I claim the ornamental design for a boat tiller, as shown and described.
FIG. 1 is a side elevation view of the present invention showing the principal components including a rudder, a rudder shaft, a rudder bearing, a mounting clamp and a tiller arm.
FIG. 2 is a rear elevation view of the present invention as shown in FIG. 1.
FIG. 3 is an opposite side elevation view of the present invention as shown in FIG. 1; and,
FIG. 4 is a front elevation view of the present invention as shown in FIG. 1.