The ornamental design for a boathull, as shown and described.
FIG. 1 is a perspective view of a boat hull according to my new design;
FIG. 2 is another perspective view thereof;
FIG. 3 is a side elevation view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is an enlarged front elevation view thereof; and,
FIG. 7 is a an enlarged rear elevation view thereof.
Where used in the drawings, the broken lines show the environmental structure only for the purpose of illustration, and forms no part of the claimed design.