Inboard Hydroplane Class
1975 - Present
letter evolution of class current class
T 72ci - 1 Litre Stock 1.5 Litre Stock
850cc - 1 Litre Modified  1.5 Litre Modified
S 145 - 2000cc - 2300cc 2.5 Litre Stock
A 2.5 Litre - 2.5 Litre Modified 2.5 Litre Modified
N 225ci - 4 Litre extinct
E 280ci - 5 Litre Stock 5 Litre Stock
F 5 Litre Modified - 6 Litre Modified National Modified
J 7Litre Div II - 7Litre Grand National Hydro
H 7Litre Div I extinct
UL 7Litre - GNH Unlimited Lights
GP 7Litre Div I & II - 500 ci Grand Prix
U Unlimiteds Unlimiteds
This evolution of a class in this chart covers from 1975 to present.
Canadian classes of 3 Litre & 360 became extinct sometime before 1992.

1.5 Litre Stock T - class 
1.5 Litre Stock
The hulls must be minimum 13' 6" long and must weigh a minimum of 750 lbs. They have engine rules that are more stock and they must run strictly on gasoline. The class runs mostly on the East coast. 

1.5 Litre Modified Y - class
1.5 Litre Modified
The hulls for this class have the same rules as the 1.5 Litre Stock. Both 2 cycle and 4 cycle motors are permitted. Engine rules vary, depending on which engine is used. No blowers, superchargers, or outdrives are permitted. This class is mostly located in the Great Lakes area, East coast, and the Northwest.

2.5 Litre Stock S - class
2.5 Litre Stock
The hulls have to be a minimum of 13' long and weigh a minimum of 975 lbs. Engines approved are the Ford Pinto 2000cc & 2300cc. Motor alterations are not permitted except the ones specified in the rules. This class is promoted for the purpose of establishing a low cost stock class engine for racing.

2.5 Litre Modified A - class
2.5 Litre Modified
The hull has to be a minimum of 16' long. Boat weight must be a minimum of 1225 lbs. if you are using 155.5 cubic inch - Ford, Chevy, Pontiac, Datsun, BMW, or Alfa Romeo motors. The boat must weigh a min. of 1325 lb. if you are using a 166 cubic inch Gaerte, Esslinger, or Fontana engine. Engines with double overhead cams are prohibited. Fuel is alcohol or aviation gas only. No form of mechanical supercharger or other auxiliary device shall be employed to increase the compression of volumetric efficiency of cylinders. This class runs mostly in the eastern half and northwestern part of the United States.

5 Litre E - class
5 Litre Class
Hull must be a minimum of 16' long and weigh no less than 1450 lbs. Engine is the stock GM 305 cubic inch V-8, with a two barrel carburetor. No grinding, polishing or blasting of internal parts is permitted. Parts must be used as furnished by General Motors. Alterations are not permitted, except as specified in the rules. This is probably the most competitive class in inboard hydroplane racing.

NM Class NM - class
National Modified class
Hulls must be a minimum of 19' long and weigh a minimum of 1600 lbs. The engines are modified and can either run a maximum of 308 cubic inch motor with alcohol and fuel injection or a 368 cubic inch motor on gasoline and one 4-barrel carburetor. Turbochargers and superchargers are prohibited. Because of their high speeds, this class has special driver requirements beyond those covered in the general racing rules.

Grand National Hydro GNH - class
Grand National Hydro
Hull must be a minimum of 20' long with a minimum weight of 2000 lb. Engines must be cast iron blocks with a maximum of 468 cubic inches. It must be normally aspirated utilizing any American made 600 CFM carburetor with a throttle bore as follows - primary 1-9/16" max., secondary 1-9/16" max. Fuel is racing gas only to APBA specifications. This class also has special driver requirements and they also have to be okay'd by at least four other drivers of that class. 

Unlimited Light UL - class
Unlimited Lights

Grand Prix GP - class
Grand Prix Hydroplanes
Hulls must be a min. of 21' long with a max. of 26' long and 12.5' wide. Engines must be auto or marine, cast iron or aluminum, 4 cycle, 2 valves per cyl., internal combustion engine with a maximum size of 511 cubic inches. A single speed gearbox is allowed, but multiple speed gearboxes are not. Supercharging and changing engines between heats is permitted. There is a special drivers license needed and you must be a licensed scuba diver with certificate card to run in this class. There are many other special rules that apply to this class. The Grand Prix run mostly in the Great Lakes region of the U.S. and the southeast area of Canada.

Unlimited Class U - class
Hull must have a min. length of 28' and max. length of 32'. They must be at least 12' wide and no wider than 14.5'. Motors can be any type of inboard mounted supercharged or unsupercharged engine or engines or 1 turbine engine. Turbine engines allowed are listed in the Unlimited technical manual. Propulsion of the boat shall be accomplished in and/or against water by propeller or screw. Driver regulations are very strict and each driver takes a physical each day of competition.

All current racers must be:

  • seat belted in.
  • separate air supply - enclosed capsule only.
  • physical exam each year.
  • No wraparound or full face helmets are permitted.
  • Many special rules require that certain metals with certain alloys are the only ones that can be used on inboards. They must have certain shear strengths, endurance limits, and yield points.

© 2001 Phil Kunz

Vintage Flatbottom class - Part I