in 1869 the LV 21 (Tybee Island Knoll) was a very busy light vessel

While in Relief status, performed temporary duty marking Wolf Trap, Jan 8-Feb 24 1869; Windmill Point Shoal, Mar 8-Sepi 1869 during lighthouse construction; York Spit, Sep 1869


http://www.the-best-light-houses.com/LV-21.html