This is a good site to see prehistoric boats of Britain http://www.abc.se/~m10354/uwa/wreck-br.htm

Compare shape of Scandinavian rock art boats (skin over frame?) http://www.megalithic.co.uk/modules.php?op=modload&name=a312&file=index&do=showpic&pid=15947&orderby=dateD with log boats and Nydam boat.