Common...... what you mention about were not "sea boats"
they were used in lakes and rivers.
besides what you said......"¥t¥~¸É¥R¤@ÂI¸ê®Æ, ¥v¤W°O¸ü¾G©M±qºÖ«Ø-¤ªêªù(¤µ¤ÑªººÖ¦{´ä)¥Xµo, §Ú§ä¤F¤@¤UºÖ¦{´ä¤ô²`¸ê®Æ,³o¬OºÖ¦{´ä¤Þ¯è¯¸ªººô¶:
http://www.fzpilot.com/yewu12.htm......"
did you know that that was almost six hundred years ago stuff?