According to the USCG per the CFRs...
Sec. 183.53 Horsepower capacity.
The maximum horsepower capacity marked on a boat must not exceed the horsepower capacity determined by the computation method discussed in paragraph (a) of this section, or for certain qualifying boats, the performance test method discussed in paragraph (b) of this section. (a) The maximum horsepower capacity must be computed as follows: (1) Compute a factor by multiplying the boat length in feet by the maximum transom width in feet excluding handles and other similar fittings, attachments, and extensions. If the boat does not have a full transom, the transom width is the broadest beam in the aftermost quarter length of the boat. (2) Locate horsepower capacity corresponding to the factor in Table 183.53. (3) For a boat with a factor over 52.5, if the horsepower capacity calculated in Table 183.53 is not an exact multiple of 5, it may be raised to the next exact multiple of 5. (4) For flat bottom hard chine boats with a factor of 52 or less, the horsepower capacity must be reduced by one horsepower capacity increment in
See Table 183.53--Outboard Boat Horsepower Capacity
(b) For boats qualifying under this paragraph, the performance test method described in this paragraph may be used to determine the horsepower capacity. (1) Qualifying criteria. (i) Thirteen feet or less in length; (ii) Remote wheel steering; (iii) Transom height (A) Minimum 19 inch transom height; or, (B) For boats with at least a 19 inch motorwell height, a minimum 15 inch transom height; (iv) Maximum persons capacity not over two persons; (2) Boat preparation. (i) The boat must be rigged with equipment recommended or provided by the boat and motor manufacturer and tested with the highest horsepower production powerplant for which the boat is to be rated, not to exceed 40 horsepower. (ii) Standard equipment must be installed in accordance with manufacturers' instructions. (iii) The lowest ratio (quickest) steering system offered on the boat model being tested must be installed. (iv) The outboard motor must be fitted with the manufacturer's recommended propeller providing maximum speed. (v) Standard permanently installed fuel tanks must be no more than one-half full. Boats without permanent tanks must be tested with one full portable tank. (vi) Portable tanks must be in their designated location or placed as far aft as possible. (vii) The outboard motor must be placed in the lowest vertical position on the transom or, if mounting instructions are provided with the boat, at the height recommended. (viii) Boat bottom, motor and propeller must be in new or almost new condition. Note: The use of the following special equipment should be considered because of the potential for exceeding the capabilities of the boat while performing the test:Racing Type Personal Flotation DeviceHelmet. (3) Test conditions. Testing must be conducted on smooth, calm water with the wind speed below 10 knots. The test must be conducted with no load other than a driver who must weigh no more than 200 pounds. The motor trim angle must be adjusted to provide maximum full throttle speed short of excessive porpoising or propeller ventilation or ``cavitation'', so that there is no loss of directional control. (4) Quick turn test procedure. Set throttle at a low maneuvering speed and steer the boat straight ahead. Turn the steering wheel 180[deg] in the direction of least resistance in \1/2\ second or less and hold it at that position without changing the throttle or trim settings during or after the wheel change. The boat completes the maneuver successfully if it is capable of completing a 90[deg] turn without the driver losing control of the boat or reducing the throttle setting. Gradually increase the boat's turn entry speed incrementally until the boat does not complete the Quick Turn Test successfully or successfully completes it at maximum throttle. Note: It is recognized that operator skill and familiarity with a particular boat and motor combination will affect the test results. It is permissible to make a number of practice runs through the quick turn test at any throttle setting. (5) Test course method. Set throttle for 30 miles per hour boat speed and run the test course set up in accordance with Figure 183.53, passing outside the designated avoidance marker for 35 to 37.5 miles per hour without contacting any of the course markers. If the boat successfully completes this run of the test course, increase the throttle setting to 35 to 37.5 miles per hour boat speed and run the course passing outside the designated avoidance marker for that speed without contacting any of the course markers. If the boat successfully completes this run of the test course and the motor was not at full throttle, increase the throttle setting to 37.5 to 42.5 miles per hour boat speed and run the course passing outside the designated avoidance marker for that speed without contacting any of the course markers. If the boat successfully completes this run of the test course and the motor was not at full throttle, increase the throttle setting to 42.5 miles per hour or more and run the course passing outside the designated avoidance marker for that speed without contacting any of the course markers. If the boat successfully completes this run of the test course and the motor was not at full throttle, continue to increase the throttle setting and run the test course passing outside the designated avoidance marker for 42.5 miles per hour or more until the boat fails to complete the test successfully or the boat completes the test course maneuvers successfully at full throttle. The boat successfully completes the test course if the driver is able to maneuver it between the designated avoidance markers without striking the markers and without losing control of the boat or reducing the throttle setting. There must be no change in position of any equipment on board and there must be no change of position of personnel in order to influence the test results. There must be no instability evidenced by oscillating motion in the roll or yaw axes exhibited while negotiating the course. Note: It is recognized that operator skill and familiarity with a particular boat and motor combination will affect the test results. It is therefore considered permissible to make a number of practice runs through the test course at any throttle setting. (6) Maximum horsepower capacity. (i) For boats capable of less than 35 miles per hour, the maximum horsepower capacity must be the maximum horsepower with which the boat was able to successfully complete the Quick Turn Test Procedure in Sec. 183.53(b)(4) at full throttle or the maximum horsepower determined under the calculations in Sec. 183.53(a) of this section. (ii) For boats capable of 35 miles per hour or more, the maximum horsepower capacity must be the maximum horsepower with which the boat was able to successfully complete both the Quick Turn Test Procedure in Sec. 183.53(b)(4) and the Test Course Method in Sec. 183.53(b)(5) at full throttle or the calculations in Sec. 183.53(a) of this section. (iii) The maximum horsepower capacity determined in accordance with Sec. 183.53(b) must not exceed 40 horsepower. Figure 183.53--Boat Horsepower Capacity Test Course--35 MPH or More[GRAPHIC] [TIFF OMITTED] TC18OC91.021
Subpart J_Fuel SystemsSec. 183.526 Carburetors. (a) [Reserved] (b) Each carburetor must not leak more than five cubic centimeters of fuel in 30 seconds when: (1) The float valve is open; (2) The carburetor is at half throttle; and (3) The engine is cranked without starting; or (4) The fuel pump is delivering the maximum pressure specified by its manufacturer. (c) Each updraft and horizontal draft carburetor must have a device that: (1) Collects and holds fuel that flows out of the carburetor venturi section toward the air intake; (2) Prevents collected fuel from being carried out of the carburetor assembly by the shock wave of a backfire or by reverse air flow; and (3) Returns collected fuel to the engine induction system after the engine starts,Subpart J_Fuel SystemsSec. 183.524 Fuel pumps. (a) Each diaphragm pump must not leak fuel from the pump if the primary diaphragm fails. (b) Each electrically operated fuel pump must not operate except when the engine is operating or when the engine is started. (c) If tested under Sec. 183.590, each fuel pump, as installed in the boat, must not leak more than five ounces of fuel in 2\1/2\ minutes, inclusive of leaks from fuel line, fuel filter and strainer.Subpart I_Electrical SystemsSec. 183.410 Ignition protection. (a) Each electrical component must not ignite a propane gas and air mixture that is 4.25 to 5.25 percent propane gas by volume surrounding the electrical component when it is operated at each of its manufacturer rated voltages and current loadings, unless it is isolated from gasoline fuel sources, such as engines, and valves, connections, or other fittings in vent lines, fill lines, distribution lines or on fuel tanks, in accordance with paragraph (b) of this section. (b) An electrical component is isolated from a gasoline fuel source if: (1) A bulkhead that meets the requirements of paragraph (c) of this section is between the electrical component and the gasoline fuel source; (2) The electrical component is: (i) Lower than the gasoline fuel source and a means is provided to prevent fuel and fuel vapors that may leak from the gasoline fuel source from becoming exposed to the electrical component; or (ii) Higher than the gasoline fuel source and a deck or other enclosure is between it and the gasoline fuel source; or (3) The space between the electrical component and the gasoline fuel source is at least two feet and the space is open to the atmosphere. (c) Each bulkhead required by paragraph (b)(1) of this section must: (1) Separate the electrical component from the gasoline fuel source and extend both vertically and horizontally the distance of the open space between the fuel source and the ignition source; (2) Resist a water level that is 12 inches high or one-third of the maximum height of the bulkhead, whichever is less, without seepage of more than one-quarter fluid ounce of fresh water per hour; and (3) Have no opening located higher than 12 inches or one-third the maximum height of the bulkhead, whichever is less, unless the opening is used for the passage of conductors, piping, ventilation ducts, mechanical equipment, and similar items, or doors, hatches, and access panels, and the maximum annular space around each item or door, hatch or access panel must not be more than one-quarter inch.