The 225-foot long, 1309 dwt motor vessel Arco Dee allided with the harbour walls at Littlehampton, England.   The vessel had been blown off course by strong winds as it entered the harbour causing it to strike the harbour walls.   Both the Arco Dee and concrete wall suffered minor damage.  No one was injured.   The Arco Dee was entering the harbour for routine maintenance.  Officals stated that limits were placed for entry to the harbour in weather with strong winds especially those who are sailing in ballast only.