The next battle takes place 80 years after the first,
      yet Rackham has only aged 10 years.

   How fast was his ship travelling?

                        Earth time      80
            gamma  =   -----------  =  ----  =   8
                        ship time       10


                                           0.5 
                       [      1          ]    
            v/c    =   [ --------------- ]  
                       [  1 - 1/gamma^2  ]


                   =   0.992


    Rackham's ship must have been flying at >=  0.992 c.