The aforementioned prohibition on setting sail within three days of Shabbat in order to avoid its desecration or the negation of the mitzva of oneg Shabbat is limited to cases where the trip is not undertaken in service of a mitzva. If, however, the trip is for the sake of a mitzva, and the boat belongs to non-Jews, then one may set sail even on Friday. Some maintain that this permission is contingent upon the owner of the boat agreeing that he will drop anchor on Shabbat; if he is unwilling to agree to this stipulation, it is forbidden to set sail. However, most authorities maintain that even if the non-Jew did not commit to drop anchor, one may still set sail with him for the sake of a mitzva.
On Shabbat itself one may not set sail, even if the boat belongs to a non-Jew and one is on a mitzva mission. The Sages forbade sailing on Shabbat out of the concern that one might come to fashion a raft (Beitza 36b; SA 339:2). If the boat is scheduled to set sail on Shabbat, one may board it before Shabbat starts and remain on it until departure time. Some are lenient, allowing one to accept Shabbat while on the boat and then return home until the departure time; one should not object if people choose to follow this practice (SA and Rema 248:3).
So far we have been discussing boats that belong to non-Jews. If the boat belongs to Jews who are Shabbat desecrators, there is disagreement about the proper practice. Some maintain that on the first three days of the week one may set sail in these boats, because during this time one does not have an obligation to take into account what will happen on Shabbat (Tzitz Eliezer 5:7). However, in practice one may not support Shabbat desecration, and even at the beginning of the week one may not set sail on a boat owned by Jews who desecrate Shabbat (R. Menachem Mendel Schneerson; Minĥat Yitzĥak 3:39; Yeĥaveh Da’at 6:16).
. Shabbat 19a records a disagreement. According to R. Yehuda Ha-nasi, one must stipulate with the non-Jewish owner that he will drop anchor on Shabbat; according to R. Yehuda’s father, R. Shimon b. Gamliel, this is unnecessary. The Rishonim disagree whom the halakha follows. Rambam and SA follow R. Yehuda Ha-nasi, while Rabbeinu Ĥananel and Tur follow R. Shimon b. Gamliel. Furthermore, the Aĥaronim disagree about R. Yehuda Ha-nasi’s position in a case where the non-Jew refuses to accept the stipulation. According to MA, if the non-Jew says that he will continue sailing on Shabbat, one may not board the boat within three days of Shabbat even for the sake of a mitzva. Only if there is a possibility that the boat will drop anchor on Shabbat may he board. However, Eliya Rabba and SAH maintain that R. Yehuda Ha-nasi’s position is that the Jew must request that the non-Jew drop anchor, but may set sail with him even if he refuses. MB 248:2 and SHT ad loc. 1 state that most Aĥaronim adopt the latter position.
What qualifies as a mitzva for these purposes? Studying Torah, collecting charity, and the like. Rema writes that some maintain that traveling for the sake of one’s livelihood is also considered a mitzva mission, even if one has enough to survive and is traveling to earn more. Those who choose to be lenient in this case have an opinion to rely on. However, according to MB 248:36, where there is no custom to be lenient, one should not be lenient le-khatĥila, since many authorities feel that it is only for the sake of an uncontestable mitzva that one may set sail before Shabbat. Moving to Eretz Yisrael is definitely considered a mitzva. However, the status of one traveling to visit Eretz Yisrael is disputed. Pri Megadim maintains that this too is a mitzva, whereas MA maintains that it is not.
. In the case of a Jewish-owned boat that is setting sail for over a week, if it is safe for them to drop anchor at sea and stay still on Shabbat, they must do so. R. Menachem Mendel Schneerson writes that he consulted with experts and was told that it is not dangerous to drop anchor at sea for 25 hours. He is quoted in Minĥat Yitzĥak 3:39 and Yeĥaveh Da’at 6:16. However, there have been experts who felt that this would be dangerous, and that one would have to continue sailing on Shabbat. If they are correct and it is not possible to drop anchor at a port on Shabbat, Tzitz Eliezer 5:7 declares that for a mitzva one may set sail even during the three days preceding Shabbat. If it is not for a mitzva, then one may set sail only during the first three days of the week. His position is based on the opinions of Ha-Ma’or and Rivash, who maintain that even if there is no mitzva involved and it is clear that one will be forced to desecrate Shabbat due to danger to life, one may set sail during the first three days of the week because one does not have to take into account then what will happen on Shabbat. During the three days preceding Shabbat, one may set sail only for a mitzva mission, since at that point he is required to take Shabbat into account, though involvement in a mitzva exempts him from this concern. This is the decision of SA 248:4. See Igrot Moshe OĤ 1:39. However, it seems that one can in fact drop anchor at sea, in which case one must refrain from aiding transgressors. Furthermore, according to Radbaz and Mahari b. Lev, if it is certain that the Jewish owners will desecrate Shabbat, it is rabbinically forbidden to set sail even during the first three days of the week and even for the sake of a mitzva. Some believe that one must follow their view and be stringent (Minĥat Yitzĥak 3:39). See the next section.