Add these variables to the report:

          Value to store                     Initial Value    Reset at
          ---------------------------------------------------------------
prevShip  currShip                           currShip         End of report
currShip  Ship                               Ship             End of report
nShip     nShip+iif(prevShip<>currShip,1,0)  1                End of report

nShip will count the groups.
The variable prevShip must be before currShip in the report variable list.
Not very elegant, but works for me...