It is Rabbinically prohibited to carry from one Torah domain to another (reshus harabbim to reshus hayachid or vice versa) on Shabbos via a makom petur (an insignificant domain) as a safeguard to prevent transgressing by carrying directly from one domain to another. The Shulchan Aruch 346:1 cites two opinions regarding carrying an object from a Rabbinically forbidden domain to another (karmelis to reshus hayachid or reshus harabim and vice versa) via a makom petur. The first opinion is that it is forbidden just like from Torah domains, but the second opinion maintains that the Rabbis did not enact this safeguard for Rabbinic domains.
The Shulchan Aruch (355:1) discusses carrying water from a water source that is a karmelis to a reshus hayachid, which, Rabbinically, is considered carrying from one domain to another and rules that although, usually, some measure of mechitzos (halachic barriers, albeit of lesser quality than is usually acceptable) are required to convert the area from which the water is drawn to a reshus hayachid, on a boat [where one cannot construct as on land], if its walls are ten tefachim above the surface of the water [necessitating lifting the water in the air over ten tefachim above the karmelis, which is a makom petur], he may extend even a small board over the top of the wall of the boat, and draw the water through a hole in the board.
The Biur Halachah (355:1 s.v. Avir) cites the Mogen Avrohom who explains that this leniency for a boat is even according to the stringent opinion regarding carrying from Rabbinic domains via a makom petur, because the required protrusion serves as a halachic differentiation from normal cases of carrying via a makom petur and is not subject to the safeguard. However, the Gra disagrees and holds that the leniency of drawing water via the protrusion on a boat is only according to the lenient view in Siman 346 who permits carrying from Rabbinic domains via a makom petur.
However, the Mishnah Berurah (355:3 §30) writes that the opinion of the Gra is that even those who permit carrying from Rabbinic domains via a makom petur require the object to
‘rest’ in the makom petur before continuing to the second domain. If so, how can the Gra explain that these opinions permit drawing water via the protrusion over the walls of a boat because it is carrying from a Rabbinic domain via a makom petur – he does not seem to fulfill the requirement that the water ‘rest’ on the protrusion before entering the boat?
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