The ornamental design for a substantially ovoid shaped towbobber, as shown and described.
FIG. 1 is a perspective view of a substantially ovoid shaped tow bobber;
FIG. 2 is a top view of a substantially ovoid shaped tow bobber;
FIG. 3 is a front view of a substantially ovoid shaped tow bobber;
FIG. 4 is a left side view of a substantially ovoid shaped tow bobber;
FIG. 5 is a rear view of a substantially ovoid shaped tow bobber;
FIG. 6 is a right side view of a substantially ovoid shaped tow bobber;
FIG. 7 is a bottom view of a substantially ovoid shaped tow bobber; and,
FIG. 8 is a perspective view of a substantially ovoid shaped tow bobber in an aquatic towing system environment.
The broken lines showing stitching on a substantially ovoid shaped tow bobber, a front target pattern on a front end of the substantially ovoid shaped tow bobber, a rear target pattern on a rear end of the substantially ovoid shaped tow bobber, straps, tow line sections, a boat, and a towable of FIGS. 1-8 form no part of the claimed design.