Athwartships tune Fore and aft tune involves mast rake and mast bend. Athwartships tune involves making the mast straight sideways and minimizing lean to leeward. Rake Mast rake is controlled by headstay length. Tightening the headstay induces forward rake, easing headstay increases aft rake. Rake controls helm.
Forward rake adds lee helm.
Aft rake induces weather helm. A normal 40-footer has 12 to 18 of rake. Rake is measured with a plumb. Rake may vary with boat type and normal sailing conditions. It keeps the mast bending positively, preventing mast inversion and reducing pumping in choppy conditions.