I read it as he was jumping back on to empty the cargo hold and it started moving...as it might do but should never float away once he jumps off again. It may indeed 'slide' back into the water if beached.

As for beaching it at speed...it may take a second for the sail to furl if you're charging in at speed, and if you're doing that it may well take longer for it to stop due to collision physics with land...but that's just unsafe seamanship :)