Outboard Terminology & Definitions

A glossary of important terms and their meanings

Outboard motor terminology glossary definitions

This article lists many of the most important terms and parts of an outboard.


Alternator: An electrical production device that produces AC current for the use of marine accessories for recharging the outboard battery,

Anode: A galvanically active piece of metal i.e. it dissolves instead of your engine and hence helps prevent corrosion of the expensive outboard components. Note: These should be checked often and replaced as required.

Anti-Cavitation Plate: See Anti-Ventilation Plate.

Anti-Ventilation Plate: 1. A plate case into the lower unit, which helps prevent “propeller speed-up” and loss of boat speed by keeping the propeller better surrounded by water.

Atomise: To make things like oil/ petrol into fine particles or mist.

Balanced Carrying Handle: Provide lower HP engines to increase portability and transportation. Yes this might sound silly but when lifting 18kg’s or more when your in a tender or small boat it is very handy!

Blowout: This happens when bubbles reach the back of the gear case and cause the propeller to free-wheel and forward-thrust to decrease dramatically.

Camshaft: A shaft turned by the crankshaft, with lobes which open and close the intake and exhaust valves at the proper time. 2. Shaft on which suitably phased cams are mounted, as for example to operate intake and exhaust valves of an engine. In four-stroke engines, the camshaft rotates at half-crankshaft speed.

Carburettor: A complex mechanical engine component that mixes gasoline and air to the required proportions in order to supply that mix to the inlet manifold and subsequently the combustion chamber.  They typically incorporate a manual or automatic choke to adjust to climatic conditions i.e. hotter and colder air temperatures and come with minor external adjustment capabilites. (see 'Fixing 2 stroke outboard fuel problems' Guide)

Cavitation: This is when your prop breaks loose and “free wheels” at low speed, due to a stream of bubbles interrupting the flow of water.

CD Ignition: Capacitor Discharge ignition stores primary energy in a capacitor, providing automatic spark advance for sure starts, and longer plug life.

Clamp Bracket: The assembly which secures an outboard motor to the boat and should be checked for corrosion / damage. The clamp screws should also be greased as otherwise they often seize

Combustion Chamber: This is the upper part of the cylinder where fuel is ignited.

Connecting Rod: This is the vital linkage that connects the crank to the piston in an engine or other reciprocating part/ machine.

Cool Fuel System: This is standard on all fuel-injected products, this significantly lessons the chance of vapour locking.

Control Box: This is the unit that enables you to start, and put the engine into gear, tilt the engine etc that is mounted near the steering wheel for engines using remote steering. Note: if you hear sa beep from it call us as it means something is wrong.

Counter-Rotation: Installation of multiple engines allows the propellers to turn counter to one another, with the torque of the left-hand engine being balanced with the torque of the right-hand engine.

Cowl: The removable covering on an outboard engine. Note: new plastic cowlings need the screws removed and greased because if the seize you cannot use a heat gun to free them and they often break

Crankcase: The part of the cylinder block, that encloses the crankshaft area.

Crankshaft: A shaft with offset throws that converts piston reciprocating motion to rotary motion.

Cylinder: A tubular component, with a closure at one end and a moveable piston inserted from the other end. The movement of the piston toward the closed end creates compression.

Cylinder Block: Part of the engine which houses the cylinders.

Cylinder Head:Part of a reciprocating engine that seats or closes the upper ends of the cylinder. Also contains most of the combustion chamber and may contain the valves.

De-compression Relief Ports: Exhaust outlet holes in the exhaust manifold near the cylinders that reduce back pressure for easier starting and improved idling.

Detonation: Commonly called spark knock or ping. In the combustion chamber, an uncontrolled second explosion of the remaining compressed air/fuel mixture, resulting in a pinging noise and possible loss of power and reduced engine life.

DFI (Direct Fuel Injection): A technology whereby a precisely shaped charge of fuel is injected directly into each cylinder through the top of the cylinder head rather than at the point of the air intake into the crankcase.

Displacement: The capacity of an engine based on the volume of a cylinder with the piston at its lowest point, multiplied by the number of cylinders. Displacement is usually measured in the cubic inches, cubic centimetres or litres.

DOHC (Double Overhead Cam):An engine featuring one cam for intake valves and one cam for exhaust valves, thus providing more precise valve timing for increased power and top end speeds.

Clutch Dog: Clutch which transmits power by engaging metal teeth or dogs. It allows only direct mechanical engagement or disengagement, without slipping or progressive torque transmission.

Drive Hub: The central housing of the propeller, to which the blades are attached.

Driveshaft: The shaft, usually vertical, which transmits power from the engine or powerhead, to the drive unit.

DTS (Digital Throttle & Shift):The SmartCraft Digital Throttle & Shift System brings digital precision to engine throttle and shifting, for unprecedented smooth shifting; precise throttle control; and superior, reliable performance. Lightning fast throttle response is accomplished by a fully integrated digital system.

Dual Water Pick-Ups: The lower unit is equipped with a secondary water pick-up thus removing a single point of failure for something that can easily get blocked and normally located under the anti-cavitation plate. Note: water intakes should be checked to be free and clear every time you use the Outboard.

ECM (Electronic Control Module):Through its internal circuitry, the ECM electronically controls the engine by monitoring input sensors and electronically altering the engine’s i.e. fuel, timing, warning, etc.

EFI (Electronic fuel injection): A system that injects fuel into an engine when electronic cicuitry controls the gasoline to air mixture rather than the venturi effect through a carburettor. Often incoporating ambient air temperature detection to automatically adjust the mixture strength, negating the new for a manual choke.

Electronic Control Module (ECM) to time and meter fuel flow rather than by the use of carburettor’s

EPA (Environmental Protection Agency) Starting with the 2006 model year outboard engines manufactured will have to have their emissions reduced by 75%.

Ethanol: Ethyl-alcohol fuel or fuel additive. For more issues relating to Ethanol fuel here

Exhaust Ports: Holes in the cylinder wall of a two-stroke engine that discharge exhaust gases into the exhaust system or an exhaust valve to the exhaust manifold in a four-stroke engine.

Flywheel: A heavy wheel attached to a crankshaft to provide momentum and reduce vibration when running.

Flywheel Magnets: Permanent magnets cast into the flywheel to energize various types of coils.

Four-stroke Engine: Four-stroke engines operate with one power stroke for every four strokes of the engine. In addition to the regular combustion and ignition, the piston has to make four movements to complete a combustion cycle. This is a process that normally provides smoother operation and fewer overall emissions. More detail explanation of differences between two-stroke vs. four-stroke outboard engines

Fresh Water Flush Port: On selected engine models, the port is either front or rear mounted for convenient attachment to garden type hose. The engine can be effectively flushed without running it. Recommended for engines used in sand or silt laden water

Fuel Injection: Injection of fuel under pressure, into the intake-tract, directly into the cylinder or indirectly into a cylinder pre-chamber.

Fuel Injector: The device within the fuel management system that meters and atomizes the fuel before it enters the cylinder.

Fuel Rail: An assembly that receives fuel from and is pressurized by a powerful electric fuel pump (from the vapor separator assembly). The fuel trail stores the fuel until the fuel injectors (attached to the rail) open.

Full Gear Shift (F-N-R): A noutboard equipped with a gear-case that has Forward and Reverse gears with a neutral intermediary.

Full Throttle: 1 The condition whereby the throttle jet of the carburetor or throttle body is fully open (see video here) and/or at which maximum performance for a specific engine can safely be obtained governed by the maximum revolutions per minute.

Fully Regulated Alternator: Alternator that has a regulator to prevent overcharge.

Galvanised:A chemical process an involving electrolysis or molten metal dip process to coat or plate a metal which protects from rust and corrosion.

Gear Housing:The lower housing of a drive unit which converts thrust from a vertical shaft to the horizontal propeller shaft. In most cases, the gear housing houses forward/reverse shafting mechanism.

Gear Ratio:The relative number of turns between a drive gear and a driven gear.

Harness:A group of wires bundled together into a single assembly.

Heat Exchanger Baffles: The cooling fins located in a heat exchanger.

High-Speed Gear case: A gear case designed to be hydro-dynamically efficient at high speed.

Horsepower: A measure of mechanical power or the rate at which work is done. One horsepower equals 33,000 pound-feet of work per minute; it is the power necessary to raise 33,000 pounds a distance of 1 foot in 1 minute.

Hour Meter: An instrument which records and displays the amount of time an engine has actually run. Note: diagnostic readings are the only way to get accurate hours as these can be disconnected.

Idle Relief Ports: Openings in the upper rear portion of the mid-section that allow exhaust to exit during low speed operation and idling.

Ignition Coil: A transformer using primary (low voltage) windings to induce current in secondary (high voltage) windings. This secondary voltage is then directed to the spark plugs.

Ignition System: In the outboard engine, it is the system that provides very high voltage sparks to the cylinders to fire the compressed air/fuel mixture.

Impeller: A small often rubberised fin drive that draws water up the cooling pipes and pushes it round the power head and drive train to cool various engine components.

Integral Fuel Tank:Fuel tank that is mounted under the engine cowling.

Intercooler: Heat exchanger that removes heat from pressure charged air. The process of the intercooler is to take the heat compressed air from the supercharger and cool it down. By cooling air down before it enters the cylinder, more horsepower can be created.

Isolator:  An electrical device used to keep battery-charging circuits separate from one another or on inbound engines a flexible mount used to reduce a components exposure to vibration.

Kill switch or Kill cord: if pushed (or removed) it will cut all power to the engine. Often works in conjunction with a lanyard attached to the helmsperson

Lanyard Stop Switch: Stop switch with a cord designed to be attached to the driver, which stops the engine if the operator leaves the helm.

Lean Mixture: An air/fuel mixture that has a relatively low proportion of fuel and a relatively high proportion of air.

Lean-Out: To reduce the fuel-to-air ratio.

Leg: often used when lifting or moving an engine i.e. someone grab the head (the heavy top part where the power head is_ and someone grab the leg i.e. where the gearbox / prop is.

Magnetic Drain Plug: A lubrication drain plug that is magnetized to retain small metal filings from normal gear wear prolonging drive unit life.

Manifold: A metal casting with openings which direct fuel and air from the carburetor to the cylinder head (intake manifold) or which carry exhaust gases away from the engine (exhaust manifold). Intake and exhaust manifolds may be combined into one part.

Manual Start: Operator must pull on a rope handle to manually crank the outboard engine to get it running.

Manual Trim: A mechanical system which allows the outboard engine to be tilted to 5 or 3 respectively different angles with respect to the transom.

Octane Rating: A measure of the anti-knock properties of a fuel. The higher the octane rating, the more resistant the fuel is to detonation.

Oil Pan: he reservoir at the bottom of a four-cycle engine which houses the oil pump and holds the oil supply.

OptiMax: Mercury Marine’s line of direct fuel injected (DFI) 2-Stroke outboards.

Overwintering: The process to preparing to store or 'lay up' an outboard engine of the a prolonged period of inactivity, i.e. over the winter. Typically involves drainging down fuel, clean and protecting key components and covering.

Piston: 1. The round component which moves in a cylinder to create, or to react to, pressure in the cylinder. 2. Reciprocating component, usually in the form of a cylinder closed at one end, it operates under fluid pressure within a smooth walled cylinder. In a reciprocating engine, gas pressure on the piston crown provides the prime force that is converted into rotating mechanical by the crankshaft.

Piston Head:The top of the piston.

Piston Pin: A steel pin that connects the piston to the connecting rod. Also called wrist pin.

Piston Rings:A split ring installed around the piston to seat the space between the cylinder wall and piston.

Piston Slap: Noise made by contact between an excessively loose or worn piston and the cylinder wall of an engine.

Power Tilt: Enables the outboard engine to tilted up or down by the push of a switch. Power Trim: A mechanism which hydraulically adjusts the angle of the outboard thereby changing the angle of thrust from the propeller for better handling and performance.

Primer Bulb: A hand pump in the fuel tank hose used to fill the engine’s fuel system.

Primer System: Mechanical or electrical, used to introduce extra fuel into an engine, for quicker cold starts.

Propeller Diameter: Distance across the imaginary circle that is made when the propeller rotates.

Propeller Pitch: The theoretical distance, in inches, that a propeller travels forward during one revolution.

Propeller Rake: The backward slant of a propeller blade. Higher rake props hold the water better.

Remote Steering: An engine equipped with a steering wheel rather than a tiller handle.

Reverse Lock: A lock which prevents the engine from tilting out of the water when operating in reverse.

Rich Mixture: Fuel/air mixture in which the proportion of fuel exceeds that necessary for combustion. Is more than is necessary, hence, “rich” or “excessive”.

Rocker Arm: A lever which transmits upward motion of the pushrod to downward motion on the intake or exhaust valve, thus opening the valve.

Rubber Hub: This refers to the rubber shock absorbing device between the splined hub that slides into the propshaft and the propeller hub port.)

Shallow-Water Drive: System that can be engaged to tilt an outboard’s drive unit so it can be operated in very shallow water.

Shear Pin:A safety device used to fasten a propeller to its shaft, it breaks when the propeller hits a solid object, thus preventing further damage.

Shift-In Tiller Handle:Style of tiller handle which mechanically controls the shift as well as the throttle.

Single Overhead Cam: A four cycle engine design whereby a single camshaft to actuate the intake and exhaust valves is located above the cylinder heads. This camshaft is bell driven by the crankshaft, and it provides more efficient operation by eliminating friction robbing components associated with conventional camshaft designs (i.e. push rods, etc) thereby allowing for maximum horsepower or output.

Single-Point Injection:An oil injection system that uses only one discharge port as opposed to up to six ports.

Skeg: A fin at the bottom of the gear housing, shaped in such a way that hydrodynamic efficiency is increased.

Solid Hub: Splined hub prop with no rubber shock-absorbing device.

Sound Attenuator:A silencing device found on outboards. It works by utilizing unique materials with high-sound dampening characteristics, reducing natural intake frequencies and moving them out of the engine’s operating range.

Spark Advance: The amount of time the spark occurs in a cylinder before the piston reaches Top Dead Center. Expressed in degrees of crankshaft rotation, if allows for the relative speeds of combustion compared to engine rotation.

Squeeze Bulb: Small fist shaped rubber ball placed inline with the fuel supply hose and used to prime the hose with fuel. Typically used with external gasoline fuel tanks.

Stainless Steel Drive/Propeller Shaft:Shafts that are made from a high grade of stainless steel, to extend reliability and durability to these critical components and moving parts susceptible to corrosion.

Stainless Steel Grounding System: Continuity circuit that bonds all the outdrive and engine components together to help prevent corrosion between dissimilar metals.

Steering Torque:1. The pulling to one side (usually the propeller rotation direction), while steering a boat. 2. The tendency for a drive unit to pull to one side while the boat is powered.

Supercharger: Mechanical pump or compressor for increasing the pressure of induction air or gasses. Dramatically improves low end torque and acceleration.

Swivel Bracket: A housing mounted between the clamp brackets that allows the engine to pivot and tilt.

Thermostat: A temperature-sensitive switch that controls the circulation of water in the powerhead.

Throttle Friction: A device that allows the user to set and hold a desired boat speed.

Thru-prop Exhaust: A system whereby the exhaust gases exiting the engine are routed down through the drive unit and allowed to exit through the propeller for efficiency and noise reduction.

Tiller Handle: A device used to grip an outboard in order to steer. (Some models propel, stop, etc). Note: Many people like longer tillers and hence attaché tiller extension

Tilt Lock:1. A lever that releases the reverse lock allowing motor to be tilted using tilt gap. 2. A device which supports the engine in its full-tilt up position for secure trailoring.

Tilt Pin:An adjustable assembly on the clamp bracket for setting the angle between the propeller, boat and water.

Tilt Tube: A tubular shaft that connects the clamp brackets to the swivel brackets. It permits the engine to be tilted.

Torque: A force that produces or tends to produce rotations or torsion. A characteristic of an engine with good torque levels is a strong holeshot and strong acceleration when the throttle is opened.

Transmission: A mechanism attached to the engine which uses engine rotation to provide forward, reverse or neutral rotation at the propeller.

Transom: The stern of the boat that the motor or outdrive is connected to.

Trim: To adjust the position of a boat moving in the water by altering the angle of the drive unit (or motor) to the boat.

Trim Tab: An adjustable fin or rudder attached to the gear housing which serves to equalize steering torque and acts as a sacrificial anode.

Turn-Key Starting: Refers to engines that start without the need of priming, usually associated with Electronic Fuel Injection (EFI) and Direct Fuel Injection (DFI).

Twist Grip: A throttle control on the end of a steering handle whereby twisting the gripchanges engine speed. Note: some engines allow you to set the speed which is very handing for some tasks i.e. trolling for fish etc.

Two -Stroke: An engine which delivers one power pulse for every 2-strokes of the piston, during one revolution of the crankshaft and requires oil to be mixed with the fuel. Note: if you have an older engine it could be worth turning off the auto oil mix and mixing in the petrol tank as if the auto-mixer fails it will often damage the engine beyond economic repair. See differences between two-stroke vs. four-stroke outboard engines

Voltage: A difference in electrical potential between two ends of a circuit which causes current to flow. Unit of measurement is the volt.

Water-Separating Fuel Filter: A filter that helps keep water out of the engine for smoother running and increased engine reliability.


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