Complex sentence: It is partly for this second reason that centerboards are often lifted while sailing downwind even in non-planing hulls, the main reason being that a centreboard/keel is not needed for sailing downwind and simply adds to the drag of the hull.
Split into simpler sentences: It is partly for this second reason that centerboards are often lifted while sailing downwind even in non-planing hulls. The main reason being that a centreboard/keel is not needed for sailing downwind and simply adds to the drag of the hull.