VHF marine FM narrowbanding impacts
12.5 kHz channel spacing is
permitted
in certain cases by the FCC in VHF marine radio.
Despite
experiments
demonstrating the practicality of using 12.5 kHz channel spacing on VHF marine radio, currently available radios and practice appear to remain at +/- 5 kHz deviation and 25 kHz channel spacing.
Unlike the vocal
controversy around 1969 over marine VHF narrowbanding
from +/- 15 kHz deviation to +/- 5 kHz deviation, there appears to be little controversy over going to 12.5 kHz marine VHF channel spacing.
Assuming there isn’t adjacent channel interference, the communications range penalty from narrowbanding 5 kHz FM deviation systems to 2.5 kHz FM deviation can be represented to first order as 20*log10(2.5/5) + 20*log10(16/11) ~ -3.3 dB, which is measurable but not necessarily devastating for a system that already has adequate coverage.
This comes from the disadvantage of reduced FM modulation index (2.5 / 5) and the advantage of narrower IF receiver filter (11 kHz vs. 16 kHz typical).
The true significance must be measured and modeled for the particular communications systems.
Keep in mind the reduced ability to withstand co-channel interference due to the reduced FM capture effect, which can be expressed in dB as the cube 10*log10((5/2.5)**3) (9 dB C/N reduction in resilience to co-channel interfering FM signals).
Unlike in commercial PMR / LMR / SMR systems where digital modulation gains of 3-4 dB can be achieved by say APCO25, DMR, NXDN or similar, VHF marine radio like airband VHF needs to keep compatible analog modulation schemes indefinitely due to the large number of casual spectrum users that still need life-critical communications access.