Only because after Terry Darlington made it sound dangerous in his book (it wasn't), no-one would insure narrowboats crossing the Channel any more. We did take Snail out to sea in Europe a few times where she behaved very well, even in chop. Unlike Chris Coburn who has achieved all kinds of exploits in his very ordinary nb, we had ours especially made with a 15mm bottom plate, ballast cut to fit so they couldn't slide, 67hp engine, PRM260 gear box, large prop (22x22), double sized skin tanks, all portholes, trad stern, tender that was always available for use if needed, easy to tow along behind on the big European waterways. Snail easily makes 10 kts, (we tested this on the Gent-Terneuzen Canal,built in 1823 which is 200m wide and capable of accomodating ships up to 125,000 tons, with a draught of up to 12.5 m) not a lot of use now we've come back to the UK canals. My blog and book illustrates what's achievable.