Contractors prepare to exchange the 16-foot-diameter propellers on the Coast Guard Cutter Polar Star while the cutter undergoes repair work at a dry dock in Vallejo, Calif., June 14, 2018. The Polar Star is the nation’s only heavy icebreaker and one of only two Coast Guard cutters capable of accessing the Polar Regions. U.S. Coast Guard photo by Petty Officer 1st Class Matthew S. Masaschi