Wash and stem grapes. Measure 10 cups. Remove skins from half of the grapes; set aside.
In an 8- or 10-quart pot combine the skinned and unskinned grapes. Cover and cook 10 minutes or until very soft. Press grapes through a sieve; discard seeds and cooked skins. Measure 3 cups of strained pulp; return to pot. Stir in the uncooked grape skins and water. Cook, covered, for 10 minutes. Uncover; stir in sugar. Bring mixture to a full rolling boil, stirring often. Boil, uncovered, about 12 minutes or until jam sheets off a metal spoon.