The 151 meter long, 10113 dwt reefer Spring Bok collided in heavy fog with the 88 meter long, 3590 dwt LPG tanker Gas Arctic.   The Spring Bok struck the stern of the Gas Arctic while attempting to overtake it.  Both vessels suffered damage above the waterline, but were able to proceed under their own power to Portland, England.  Both vessels will be inspected.   No reports of injuries to the 14 crew on the Gas Arctic or the 22 crew on the Spring Bok.  Authorities are conducting an investigation into the incident.