work out distance (eg use google maps) then estimate sailing speed of around 10mph (more with a good wind behind you - less if having to tack) - summer days are quite long 3am to 10pm is light but you could sail by moonlight + a knowledgable sailor could go on an overcast night in a small boat purely on sound of the sea + the 'feel' of the boat as the Irish sea is quite shallow in places (eg even coming back on the Ben you know when you are approaching Douglas as the boat behaves differently in the shallower water