Steamboat Terminology

This glossary is based on the one in Steam Engine Principles by NG Calvert.  I've selected the terms relating to the use of steam afloat and changed many of the definitions to emphasise nautical applications.  There are additions from several other sources.  I would be very grateful for your contributions and corrections. 

In this glossary I have used "Subject, Qualifier" notation to group associated terms.  Please use your browser's page search facility to find words of interest. If the glossary ever gains a consensus of approval I intend to transfer it to the SBA web site.

Word	Definition
Adiabatic	A process in which heat is neither absorbed nor emitted.
Admission, Inside	Live steam supplied to the waist of a piston valve and the exhaust passes at either end.
Admission, Outside	Live steam is supplied to the steam chest behind the slide valve and the exhaust passes through the cavity in the face.
Advance, Angle of	The angle in excess of 90o by which the eccentric leads the crank (assuming an outside-admission slide valve).
Angularity	The distortion to the transmitted motion caused by a rod changing angle as well as moving axially. E.g.. At constant speed, the crank pin describes a sine wave in the vertical plane . The connecting rod is vertical at both dead-centres but inclined at mid-stroke and therefore it is effectively shorter in the vertical plane so the movement of the piston is not purely sinusoidal and the valve events are disturbed. Also known as "Obliquity".
Auxiliary Machinery	The various pumps etc. used to support the main engine(s).
Blow Down	A procedure using steam pressure to discharge the contents of the boiler through low level vents in order to clear sediment from the mud drums.
Boiler	The pressure vessel in which water is converted into steam. Common forms are "fire tube" and water tube". Each can be "horizontal" or "vertical" according to the orientation of the main drum. Loosely used to include the pressure vessel, its casing, the firebox and ancillary equipment which in a small launch form an integrated system.
Boiler Stays	Rods inside a boiler restraining flat surfaces such as the end caps against steam pressure.
Boiler, Field Tube	A boiler design based on tubes that are closed at one end. Water circulates in and out at the other.
Boiler, Fire Tube	A boiler where the heating surface consists of a number of tubes which carry the hot smoke through the water space.
Boiler, Flash	A high-temperature Monotube boiler. The feed water 'flashes' to steam immediately.
Boiler, Monotube	A boiler consisting of a single tube. Feed water enters at one end and steam leaves at the other.
Boiler, Shell	A general term for any boiler which mainly consists of a single large pressure vessel.
Boiler, Water Tube	A boiler having a heating surface consisting of a large number of relatively small diameter tubes which contain the water.
Brasses	The plain bearings in the ends of the connecting rod. Originally made from brass now more commonly bronze or "white metal". (Sometimes replaced with roller bearings).
Clearance	The dead space between the piston and the cylinder cover at the end of the stroke.
Cock	A quick-acting valve consisting of a transverse cylindrical plug with a bore through its diameter. The valve is closed by turning the handle a quarter turn.
Cock, Test	A cock that allows the nature (steam or water) of the contents of the boiler or a pipe to be determined at a specific point.
Compression	That part of the engine cycle where the exhaust valve is closed before the end of the piston stroke. It equalises the pressure in the cylinder with that in the steam chest and reduces engine vibration. Also known as "cushioning".
Condenser	A device which cools exhaust steam back into water, either to conserve water or to lower the back pressure by creating a partial vacuum in the exhaust.
Condenser, Air	A surface condenser which uses the atmosphere for cooling.
Condenser, Evaporative	An air condenser which is augmented by evaporating a spray of cold water.
Condenser, Keel	A surface condenser fitted outside the hull, below the water level to use the sea for cooling.
Condenser, Mixing	A condenser where water is injected into the exhaust steam to cool it.
Condenser, Surface	A condenser which draws cooling from its surrounding environment
Conduction	A mechanism of heat movement, principally through solids.
Convection, Free	The natural circulation within a fluid due to variations of temperature and pressure. Important in heat transfer within fluids and between a fluid and a solid surface.
Crank	A mechanism for converting reciprocating motion to rotational motion and vice versa.
Crank, Overhung	A crank on the end of a shaft so that it has bearings on one side only.
Crank, Return	A light crank arm used for driving valve gear, mounted outside an overhung crank.
Critical Point	The temperature above which no pressure, however great, can liquefy the vapour.
Crosshead	The structure at the lower end of the piston rod which resists the lateral forces applied by the connecting rod.
Crosshead Guide	The slide that restrains the crosshead to a linear movement and absorbs lateral thrust.
Cut Off	The closing of the inlet port before the piston reaches the end of its stroke so that the steam can work expansively.
Cylinder	The tubular body of the engine in which the piston moves.
Cylinder Covers	The removable end caps of the cylinder.
Cylinder Drains	Cocks that release the condensate that forms in the cylinder as the engine warms up.
Damper	A device for limiting the air supply to the firebox in order to reduce combustion.
Dead Centre	When the piston is at either end of the stroke the connecting rod is inline with the crank and neither push nor pull can start rotation. Inner (IDC) and outer (ODC) dead centre are relative to the crankshaft. Top (TDC) and bottom (BDC) dead centre are used for vertical engines.
Diagram, Indicator	A record of the variations in pressure against piston position. Used to analyse engine performance.
Diagram, Reuleaux	A diagram showing the timing of valve events.
Diagram, Zeuner	A diagram showing the timing of valve events.
Draught	The pressure differential that draws air through the firebox. Usually measured in inches of water.
Draught, Forced	Increasing the draught by mechanical means to increase combustion.
Draught, Induced	The use of steam jets or fans in the funnel to draw air through the firebox and increase combustion.
Draught, Natural	The use of a funnel to draw air through a the firebox by convection.
Dryness Fraction	The fraction of dry steam in a steam/water vapour mixture (by weight).
Eccentric	A sheave set off-centre on the crankshaft to cause a reciprocating motion in the linkage. This drives the slide valve or a pump.
Eccentric Strap	The fitting that imparts the motion of the eccentric to the linkage.
Eccentric, Slip	A reversing system where the eccentric is moved on the crankshaft to change the slide valve timing.
Economiser	An arrangement for preheating the boiler feed water by recovering heat from the smoke in the funnel.
Efficiency	The amount of energy delivered by a machine as a fraction of the total energy put into it.
Ejector	A jet pump used for suction e.g.. an air pump or a bilge pump.
Emulsion	An mixture of oil and water.
End, Big	The end of the connecting rod which attaches to the crankshaft.
End, Little	The end of the connecting rod which attaches to the piston rod.
Endothermic	A process which absorbs heat.
Energy	The capacity for doing work.
Engine, 'Ordinary' Steam	A simple-expansion, double-acting, slide-valve engine. Normally with exposed cranks and linkage.
Engine, Atmospheric	An early form of steam engine in which the piston is moved by the suction of a vacuum acting on one side and atmospheric pressure acting on the other. The vacuum is created by the condensation of low pressure steam.
Engine, Barring	A small auxiliary engine used to rotate a much larger one to a position where it will self-start.
Engine, Beam	An early form of stationary steam engine which produced an oscillating motion rather than a rotary one.
Engine, Compound	An engine with more than one cylinder (commonly two) where the steam passes through each in series so that there are several stages of expansion. The cylinder diameter usually increases at each stage.
Engine, Double-Acting	An engine in which the steam acts alternately on each side of the piston.
Engine, Heat	Any engine which converts heat into work such as a steam engine.
Engine, Reversing	A small auxiliary engine which actuates the reversing gear of a much larger one.
Engine, Simple	An engine in which the steam expands only once (in contrast to a compound).
Engine, Single-Acting	An engine in which the steam acts on one side of the piston only. The piston returns up the cylinder due to the momentum of the flywheel.
Engine, Tandem	A compound engine whose two pistons share a common piston rod and work on the same crank.
Engine, Triple Expansion	An engine in which the expansion is carried out in three successive stages. High pressure steam first enters a small cylinder, the lower pressure exhaust then passes into a larger cylinder and the exhaust from there passes into an even larger one.
Engine, Twin	A two cylinder engine where high pressure steam is supplied to both directly.
Engine, Uniflow	An engine in which steam is admitted at the cylinder head and exhausted through a belt of ports exposed in the cylinder wall when the piston nears the end of its stroke. The most efficient of all piston engines for constant speed duties.
Engine, Vee	An engine where the cylinder are at an angle rather than parallel to each other.
Enthalpy	The energy in steam due to its temperature and pressure.
Entropy	A concept in the theory of thermodynamics associated with disorder or lack of structure. A measure of the inability to do work. Reversible changes do not change the entropy, irreversible ones increase it.
Exothermal	A process which gives out heat.
Expansion	The practice of stopping steam admission before the end of the stroke and allowing expansion of steam in the cylinder to drive to piston to the end.
Expansion Ratio	The ratio of the maximum to the minimum volume of the cylinder to one side of the piston. Equals (Clearance) : (Clearance + Swept Volume)
Expansion, Compound	The steam passes through more than one cylinder undergoing successive expansion in each.
Expansion, Quadruple	An engine in which the expansion is carried out in four successive cylinders. Used in large, high-pressure marine engines.
Expansion, Ratio of	The ratio of the volume of steam in the cylinder at release to the volume at cut-off.
Feed Water	The water supplied to the boiler to replace the steam that has been used.
Feed Water Bypass	A mechanism used to control the amount of feed water that enters the boiler. The feedwater pump normally supplies at a constant rate equal to he maximum expected demand. Any excess is fed back to the hotwell.
Feedwater Heater	A device for warming the feed water with heat from the exhaust steam before input to the boiler.
Fire Bars	The grate which supports the fuel in the firebox so that air for combustion can be drawn through it.
Firebox	The furnace of the boiler.
Flues	Ducts which control the movement of hot smoke in the boiler.
Flywheel	A wheel or disc with a heavy rim mounted on the crank shaft. Its momentum helps ensure uniformity of rotation.
Force	A vector capable of changing the momentum of a body.
Funnel	The chimney of a boiler. Convection within it induces a draught through the firebox.
Furnace	The combustion chamber of the boiler.
Fusible Plug	A safety device consisting of a plug of low melting point alloy set into the boiler. In the event of overheating, the plug melts and water is dumped onto the fire.
Gauge, Bourdon	A common form of pressure gauge based on the tendency of an curved tube to straighten out when subject to internal pressure.
Gauge, Pressure	A device used to indicate the steam pressure at various points in the pipe system.
Gauge, Vacuum	A device used to indicate pressures below atmospheric.
Gauge, Water	A glass tube connected to the boiler at both top and bottom so that it indicates the water level in the boiler.
Gland, Stuffing Box	A seal on a rotating or sliding shaft where it passes through the wall of a pressure vessel.
Governor	A device used to maintain a constant engine speed despite changes in load.
Grate	The support for the burning fuel that permits air to flow up through the fire.
Gudgeon Pin	The bearing pin which attaches the little end to the crosshead. Also known as "Wrist Pin"
Heat, Latent	The energy required to change the state of a substance without changing its temperature. The latent heat of vaporisation is the energy required to convert water to steam.
Heat, Sensible	Heat which causes a change in temperature rather than a change in state (latent heat). See enthalpy.
Heat, Total	The term formerly used for the heat required to produce unit mass of steam from water at freezing point. Superseded by 'enthalpy'.
Heating Area	The total surface area of the boiler that can capture heat from the fire.
Horsepower	A measure of power output. Now superseded by the kilowatt. 1 HP is equivalent to 0.746 kW.
Horsepower, Brake	Power as measured by a brake on the propeller shaft. Often called Shaft Horse Power.
Horsepower, Friction	The horsepower needed to overcome the internal friction of a machine.
Horsepower, Indicated	Horsepower calculated from the indicator diagram.
Hotwell	The tank which receives condensate from the air pump and supplies feed water to the injector.
Hunting	A regular oscillation in engine speed caused by the governor overcorrecting for each change.
Imperial	An anachronistic system of measurement based on the foot and the pound. Much used in steamboating and certain backwaters of the former British Empire.
Indicator	An instrument which draws a graph of steam pressure in the cylinder against piston position. The graph is used to determine engine performance. See Indicated Horsepower.
Injector	A form of jet pump used as a boiler feed pump.
Isentropic	A theoretically ideal engine cycle that is adiabatic and frictionless.
Isothermal	A process in which the temperature remains constant.
Kinetic Theory	A theory which accounts for many fluid properties in terms of molecular motion.
Lap, Steam and Exhaust	The amount by which the faces of the slide valve overlap the steam and exhaust ports when at mid-stroke. The same as 'outside' and 'inside' lap respectively
Link Motion	The system of rods which transmits the motion of the eccentric to the slide valve and controls reversing.
Linking Up	A means of reducing power once cruising speed is attained, by setting the link motion towards the central position. Also called "notching up".
Lubricator, Displacement	A container of oil connected to the steam pipe near to the engine. As steam enters and condenses, oil is displaced into the engine for the lubrication of the valve and piston.
Manhole	A maintenance opening in a boiler.
Manifold	A multiple junction in pipework for collecting or distributing fluids from or to several places.
Motion, Radial	A regulating, controlling and reversing valve gear of the more precise type. In particular, if the motion is moved from the ahead to the astern position while the piston is (at rest) at top dead centre, the valve does not move.
Motion, Watt Parallel	A linkwork used instead of a crosshead to hold the end of the piston rod to a nearly straight path.
Mud Plug	A plug in the end of the mud drum which can be opened to remove sediment from the steam boiler by blowing down.
Nozzle	A pipe-fitting which produces a controlled jet of fluid.
Pipe	A long cylindrical hole covered by a thin layer of metal. Before use it is known by its construction; after use by its application ("15mm seamless copper" becomes "feedwater"). Hot pipes are often wrapped in bandages to remind passengers that they will need wrapping in bandages if they touch the pipe.
Pipe, Blast	A pipe which discharges a jet of exhaust steam up the funnel to induce a draught through the firebox.
Piston	The principle moving part in the cylinder on which the steam acts.
Plug	A semi-permanent method of closing an opening in a pressure vessel.
Port	The openings in the valve face of a cylinder block. The steam ports connect with the cylinder via the steam ways. The exhaust port connects to the exhaust pipe.
Port Bridges	The parts of the port face between the inlet ports and the exhaust port
Pressure	Force per unit area.
Pressure, Back	The pressure which resists the free discharge of exhaust steam from the cylinder.
Pressure, Friction Back	The internal friction of an engine expressed in terms of the equivalent back pressure.
Pressure, High	Steam as it arrives at the engine prior to entering the first cylinder of a compound.
Pressure, Intermediate	Steam in the receiver between the cylinders of a compound engine.
Pressure, Low	Steam as it enters the last cylinder in a compound engine.
Pressure, Partial	The contribution of one component of a mixture of gases to the pressure of the whole. The pressure of the mixture is the sum of the partial pressures.
Priming	(In boilers) The foaming of boiler water caused by a sudden release of steam throughout the volume of water. Produces a very wet steam-water mix at steam chest. Can be due to impurities in the feed water or to a sudden fall in pressure.
Pump	A device for moving fluids. Identified by its application or its operating principal.
Pump, Air	A pump used to extract condensate (strictly just the air fraction) from a condenser.
Pump, Duplex	Two direct-acting pumps (i.e. pumps with the steam and water pump pistons on the same piston rod) side by side. The are made to work alternately by using each to operate the slide valve of the other.
Pump, Edwards Air	An effective reciprocating air pump which makes use of inertia forces to dispense with foot and bucket valves.
Pump, Extraction	A pump which removes condensate the condenser. This role is often fulfilled by the air pump.
Pump, Feed	The pump which supplies water to the boiler.
Pump, Impeller	A pump where the fluid is moved by a rotating paddle-wheel.
Pump, Jet	A pump working by entraining a fluid in a jet of steam. See injector, ejector.
Radiation	Linear transmission of (heat) energy, as from a fire.
Receiver	The chamber which holds the steam between one stage of expansion and the next. Frequently just a pipe connecting the cylinders.
Rod	A link that transmits motion along its length.
Rod, Connecting	The rod which connects the reciprocating piston rod to the rotating crank.
Rod, Piston	A rod connecting the piston to the crosshead in a double-acting engine. It transfers the motion of the piston outside the cylinder.
Rod, Tail	An extension of the piston rod through the cylinder head. Used on very large pistons to reduce loading on the cylinder walls.
Shaft	A link that transmits rotational movement.
Shaft, Weigh	The shaft connecting the reversing links on a multi-cylinder engine so that they operate together.
Steam	Water in a gaseous state at a temperature above its boiling point
Steam Chest	The chamber on the side of the cylinder which houses slide valve.
Steam Lead	The amount by which the steam port is open at the start of the piston stroke.
Steam, "Stuff"	Steam of unknown condition.
Steam, Dry	Steam containing no unevaporated water droplets
Steam, Saturated	Steam at the boiling temperature of water, at the prevailing pressure.
Steam, Superheated	Steam at a temperature above that required to evaporate water at the ambient pressure.
Steam, Wet	Steam with water droplets dispersed in it.
Steamways	The passageways which carry the steam from the ports in the steam chest to the ends of the cylinder.
Stuffing Box	The assembly which prevents leakage when a sliding or rotating shaft works through a pressure vessel wall. E.g.. the piston rod and lower cylinder cover: the valve rod and steam chest
Thermodynamics	The science of energy, work and heat.
Thermodynamics, Second Law of	The most fundamental of all natural laws: "everything tends to chaos". Energy is consumed when temperature is raised.
Torque	A turning force. Steam engines produce significant torque at very low speeds and so will drive from rest and do not usually require gearing down.
Vacuum	A term commonly used to refer to any pressure below atmospheric.
Valve Gear	The mechanism that drives the slide valve and often controls reversing. Reversing is usually accomplished by changing the timing of the valve by 180o which reverses the direction of rotation of the whole engine.
Valve Gear, Hackworth	A reversing and regulating gear based on one eccentric and on a sliding block of variable inclination, formerly popular for marine uses. The parent of many other types of valve gear.
Valve Gear, Joy	A radial valve gear which derives its motion from linkwork attached to the connecting rod and therefore needs no eccentric. Thence similar to the Hackworth.
Valve Gear, Kitson	A radial valve gear, similar in principle to the Walshaert, which picks up a rotary motion from a point on the coupling rod; formerly used on steam trams.
Valve Gear, Marshall	A radial gear very similar in its action to the Hackworth.
Valve Gear, Stephenson	A popular reversing and regulating gear.
Valve Gear, Walshaert	A radial valve gear of Belgian origin which became very widely used for railway locomotives.
Valve, Bash	A quick-acting valve operated by some reciprocating mechanism (such as the piston) as it nears the end of its stroke.
Valve, Butterfly	A quick-acting valve consisting of a circular disc, pivoted along its diameter.
Valve, Check	A non-return valve which closes automatically when the flow stops or tends to reverse.
Valve, Clack	A non-return valve which closes automatically when the flow stops or tends to reverse.
Valve, Corliss	A rapid-acting semi-rotary slide valve linkage.
Valve, Double Beat	A valve so designed that pressure forces are almost in equilibrium; hence it is easy to operate.
Valve, Double-ported	A valve having a duct in addition to the cavity to adjust the timing of the events.
Valve, Drop	See Poppet Valve
Valve, Mitre	See Poppet Valve
Valve, Piston	A circular form of slide valve. This avoids lateral forces which increase friction.
Valve, Poppet	An axi-symmetric valve generally worked by a cam, similar to those in an internal combustion engine. Used in high superheat engines.
Valve, Relief	A valve which controls boiler pressure by releasing excess steam.
Valve, Safety	An automatic valve which dumps steam from the boiler if the pressure rises above the safe working limit. In practice, often combined with the relief valve.
Valve, Shuttle	A slide valve operated by steam, used when rapid action is required at the end of a piston stroke. Used particularly for direct acting boiler feed pumps.
Valve, Simpling	A valve which temporarily converts a compound to work as a twin engine to assist warming up.
Valve, Slide	A valve which works by sliding over ports to cover and uncover them. The type used to control the flow into and out of the cylinder of an ordinary steam engine.
Valve, Stop	The valve which controls the release of steam from the boiler.
Valve, Throttle	The valve which controls the speed of the engine by restricting the steam supply to the steam chest.
Vessel, Air	A device to reduces the shock (water hammer) caused by reciprocating pumps.
Volume Ratio	The volume of saturated steam produced from a unit volume of water.
Weights, Balance (or Bob)	Weights attached to the crankshaft which counteract the lateral forces imposed by the con rod.
Willans Line	The graphical representation of constant speed engine characteristics, found experimentally to be very close to a straight line.
Windermere Kettle	A tea kettle that is heated by steam from the boiler. (Essential creature comfort).
Wiredrawing	A gradual fall in steam pressure in the cylinder due to the slow rate of closure of the slide valve. May erode valve or port face.
Work	a) An action which absorbs energy. b) An often unavoidable distraction from steamboating.

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This web page is maintained by Bob Shearer. Created: 10/Feb/99. Last updated: 10/Nov/00.	Page Head