This rudder blade is designed specifically for Mac 21 boats built 1982 - 1987, and using the all-aluminum rudder-head. Boats older than 1982 should order the Venture 21 rudder. This is a drop-in replacement and requires no modification to your boat, just install it and go sailing. The blade comes to you with all holes not drilled, allowing you to install the blade for a custom fit to your rudder castle. Just fit it to your rudder castle and drill your pivot hole along with the uphaul/downhaul. The blade is our solid HDPE (no swelling), true NACA 0012 airfoil design, offering maximum lift with minimum drag.