The ornamental design for a boathull, as shown and described.
FIG. 1 is a bottom perspective view from above one side of a boat hull.
FIG. 2 is a front view of a boat hull.
FIG. 3 is a side view of a boat hull; and,
FIG. 4 is an underneath plan view of a boat hull.
The dash-dot broken lines represent the bounds of the claim and all other broken lines are for the purpose of illustrating portions of the article that form no part of the claim. None of the broken lines form a part of the claimed design.