54363 isn't free
After I wrote 'is this the shortest URL ever?', Nic Chilton says he has registered 'http://nboat.tk'.
How short can a canal boat address get?
You could try checkdomain.com, which will tell you which letters relate to which countries. (TK is Turkmenistan, a shady extension by my reckoning). There's no country ending in '.nb'.
But, of course, length doesn't count as much as thickness. What's needed isn't brevity but sense.
If you want a short domain for your boat, try the boat's registration number. Granny Buttons is 54363. Strangely, 54363.com was registered only a few months ago and I've missed out. And the '.co.uk' extension is a bit too long-winded.
I registered grannybuttons.com nearly ten years ago. Don't expect narrowboat.com or kingfisher.co.uk to be still available.
But you might find your boat number is still free.