Question: In the photo below you can see the large diameter hose near the center of the photo is marked “Type A2.” The hose is the fill pipe on my diesel-fueled boat. I thought all fuel hoses needed to be rated as Type A1 to be compliant with USCG regulations. What’s up here?


This is an A2-rated hose that connects a deck fill opening to a diesel tank. It would not normally be full of fuel for long periods of time.

Answer: This is a confusing issue for a lot of folks. First of all, the USCG doesn’t say anything about diesel fuel systems on recreational boats. Gasoline-fueled boats for sure, they have lots to say, but on diesel, nothing. That said, the builder in this case may have been trying to comply with ABYC recommendations when building your boat. The ABYC basically mirrors its gasoline fuel standard with its diesel standard. Here’s the way fuel hose designations play out:

Diesel fuel fill and vent hoses are minimally rated A2 if the hoses are routed through engine room spaces. The “A” designator is for a fire-test rating, only required in engine room spaces. A2 has a lower rating in terms of fuel vapor permeability when compared to the A1 rating, which is required for fuel distribution lines. The thinking here is that because fuel fill and vent hoses are not constantly filled with fuel, a less stringent permeability rating is acceptable. The actual difference in permeation rating is quite considerable: A1 has a maximum permeation rating of 100 grams/meter squared over a 24-hour period. For A2 it’s 300 grams/meter squared in 24 hours.

Fuel hose also has a new designator of A1-15, required on new boats using gasoline fuel that is in accordance with new evaporative emissions requirements. Its maximum permeability rate is 15 grams/square meter over a 24-hour period, a much more stringent standard.

Note that there are also “B” rated hoses. These may be used in non- engine room spaces only, and do not carry a fire test rating. The B-series hoses do share the same permeability ratings, however.