The ornamental design for a rowingmachine, as shown and described.
FIG. 1 is a perspective view of a rowing machine showing my new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is bottom view thereof.
The dot-dash broken line defines the boundary of the claimed design, which extends to the boundary but does not include the boundary, all other broken lines in the drawings depict portions of the rowing machine that form no part of the claimed design.