Courtesy of Scream And
The online Boating performance magazine

Version 1.1

Adapter Plate:
 "Adapts” and connects the powerhead of an outboard engine to the center section, and usually contains the tune

Aerodynamics:  Pertains to the forces generated by flow of air over a surface, used to lift the hull out of the water, also responsible for generating drag, among other things.

Afterplane:  A device that is attached to the anti-ventilation plate that increases its working area, thereby increasing its effectiveness.

Anti-Cavitation Plate:  See Anti-Ventilation plate.

Anti-Ventilation plate:  The horizontal web or plate above the Lower Unit. It is intended to prevent the Prop from sucking surface air, Ventilating, into it and causing a Cavitation like effect. Useful on a deeply mounted outboard or drive unit. On high-performance or surfacing setups, it is of limited value due to it being out of the water while on plane.

Backing Plate:  Any type of plate that lies on the inside of the hull.  It helps distribute the load from the fasteners over a wider area than otherwise covered by a washer.

Blowout:  occurs when the ratio of air to water around the propeller gets so high that the propeller is no longer grabbing water, but is trying to propel itself through air (or a relative vacuum).  This causes the propeller to lose "bite", and then a chain of events occurs that can range from merely a "loose" steering feeling, to a vicious turn to the right (typically).  The speed at which this occurs varies with boat design, gear case design, and propeller design. 

Blowover:  An inherent dynamic instability in high-speed powerboats, a potentially dangerous tendency to pitch up, when under significant aerodynamic lift.  Often seen when they meet a wave or a wind gust they may suddenly flip backwards – or “blow over.”

Blue Print, or Blueprinting:  On an engine, it is bringing the specifications to a tighter tolerance range. On a Hull, it is the process of making the bottom straight for the entire running surfaces and bringing any edges to a sharp edge, especially the hull - transom interface. Check with manufacturer before removing hooks and rockers designed into the hull.

Bucket or Bucketing:  An effect like swinging a bucket of water over your head. The bucket is the helmet, the water your head, and the handle your neck. In this case, it works in reverse. The helmet tends to put more strain on your neck. See Helmet Restraint.

Bullet:  See Nosecone.

Cavitation: The formation of “voids” or air inside the water stream, caused by low pressure near a surface. Encountered when the water separates from the propeller.  Also called “ventilation”; causes propeller burn, and can lead to “blow-out”.

Cavitation Burn:  Metal erosion on the propeller blades caused by air or gaseous bubbles on the positive side of the propeller blades.  Usually caused by excessive ventilation of the propeller.

Cavitation Plate:  See Anti-Ventilation plate. 

Catch Can: Sometimes referred to as a “Puke Can.”  Small tank that holds a “buffer” supply of fuel to prevent fuel starvation due to fuel slosh in the main tank. Sometimes used to return fuel for electronic fuel injection (EFI) systems.

Center of Gravity:  The STATIC balance point of the deadweight of entire rig. Fore and aft, starboard and port. One of the most critical aspects of the setup; generally, a lower center of gravity is more desirable.  Not the same as, and not as important as, the Dynamic C of G, which accounts for dynamic forces that are in play during all phases of operation.

Center Steer:  A hull setup that positions the steering helm (and driver) in the center of the boat, laterally.

Chine Walk:  Severe side-to-side rolling of the boat.  It actually means to roll from chine to chine but it is also used for less severe rolling, although always severe enough to scare or cause a loss of control. Essentially, this is a characteristic of high speed V-bottom boats, where the boat is balancing on the pad or very bottom, further aggravated by the drag of the lower unit.  Chine walking must be compensated for by steering input from the operator.

Chopper Propeller:  Also known as a “Round Ear” propeller.  A high-rake propeller design utilizing rounded leading and trailing edges.  This design is useful for V-bottom and heavier hulls that do not generate their own bow lift.

CLE:  “Crescent Leading Edge” lower unit.  An older-generation Mercury high-performance lower unit with integral nosecone and low water pickups.  There were two designs of the CLE, both offering the same performance.  Designed for high-speed surfacing applications, and originally equipped on 2.4 Bridgeport engines.  No longer in production.  2.0/2.4/2.5L applications only.

Cleaver Propeller:  A stern-lifting propeller design utilizing thin leading edges and square trailing edges, primarily used for stern lift/tunnel boats.

Cone or Nose Cone:  The foremost part of the gear case on a Lower Unit. It is also the after market add-on to create a longer aspect ratio out of the gear case. It is the location for the Low Water Pickups.

Crossflow: A type of two-stroke technology.. The incoming fuel mixture is deflected by a dam on the piston and is forced up into the combustion chamber.

Cup: modified area of the trailing edge of the propeller that will and can affect hole shot and bow lift.

Crab The tendency of a high performance single engine boat to track to one side. The cause is surfacing of the propeller causing a paddle wheel effect. Can be noticed on dual engine boats that do not have counter rotating gearcases.

Displaced Transom:  Transom design increasing the effective setback.  Usually implemented by the addition of a standoff box incorporated into the transom design.

Dual Cable Steering:  Safety Item. Two steering cables that offer redundancy if one fails.  They are also used to eliminate excess slack in the steering system, offering greater control. The cables can attach to the engine from the same side or opposing sides.

Detonation:  Internal deterioration of aluminum components (usually the piston) resulting from lean mixtures, the collision of multiple flame fronts (excessive timing advance), and poor quality fuel.

Dockbuster:  Dockbuster refers to the Merc motors that were made in the 50's and
early 60's that had no shifting in the lower units.  They had no neutral and
when you cranked the motor to start it the prop turned and when it fired
you were underway.  You would pull the single lever back to the middle postion
and it killed the engine.  You would pull the lever to the rear and hit the starter
button and the starter would spin the motor in reverse direction and it would
run the motor in reverse ..  Mercury called this Direct Reversing and you often
see these motors referred to as DR's  A motor that has a shifting lower unit is
called a Full Gear Shift and referred to as FGS.  When the DR's were tuned
well and used by someone that had learned to manipulate the controls they
actually handled a lot better than this sounds.  But becuase they often fell into
unskilled hands the name Dockbuster became pretty common.

ECU:  Electronic Control Unit – the “brains” of an electronic fuel injection system.  A computer which monitors engine systems and controls fuel delivery.

Edge:  Refers to the corner formed by hull angle changes.

Foot Pedal:  Safety Item. A foot operated throttle pedal as in a car. It allows both hands to be used for steering, and offers faster and more precise throttle control.  Sometimes referred to as a “Hot Foot.”

Gauge:  An instrument for monitoring information – speed, engine rpm, water pressure, and so on.

Galling:  Occurs when the threads of a bolt are damaged from lack of lubrication or excessive force of tightening.  This is not cross-threading. 

Gelcoat:  The outer cosmetic layer in a fiberglass hull or product that provides a protective, high-luster finish.  Similar to paint, but epoxy–based and much thicker.

Goggles:  Safety Item. Eye protection that is almost essential in high-speed applications.  Goggles shield the eyes from wind and tearing, allowing the operator full field of view.

Helmet:  Safety Item. Offers head protection and eye protection if it is the full-face type.  Can be open face or closed (full) face.  D-Rings allow use of a helmet restraint.   A helmet should be Snell approved.  Once used in an impact situation, the helmet must be replaced. See Helmet Restraint and Jacket.

Helmet Restraint:  Webbing worn under the arms with 4 or more straps that attach to the helmet via D-rings.  Provides some neck from Bucketing during an accident situation.

Hole-Shot:  How the boat performs from a dead stop to planing speed.  Generally, a faster hole shot is more desirable.

Hook (noun):  The shape of the hull where the bottom is concave, rather than at or near the transom.  Opposite of a Rocker.  A hook can be caused by insufficient support from the Trailer, failure of the Hull, or actually designed into the hull.

Hook (verb):  The tendency of some hulls to dart to the right or left due to drastic weight transfer to the bow, usually under extreme deceleration. Also can be noticed in tight turns when one side of the hull wets out.

HP: Horse Power:  The power output rating of an engine or motor.  Computed by using the following formula:  Torque X RPM / 5252.

Hydrodynamics:  Pertains to the properties and forces of the flow of water over or around a surface.

Hydraulic Steering:  A steering system that utilizes fluid pressure to assist in steering action.  The system is comprised of a fluid pump, pressure lines, and the steering cylinder.

Jacket:  Racing jacket. Used for high speeds.  Offers a face up turning collar to prevent helmet-scooping water into it.  May include impact protection for the torso.  Will stay on and remain in one piece at high speeds.  Not Coast Guard approved.

Jackplate:  Also known as a Transom Jack or Lift Plate.  A mechanism that allows vertical adjustment of the outboard on the transom.  It also adds set back. There are manual and hydraulic types.

Kill Switch:  Emergency cut-off switch, usually actuated by a lanyard attached to the operator.  When pulled, the lanyard will actuate the switch, cutting the engine and fuel.

Kite (verb): The affect on a boat when too much air is packed under it at high speed; usually due to an inherent dynamic instability in high-speed powerboats. Caused by excessive trim, gusts of wind, launching off a wave. Many times can result in a blowover.

Layup:  Referring to the application layers, thickness, and materials used in the construction of a fiberglass hull.

Lean (Lean Mixture)A lower-than-normal fuel content in the engine’s air/fuel mix.  Often results in higher operating temperatures, overheating, detonation, and increased power output.

Looper or Loop Charged:  Another type of two-stroke technology, different than the Crossflow. The fuel mixture enters the cylinder on opposite sides and when it hits in the middle it is forced up into the combustion chamber.

Low Water Pick-Ups:  Cooling water inlets on a lower unit. These are on the nose, usually below the point. Allows for high jack height and still delivers water. The normal location of the water pick up for the water pump is on the side of the lower unit above the gear case.

Midsection: The part of an outboard that connects the lower unit and powerhead together. Contains the steering mechanism as well as the Tilt/Trim mechanisms.

Outboard: A drive package consisting of: A powerhead, midsection, and a lower unit. It normally has steering integrated into the package and usually but not always a trim mechanism.

Opposed Steering:  The steering cables attached to the outboard from the starboard and port sides.

Pad:  The flat, center component of a V-bottom hull.  Pads offer V-bottom hulls greater lift compared to non-padded V-bottom designs.  Also refers to the running surfaces of the outboard sponsons on a tunnel boat.

Planing:  When the boat has achieved sufficient speed to ride onto its bow wave, a function of the hydrodynamic design of the hull. Phase of operation that comes after “submerged” motion, has characteristic of significantly less “drag”.  Tunnel type boats ride on a rammed cushion of air and thus plane twice, once for the water and again for the airlift.

Pitch (Propeller):  The theoretical distance a propeller travels in one full rotation.

Porpoising:  A constant rhythmic longitudinal pitching action of the boat, caused by a dynamic longitudinal instability.  Often results from improper dynamic balance of weight and aero/hydro-dynamic forces, and usually occurs at specific speed for a unique hull setup. Over trimming the engine might cause this, whereby the bow is being held up by prop thrust, but not enough to stabilize the condition. Trimming 'in' can eliminate it at low speeds. The same action from the boat caused by not enough hull lift and subsequent falling of the bow back into the water.  On some particular rigs and setups, this could be a transition zone from hydrodynamic lift to aerodynamic lift that needs to be driven through in some manner before the hull stabilizes.  Can also be caused by rocker

Powerhead:  The internal combustion engine that powers an outboard.

Propeller Shaft:  The output shaft of the lower unit, where the propeller is mounted.

Prop Shaft Centerline:  Referenced to the boat bottom for prop height measurement. Measured at zero trim. For high performance use is within two inches of the boat bottom- above, equal to, or below it. Can be measured with a straight edge placed on the hull; the distance between the center of the prop shaft and straight edge is then measured.

Prop Walk:  A “paddle wheel” effect whereby the propeller will ‘walk’ across the water, sometimes caused by excessive engine height, not enough compensating angle in the torque tab, and high-rake propellers, often occurs with 2 and 3 blade propellers less than with 4-5 blades.

Rake (Propeller):  The longitudinal angle of the propeller blades.  Usually, higher-rake propellers provide more bow lift and better holding power.  Rake and Pitch are not always related.

Rich (Rich Mixture):  A higher-than-normal fuel content in the engine’s air/fuel mix.  Often results in cooler operating temperatures, black exhaust deposits (carbon), plug fouling, and decreased power output.

Rig: (noun):  The Trailer and boat package as towed.  Also used to refer to the boat/engine combination.

Rig: (verb):  To install anything not part of the hull manufacture process. Interior, gauges, wiring, so on, to ready the boat for operation.

Rocker:  The shape of the hull where, rather than being flat it takes on a convex shape. Opposite of Hook. Can be caused by insufficient support from the trailer or failure of the hull. Almost never an intentional design characteristic of the hull.

RPM:  Acronym forRevolutions Per Minute.”  Almost always referring to engine crankshaft rotation speed.

Safety Gear or Gear:  See Jacket, Helmet, Helmet Restraint, and Goggles

Safety Switch:  Safety Item. See Kill Switch.

Setback:  The distance between the transom and the outboard. Changes the center of gravity of the boat, and allows the engine to operate in less aerated water.

Set-up:  Everything in the boat that contributes to the performance required.  A setup can be geared towards top speed, holes hot, or a compromise of the two.

Skeg:  Lower rudder-like component of a gearcase.  Provides stability and tracking as well as counteracting steering torque when equipped with a Torque Tab.

Splines:  Machined grooves on a driveshaft, propeller shaft, or some other shaft that locks its mating counterpart in place, preventing rotation on the shaft.

Solid Engine Mounts:  Safety and performance Item. Solid aluminum, rather than rubber engine mounts. Reduces slack in engine mount system, offering greater control and stability.

Sponson:  The outer running surfaces of a tunnel boat.

Sportmaster:  Current-generation Mercury high-performance lower unit.  Includes a nosecone and low water pickups into the design, as well as stronger than stock internals.  Designed for high-speed surfacing applications.  Available for 2.0/ 2.4/ 2.5, and 3.0 applications.

Super Speedmaster IV:  Lower units used on Mercury 2.0 SST120/S2000 and 2.5L S3000 and Champ race outboards.  Available in 14:15 and 15:17 direct drive ratios.  Designed exclusively for racing applications.

Stator:  Wire winding assembly, usually located under the flywheel of an outboard engine.  Generates electricity for the ignition and charging system as the flywheel’s magnets pass over the windings.

Step:  A hull design characteristic whereby the running surface is stepped up to induce air into the water contact area, reducing drag.  A hull may contain one or more steps.

Stuffing:  The opposite of Kite. A serious situation that occurs when the bow digs into a wave and the water pressure drives the bow down, in many cases causing severe damage and injury.

Tongue:  The part of the trailer that continues forward to connect to the tow vehicle. Should be 10% of the weight of the entire rig as measured at the hitch connection.

Torquemaster:  A heavy-duty high performance Mercury lower unit that does not have a nosecone.  Designed for boats with heavier loads that require power trim for bow lift.  Current models include low water pickups, however, earlier models did not.  Available for 2.5 and 3.0L applications.

Trailer: The mechanism that lifts the Outboard past the supports - out of the range of Trim. May or may not be the same mechanism as Trim.

Trailer: The machine for the transport of the boat. A trailer should give excellent support to the hull.

Transom: The back of the boat. Usually concerning the attachment area for the outboard or stern drive unit, and how well it is supported to transmit the loads to the hull.

Transom wedges:  Shims place in between the outboard and the transom or jackplate that change the effective trim angles.

Trim: The angle of thrust from the propeller.  The trim angle can be adjusted in (negative) and out (positive). Zero trim is when the prop shaft is parallel with the boat bottom. Positive Trim helps lift the bow, Negative trim holds the bow down.

Trim (2):  The mechanism that allows the change in thrust angle of an outboard or stern drive unit.

Trim Pump:  Electric pumping unit that pressurizes the engine hydraulic trim system.

Tuner:  The megaphone-shaped exhaust pipes inside the midsection.  Essentially, the tuner is the exhaust “header” of the outboard engine.

Tunnel Boat:  Hull design that incorporates two or more sponsons, taking advantage of aerodynamic forces to create air lift, subsequently reducing water drag.

Tunnel Vee:  A synthesis of the Tunnel Boat and V-Bottom designs, incorporating a large center sponson, and two small outer outboard sponsons for added lift and stability.

V-Bottom:  Hull design utilizing a centrally located, single running surface for lift.  These hull designs generally do not rely on air pressure for a significant portion of their lift.

Wheel Torque: The tendency for the steering wheel to forcibly turn right. Caused by the prop rotation and increases as the engine is jacked out of the water, becoming maximum when only one blade of the prop is entering the water. See jack height & Prop Shaft Centerline.

*More items will be added, as well as photos to match descriptions.

Special Thanks to Techno, Jim Russell of Aeromarine Research, John Marles, Raceman, and Anthony Santocono.


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