Is it permissible to use a peeler on Shabbos?
Most poskim consider a peeler to be an instrument created especially for separating the unwanted peel from the vegetable, therefore it may not be used on shabbos. However there are those who say that R’ Moshe Feinstein zt”l considered a peeler, as merely “a complicated knife”, and the same way we may use a knife we may use a peeler. One may rely on the ruling of R’ Moshe if he usually follows the rulings of R’ Moshe.
Others say that one may use a peeler for fruits or vegetables that the peel is eaten anyways, since it isn’t separating good from bad.
R’ Shmuel Forst shlit”a in the name of R’ Moshe Fienstein zt”l, Shmiras Shabbos K’hilchoso 3- 32, see ftnt. 97, that he explains from Ketzos Hashulchan 125 ftnt. 42 that we don’t consider the fact that there is some fruit still attached to permit separating the peel, since that minute amount of peel is now also not edible, besides it is impossible to remove only the peel, and this is the way to remove the bad. Also see Shevet Halevi 1-84, Machze Eliyahu 51-3, Teshuvos V’hanhagos, 1-208, Ayal Meshulash pg 104, Shalmei Yehuda 6-15, The Shabbos kitchen, all prohibit using a peeler. See Nishams Shabbos 116-2, Shmiras Shabbos ibid, that permit using a peeler for edible peels, also see Orchos Shabbos chapter 3 ftnt 103, what he brings from Shvisas Hashabbos.