סימן צה סעיף יח

קָיְמָא לָן, דְּאֵין תְּחוּמִין לְמַעְלָה מֵעֲשָׂרָה טְפָחִים. וְלָכֵן מִי שֶׁבָּא לְתוֹךְ סְפִינָה בְּעֶרֶב שַׁבָּת קֹדֶם כְּנִיסַת שַׁבָּת וְהָלְכָה הַסְּפִינָה אֲפִלּוּ דֶּרֶךְ רְחוֹקָה, מִכָּל מָקוֹם כְּשֶׁהִגִּיעַ בַּשַׁבָּת לַנָּמָל וְיָצָא, יֵשׁ לוֹ מִשָּׁם אַלְפַּיִם אַמָּה לְכָל צַד, כִּי מִסְּתָמָא בַּהֲלִיכַת הַסְּפִינָה הוּא תָּמִיד לְמַעְלָה מֵעֲשָׂרָה טְפָחִים מֵהַקַּרְקַע, וְלֹא קָנָה שְׁבִיתָה עַד הַמָּקוֹם שֶׁהוּא בָּא לַיָּבָּשָׁה. אֲבָל אִם בְּתוֹךְ הַשַׁבָּת יָצָא מֵהַסְּפִינָה וְחָזַר וְנִכְנַס לְתוֹכָהּ, מֵאַחַר שֶׁהָיָה בַשַׁבָּת בַּיַבָּשָׁה, קָנָה שָׁם שְׁבִיתָה. וְאִם הָלְכָה הַסְּפִינָה אַחַר כָּךְ חוּץ לַתְּחוּם, אֲזַי אֵין לוֹ שָׁם אֶלָּא אַרְבַּע אַמּוֹת, כְּדִין יוֹצֵא חוּץ לַתְּחוּם. וְכֵן אִם הָיְתָה הַסְּפִינָה בַּשַׁבָּת בְּמָקוֹם אֶחָד שֶׁאֵין שָׁם עֲשָׂרָה טְפָחִים עַד הַקַּרְקַע, קָנָה שָׁם שְׁבִיתָה. וְאִם מִסְתַּפֵּק אִם הָיְתָה בְּמָקוֹם כָּזֶה אוֹ לֹא, אַזְלֵינָן לְקֻלָּא (סִימָן רמ"ח ת"ד ת"ה וְעַיֵּן סִימָן ת"ו).

We have an accepted rule that the prohibition of techumin does not apply above ten tefachim⁴⁸This is an undecided question in Maseches Eiruvin 43. The Shulchan Aruch 404: 1 says where techumin are derabanan we are meikil. The Kitzur Shulchan Aruch holds that techumin are always derabanan. [above the ground]. Therefore, if you boarded a ship on erev Shabbos, before the beginning of Shabbos and the ship sailed, even on a long voyage,⁴⁹More than 12 mil. nevertheless, when it reaches port on Shabbos and you go ashore; you are permitted to walk from there 2, 000 amos in any direction. Because, there is no doubt, that during the entire voyage of the ship you were always above ten tefachim from the ground, and did not acquire shevisah until you landed. However, if during the Shabbos you went ashore and returned to board the ship again; since you were on land during Shabbos, you acquired shevisah there. If the ship sails afterwards beyond the techum, then you have on the ship only four amos within which you may walk, which is the halachah regarding someone who goes beyond the techum.⁵⁰Which states that if he does so on purpose he has only four amos, even if he is in a place surrounded by walls. If he was forced to go out, or he went out accidentally and is in a place enclosed by walls he may move about the whole enclosed area. See Shulchan Aruch 405. If the ship reached a place that is less than ten tefachim⁵¹From the bottom of the ship to the ground. [see Aruch Hashulchan 405] above the ground, you have acquired shevisah there.⁵²The Ramban and Rashba say when the boat is moving you do not acquire shevisah. See Eiruvin (45). If you are in doubt whether or not the ship was at such a place you may be lenient.⁵³Rema 404. Because it is a safeik derabanan (doubt pertaining to a Rabbinical prohibition) and you may be meikil. Also see Sha’ar Hatziyun 404: 10.

תנא דבי אליהו כל השונה הלכות בכל יום מובטח לו שהוא בן העולם הבא שנאמר הליכות עולם לו אל תקרי הליכות אלא הלכות (נדה ע"ג.)