Courtesy of Scream And Fly.com
The online Boating performance magazine
Adapter Plate: "Adapts”
and connects the powerhead of an outboard engine to the center
section, and usually contains the tune
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.
Plate: See Anti-Ventilation
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.
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
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.”
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
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.
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”.
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.
Plate: See Anti-Ventilation
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.
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.
hull setup that positions the steering helm (and driver) in the center
of the boat, laterally.
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.
“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.
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
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.
area of the trailing edge of the propeller that will and can affect hole
shot and bow lift.
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.
Transom design increasing the effective setback. Usually
implemented by the addition of a standoff box incorporated into
the transom design.
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.
deterioration of aluminum components (usually the piston) resulting
from lean mixtures, the collision of multiple flame fronts (excessive
timing advance), and poor quality fuel.
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
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
and it killed the engine. You would pull the lever to the rear
button and the starter would spin the motor in reverse direction and it
run the motor in reverse .. Mercury called this Direct Reversing
see these motors referred to as DR's A motor that has a shifting
called a Full Gear Shift and referred to as FGS. When the DR's
well and used by someone that had learned to manipulate the controls
actually handled a lot better than this sounds. But becuase they
unskilled hands the name Dockbuster became pretty common.
Electronic Control Unit – the “brains” of an electronic fuel injection
system. A computer which monitors engine systems and controls
Refers to the corner formed by hull angle changes.
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.”
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.
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.
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.
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.
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.
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
HP: Horse Power: The power
output rating of an engine or motor. Computed by using the
following formula: Torque X RPM / 5252.
Pertains to the properties and forces of the flow of water over or
around a surface.
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. 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.
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.
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.
(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.
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.
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.
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.
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.
steering cables attached to the outboard from the starboard and port
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
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.
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
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 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
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 for
“Revolutions Per Minute.” Almost always referring to engine
crankshaft rotation speed.
Safety Gear or Gear: See Jacket,
Helmet, Helmet Restraint, and Goggles
Safety Item. See Kill Switch.
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.
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.
rudder-like component of a gearcase. Provides stability and
tracking as well as counteracting steering torque when equipped with a Torque
Machined grooves on a driveshaft, propeller shaft, or some other shaft
that locks its mating counterpart in place, preventing rotation on the
Engine Mounts: Safety
and performance Item. Solid aluminum, rather than rubber engine mounts.
Reduces slack in engine mount system, offering greater control and
outer running surfaces of a tunnel boat.
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.
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.
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.
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.
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
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.
Shims place in between the outboard and the transom or jackplate
that change the effective trim angles.
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.
mechanism that allows the change in thrust angle of an outboard or
stern drive unit.
Electric pumping unit that pressurizes the engine hydraulic trim system.
megaphone-shaped exhaust pipes inside the midsection.
the tuner is the exhaust “header” of the outboard engine.
design that incorporates two or more sponsons, taking advantage
of aerodynamic forces to create air lift, subsequently reducing water
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.
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.
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
*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.