The high cost of transportation is itself a fact demanding explanation. Inefficient ship-building and navigation would account for it in large part, but other factors are still to be considered. These distant voyages were carried on in the face not only of real dangers serious enough, but also of far greater dangers imagined by ignorant and credulous men. An English chronicler says that in 1406, when English ships were going to Bordeaux, they entered an unfrequented sea, and four vessels from Lynn were engulfed in a whirlpool which swallowed up the flood and vomited it forth again three times a day. The Arabian Nights have been called sober and realistic in comparison with the ideas held by the medieval mariner of the wonders and dangers of the unknown parts of the world. =86. Piracy.=—At sea, as on land, the merchant faced dangers of violence which were probably as serious an obstacle to the growth of commerce as the physical difficulties of navigation. Ships went always armed, and sailed when possible in fleets for better protection. Sometimes one that had ventured out alone could beat off its enemies, as in the case of a trading ship from Stralsund which was attacked in 1391 but won a complete victory, and brought back (it is said) 100 pirates, packed in casks with only the heads sticking out, for a bloody punishment. Pirates came from every source. An ordinary merchantman would turn pirate if it met a weaker vessel from some town which was so far distant or so weak that reprisals need not be feared. A mariner of Winchelsea in England, who had seized and plundered a vessel owned by Dorsetshire merchants, became mayor of Winchelsea a few years later, and in the fifteenth century a Canterbury abbot was convicted of plundering a wine ship and was forced to make restitution. Even ships sent out by the public authorities for protection against pirates attacked and plundered ships not only of other nationalities but of their own too. Six ships which had been organized in 1316 to protect Berwick from freebooters harried the English coast to the south, and the fleet of the Cinque Ports (five towns on the English Channel), used its spare time in preying on English commerce and attacking English towns.