The provisions of this section of Part 6 in the 2002 BSS Standards are mandatory for non-private boats where applicable.
In the event of an onboard fire the extinguishers must be easy to reach - they must be kept in readily accessible positions close to the main fire risk points, e.g. the engine space and the galley.
If there's a fire in the engine space and additional air is allowed into the area the fire will quickly escalate. To reduce the risk of this happening it's a good idea that any portable extinguisher used to protect the engine space is capable of being used without fully opening the primary access/engine hatch. [6.1]