If it were my daggerboard—and severely split—I'd have a neighbor with a table saw make a cut with a narrow blade. (Follow the split for the full length of the board). With that fresh wood material, use West epoxy, slow-set hardener, and add West's high-density adhesive to thicken it. Clamping it together is an important step, so you may want to rent or borrow some serious clamps. (Or buy Pony clamps from Harbor Freight). .