You should check out this post as there is discussion on shields on Viking ships.

There is proof of shields on Viking ships from what he have found in ship burials including the Gokstad ship (32 shields found in the burial) and there are ships in the Bayeux tapestry that have shields on the sides and some that don't. On the Gotlandic picture stones (one example here) we see boats that have them and some that don't. The point I am trying to make is that there is still debate as to whether ships had shields lashed to the sides or not. There is evidence for and against it but if shields were lashed to the sides they probably were used for fighting and storing them on the edge helped save room as well as acted as a shielding on a ship for naval battles as well as ocean spray. I know that the chests men brought to store their luggage items also functioned as seats on the ship for where they rowed as seats were not installed on viking ships so the idea of items having multiple purposes is something the Norse did.