The WNA mark ( Winter North Atlantic mark) is required by the ships lesser than 100 mtrs in length trading in North Atlantic Ocean during the Winter Season.
During the first conference on the Loadline convention it was generally agreed that the freeboard of small ships should be especially examined with a view to some increase being made ( in freeboard) . Statistics of ship losses had highlighted the safety problems associated with vessels under 300 ft long and some action was necessary.
It was recognized that it was difficult for ships lesser than 100 mtrs ( 328 feet) to provide for an even Compartment subdivision and the deck integrity for these ships should be paramount.
North Atlantic Ocean during winter season is primarily Rough and therefore the vessel shall be loaded to a loadline which is even lesser than the Winter mark ( so as to give sufficient freeboard allowance for the size of the ship lesser than 100 mtrs). This was primarily done for safety reasons.