It is actually, at just over 17 meters I'm sort of thinking that as there's not much chance of my venturing onto unsuitable waterways in my narrowboat & as the IWA say ; 'Note theat visibility requirements: many narrow boats over 12m long are fitted with navigation lights that are not legal in this respect, as many boat builders and fitters unfortunately do not seem to know or care about the regulations. ',
I think I'll go ahead and fit the mast. Funny enough I just found a set of battery powered lights I once used on an old wooden Folkboat we did a circumnavigation of the British Isles back in the 70's. They've still got the old (leaky) batteries in, so not really in working order anymore.