Unlimited Glossary of Technical Terms
By Fred Farley - ABRA Unlimited Historian
Afterplane: 1) the bottom surface aft of the break, normally flat and Parallel to the water surface; 2) the entire main hull aft of the sponsons; the "fuselage."
Air Dam: an aerodynamic device or spoiler in the tunnel under the bow or leading edge of a three-point hydroplane; typically a flat strip set on edge laterally (crosswise), it creates turbulence thus reducing lift.
Air Trap: 1) the aftward continuation of the inside vertical "walls" of the sponsons, forming tunnel sides and channeling high-pressure air forced underneath by the boat's shape and forward motion; 2) the inside vertical "wall" of sponson and air trap from bow to stern; the tunnel side.
Airfoil: The cross-sectional profile of a wing.
Angle of Attack: 1) hydrodynamically, the designed angle between the planing surface of the sponson and the surface of the water, measured fore and aft; (2) aerodynamically, the angle between the chordline of a hydroplane's wing and the apparent wind.
Angle of Incidence: the designed angle between the chordline of a hydroplane's wing and the water surface; the angle is fixed for ram wings but adjustable for auxiliary devices such as the horizontal stabilizer.
Break: the transverse juncture where the forward, angled part of a hull bottom meets the afterplane on a three-point hydroplane.
Bustle: a fixed hydrodynamic shape on a chine, which enlarges the area available for buoyancy, especially when the boat is turning.
Cabover: Rear-engined boat, with driver sitting ahead of engine.
Canard: 1) a small, forward-mounted wing, usually between the sponsons and often adjustable for trim, either by pivoting about a lateral axis or by hinged or sliding flaps on the trailing edge; also "foreplane" or "noseplane" in aeronautical terms; 2) a reverse three-point (actually four-point) hull configuration, a "tricycle," with two sponsons aft and a planing shoe forward.
Canoe: a construction element of modern three-point hydroplanes, including sponson, chines, and air trap full length from bow to stern, molded as one piece.
Chine: the inclined side of hull or sponson; also "non-trip."
Chordline: the imaginary straight line from leading edge to trailing edge of an airfoil, this distance being the "chord."
Deck: the top or upper surface of hull; decking.
Dihedral: hydrodynamically, the designed angle between the planing surface of the sponson and the surface of the water, measured laterally inside to outside; a negative angle is called anhedral.
Elevators: movable surfaces on the trailing edge of canards or ram wings that direct airflow; also ailerons, flaps, flippers.
Engine Hood: a part of the cowling that covers the engine and engine compartment.
Engine: by rule, an inboard, internal-combustion powerplant either reciprocating or gas turbine.
Fence: a small fixed aerodynamic device running fore and aft (chord-wise) on a wing or hull, usually on the upper surface; also "deck fence" or "deck trap," an upward extension of the sponson wall above the deck.
Flaps: see Elevators
Fuel: by rule, "any liquid or gas (other than air) that passes through the intake system and/or reaches the combustion chambers(s) of an engine," including "oil, gasoline, Jet A, alcohol, water, and nitrous oxide."
Horizontal Stabilizer: loosely called the "wing," the airfoil-shaped surface mounted horizontally above the transom and usually adjustable for trimming.
Non-Trip Chine: chine angled or inclined so as not to catch or trip on the water when the boat is turning, often called simply "chine" or "non-trip."
Nose Cowl: the forward-most part of cowling or superstructure, a streamlined fairing; in a rear-engined boat the nose cowl is usually an integral part of the enclosed cockpit structure.
Picklefork: the current three-point hull refinement in which configuration the two sponsons extend forward of the main hull center section.
Propeller: the device with two or (usually) three blades connected to a Shaft and turned by the engine, thereby generating thrust from rotation in the water; current unlimited hydroplanes employ a surface-piercing propeller; also called "prop," wheel," or "screw."
Prop Shaft: propeller shaft, the rotating shaft that passes through the bottom of the boat, connecting and transmitting power from engine/gearbox to the propeller; generally of Monel or high-strength steel alloy with a diameter of 1-3/4 to 2 inches.
Ram Wing: term properly given to the main hull center section of a three-point hydroplane, which has an airfoil shape and generates lift by aerodynamics and ground effects; also a secondary or tandem structure of the same sort.
Rudder: an upright blade that can be turned to left or right about a vertical axis, generally mounted on the transom and extending below the waterline to provide the means of turning a boat while underway.
Runner: the part of the sponson bottom that serves as a planing surface, touching the water at the lowest point.
Shaft Log: the fitting in the bottom of the hull through which the prop shaft passes; also called "stuffing box."
Skid Fin: a vertical blade attached to the hull generally aft of the left-hand sponson, extending below the waterline to help keep a boat from sliding in a turn.
Slat: a fixed or adjustable aerodynamic device mounted integrally with or immediately forward of the ram wing to create a "slot" effect along the leading edge; may be a canard.
Slot: a through-deck opening or passage formed by the gap between a slat and the leading edge of the ram wing, altering air flow; sometimes a "deck vent." Spar: 1) the major, span-wise, structural member of a wing, a means of attaching sponsons or canoes to the main hull; 2) the brace between the sponsons, often covered by a canard airfoil shape.
Spoiler: an aerodynamic device protruding into the airstream and "spoiling" or reducing lift; usually under the bow, such as an air dam, but occasionally topsides, such as a speed brake.
Sponson: the pontoon-like portion of the hull that provides hydrodynamic lift; in the three-point configuration common to current unlimited racing craft, one sponson is forward on each side of the main hull.
Strut: 1) a support or brace; 2) specifically the prop-shaft strut, a structural member on the hull bottom near the transom, which carries the rear bearing for the prop shaft. Stuffing Box: see Shaft Log.
Tail Fin: a vertical aerodynamic surface, part of the boat's superstructure near the transom, which aids directional stability; typical current designs employ two tail fins or uprights side by side, perhaps 4 to 6 feet apart and slightly splayed; also "air rudder."
Transom: the vertical structural element or end frame that defines the aft end of the main hull or ("sponson transom") the aft end of the sponson.
Tunnel: 1) the space or channel between the bottom of the hull center section, or main hull, and the sponson inner walls and air traps, in which air flow is directed and compressed by the boat's forward motion, generating a high-pressure air cushion to partially support the weight of the boat; 2) a hull configuration that employs full-length sponsons, a catamaran-like shape.
Uprights or Verticals: a common term for twin tail fins, which usually comprise upright trusses covered by sleeves or fairings and often support a horizontal stabilizer.
Wing: an aerodynamic surface that can generate lift; while the term is informally applied to the horizontal stabilizer, the most important wing on a modern hydroplane is the "ram wing."