The guidance comes with the Policy Letter 01-19: 'Equivalency Determination – Fire Pumps for Subchapter C and Subchapter M Towing Vessels'.
When developing the 2003 interim rule and 2004 final rule addressing fire-suppression systems and voyage planning for towing vessels, the Coast Guard assumed smaller vessels would utilize portable fire pumps, while larger vessels would utilize fixed fire pumps; therefore, vessel size was not associated with fire pump requirements.
This assumption imposed a larger burden on smaller vessels that opted to install fixed fire pumps.
Generally, the fixed pump is required to have a higher capacity to accommodate the greater hydraulic losses expected from the extensive fire main systems used in larger vessels.
In small vessels, however, the hydraulic losses of the smaller fire main systems are negligible, and the fixed fire pump need be of no higher capacity than the portable pump.
Thus, in small vessels, fixed and portable pumps of similar capacity will provide similar fire streams affording an equivalent level of safety.
Accordingly, for towing vessels 65 feet or less subject to 46 CFR 27.301 or 46 CFR 142.325, vessels may use a fixed fire pump that meets the performance requirements of the portable fire pump in 46 CFR 27.301e(1) or 46 CFR 142.325e(1) in lieu of the requirements in 46 CFR 27.301a(1) or 46 CFR 142.325a(1).