Embarking on Ships
1. It is forbidden to embark on a sea journey less then three days before
the commencement of Shabbos (1) ( i.e., from Wednesday onwards (2) ). If,
however, one is journeying for the sake of a mitzvah ("religious
obligation") (3), it is permitted even on Friday.
(1) There are varying opinions among the commentators as to the basis of
this prohibition, however, the one accepted by the Shulchan Aruch is that
travelling on the ocean upsets one's system (sea-sickness, etc.) in a way
that would detract from one's enjoyment of Shabbos ("Oneg Shabbos"), and it
can take up to three days to recover. Since this reasoning doesn't apply to
travelling on a river, these restrictions wouldn't apply (see tomorrow's
(2) This is the ruling of the Mogen Avraham (248:2) and the Shulchan Oruch
HoRav (248:1). The Vilna Gaon, however, maintains that the Shabbos is
included in these three days, and thus the prohibition begins on Thursday.
(3) Some authorities rule that if one is going on a business trip or even
to visit a friend, then it is the equivalent (within the context of this
particular prohibition) of travelling to perform a mitzvah, and one may
therefore embark on Friday. According to these authorities, only leaving
for a vacation is prohibited less than 3 days before Shabbos.