Which fire extinguishers would be best suited for use on board a boat?

Class B fires involve flammable or combustible liquids such as gasoline, kerosene, grease and oil. Nearly all powerboats are required to have at least one portable Class B fire extinguisher (i.e., USCG approved B-I or B-II).

Other types of extinguishers are optional, depending upon what types of fuels may catch fire, such as Class A for "ordinary combustibles" and Class C for energized electrical fires. Class ABC extinguishers are quite popular.

The numerical rating BEFORE the letter B, for class B extinguishers, indicates the approximate number of square feet of fire it can extinguish (i.e., with no significant depth).

The numerical rating (I, II, III) AFTER the letter B indicates Coast Guard classification. For example, a UL 5-B:C, rated for 5 sq ft of liquid fire, would be a USCG B-I extinguisher (with 2 pounds of dry chemical).

To obtain a USCG B-II rating you would need the equivalent of 10 pounds of dry chemical, such as having two UL 3-A, 40-B:C extinguishers, or a 15-pound CO2 extinguisher, or 2 1/2 gallons of AFFF "foam" (UL 3-A:20B).

A 20-pound dry-chemical (120-B:C) would meet USCG B-III requirements.

USCG ratings go as high as V, which is a big extinguisher on wheels.