Because only parts of the boats were found historians have had to make some guesses as to what the finished boat looked like. They can be almost certain how wide the boat would be because they found nearly all the bottom of one boat. They can be almost certain how long the boat was because they found over half of the bottom of one boat. They know the boats were made from planks stitched together. They know how high the boats where by the number and shapes of the side-planks. They know that the boats were paddled for all or part of their journey because they found several paddles. Some people think that to paddle a boat all the way across the river would have been such hard work that the boat might have had a mast and sail like some rock-engravings of Norwegian boats of the same time.
One or two hints in the structure point to a possible mast. From what has been found most people think the Ferriby boat was a large barge-like boat with with lots of men paddling it, the boatmen would probably make the crossing of the Humber at slack water of high tide; but they could have made good use of the strong tidal currents up or down as the barges do today.