Abstract: The wrench is adapted to impart desired tension to mast stays which are connected to the hull of a sailboat by turnbuckle members of various different conventional constructions. As stay-tensioning rotative movement is imparted to a turnbuckle by the wrench, components of the wrench undergo progressive movement relative to each other in response to the progressively increasing resistance of the turnbuckle to rotative movement. Indicia means upon the wrench indicates the extent of the relative movement undergone between the aforesaid wrench components, and therefore indirectly indicates the degree of tension in the mast stay connected to the turnbuckle. Use of the wrench permits tension of desired magnitude to be readily and consistently imparted to each mast stay of a sailboat.