The goal of these storm sails is to give the Viking ship some sailing power even when the sea is at its worst. With the mast down and these storm sails set, the Viking ship can roll over and still right itself and continue sailing.
The oars that make the support for these sails themselves rest on the upside down ships boats lashed to the deck of the mother ship. This system makes the deck cover and storm sails very strong. With the deck cover lashed securely to the oar sails, the Viking ship can roll over and not get water inside the boat. It would float high and dry upside down. Then the oar sails would catch the wind and right the boat.