The Blue Whale first appears at the end of Book 3. Stoick believed Hiccup that the Romans were stirring trouble in the Inner Isles and set out with a fleet of Hooligan ships to Fort Sinister. Before he got there however, a Roman Observation Balloon containing his son and friends crash-lands on deck of The Blue Whale.
There is no description given of The Blue Whale. Presumably it has the appearance of a typical Viking longship and is large, like a Whale. It does have more than one sail, however, indicated when the Roman Balloon becomes tangled in the "sails".
|Ships of DreamWorks Dragons and How to Train Your Dragon Books|