hand operated feed pump or an injector, should the engine driven pump fail.
The above is the basic, bare bones plant. For those who will operate in salt water or in rivers or other areas where the water may have sediment in suspension, a condenser is necessary. In most small steamboats this is nothing more than a piece of pipe - usually copper - running along the underside of the boat. The exhaust steam from the engine enters one end and, in its passage, is condensed to water. At the other end it again passes up through the hull to an air pump where it is sucked from the condenser and delivered to a small tank called a hotwell. The feed pump draws the water for the boiler from this hotwell. A closed cycle is thus created and little additional water needed.
For those of you who do not like the concept of a hole in the bottom of a boat for a propeller shaft; reside near a shallow lake or river; or who just have a yearning for something nostalgic, engines closely akin to the sternwheel engine shown above are available from at least two sources.
Or, you can choose a steampowered outboard engine. The complete unit - boiler and engine - mount on the transom. Built on a gasoline outboard pedestal and drive train, the unit clamps to the transom just like any outboard. Fuel is provided by small wood blocks and feedwater taken from a five gallon bucket. The boiler is of the monotube, or "flash" design; very simple to construct - requiring no special tools - and extremely safe. The entire unit is less complex to build than most steam plants and utilizes many "off the shelf" components. I know of FOUR steam outboards in the Pacific Northwest, and more on the way!
This is a very rudimentary explanation of a small marine steam plant and is intended for the newcomer - to help dispel the mysticism that surrounds a steam engine and boiler. A small marine steam plant, in its simplest form, is easier to understand and repair than any internal combustion engine. Because of its low operating rpm ( usually not more than 450 ) the wear factor is very low. Reversing is done through a change in valve timing - you simply move a lever - and there is no need for the complication of a clutch and gear box. More in depth information may be gained by joining a club, subscribing to steamboat periodicals, and checking out web sites. You can start with the list on this site's LINKS page.
I found the two moving engine animations on, respectively: Merchant Marine and Maritime and give thanks to Rick Boggs, its creator, for graciously allowing me to use his copyrighted animation; and the steamboat@rya-online animations page and acknowledge the copyright of Bob Shearer, its creator.