Based on an article published by Don Casey for BOAT-US
Here is a statistic you better know about.
Those 10-BC dry chemical fire extinguishers we all carry (designated B-I by the Coast Guard) will discharge for maybe 10 seconds. Thats right.....10 seconds!
For an onboard fire with a head start, that won't be long enough to put the fire out. Then what?
If your boat is larger than 26 feet, Coast Guard regulations require a second B-I extinguisher aboard, that or a larger B-II. A second 10-BC gives you another 9-second shot-if the fire hasn't put this second unit out of reach.
A B-II extinguisher is better, carrying at least a 60-BC rating, which doesn't mean the extinguisher will discharge for six times as long, but does mean it has six times the extinguishing capacity. But if your single B-II is on the other side of the flames.
Meeting Coast Guard requirements equips your boat to extinguish only the smallest of onboard fires.
For real fire protection, take aboard B-II extinguishers, or at least the larger B-I, rated 40-BC. All but the smallest watercraft should have at least two extinguishers aboard, mounted in opposite ends of the boat.