Fuel Hose:
When fuel hose is used it must be either USCG Type A1, USCG Type A2, USCG Type B1 or USCG type B2. Where you use it determines what type you use.
From the fuel inlet on the engine (usually the fuel pump) to the carburetor you must use Type A1 or A2.
Each fuel vent line or fill line must be USCG Type A1 or A2 and the line from the fuel tank to the engine inlet must be type A1 or A2;
As a practical matter, few boatbuilders want to have to stock two types of hose, so most buy only Type A1 and use it everywhere. This is fine. Just be sure that you put in your owner's manual that the hose from the fuel pump to the carburetor and the vent and fill hose must be TYPE A1 or A2.
So what is the difference between A1, A2, B1, and B2? First the difference between A and B. A hose has to pass the 2 ? minute fire test, B does not. The theory is the A is under more pressure than B, and being on the engine is exposed to more danger of fire than B. Since B is between the pump and the tank, if a leak occurs, the pressure rapidly drops to zero and the pump just sucks air, stopping the engine. Fuel doesn't get sprayed all over the place. With Type A, since it is between the fuel pump and the carburetor, it is under pressure and there is the potential that fuel will be sprayed around.
The difference between Type 1 and 2 is the permeability. All hose allows a small amount of vapor to pass out through the wall of the hose. The more permeable the hose is, the more vapor passes through. A1 and B1 pass much less than A2 and B2. About 1/3 as much. The Coast Guard Sets the standard for the amount that is allowed.
