The wind and weather are so chilling that you cannot melt or rinse off the ice. You will only build up even more of it. Sailing south means deflecting the convoy from its mission. A delay in getting supplies to Britain might cost lives or even effect the entire course of the war. De-icer fluid doesn't exist yet, and it would take a huge amount to clear off a ship even once. The best course of action is to order the men to hitch up life lines and use axes to chop off as much ice as possible. The work is very dangerous, but it is necessary.