"Chinese Junk," Collection of Original Water Colors of Chinese Junks and Other Craft, From The Library at The Mariners’ Museum, ND2068.C67 rare.
The main tool used by navigators and pilots on board a Chinese ship was the water compass, described above. For keeping time, incense, which was graded to burn a certain amount in a certain time, was used. As on later western vessels, a day was broken into watches. Unlike those western vessels, however, the day was broken into ten watches of 2.4 hours each.
During the time of Zheng He and the Treasure Fleet, the average ship could travel about twenty miles per watch, at a speed of about eight knots. Speed was determined by throwing an object over the bow of the ship, walking the length of the ship while watching the object, and measuring, by chanting a rhyme, how long it took for the vessel to pass the object.
Latitude was found using a similar theory, though slightly different method, than the European cross-staff. Navigators measured the altitude of Polaris or the Southern Cross above the horizon with an instrument called a qianxingban. The qianxingban was a board consisting of twelve pieces of square wood; the board would be aligned with the horizon, and navigators used the lengths of their arms to calculate the position of the stars. Another, simpler instrument used for this purpose was the liangtianchi, a vertical ruler.
Captains also used sailing charts, which were much larger than their western counterparts. It was unrolled in sections, depending on where the ship was. The charts used by the Treasure Fleet were a series of sailing directions in the form of compass bearings and lengths of watches from port to port, across the Indian Ocean. It also showed any landmarks that might help the captains recognize their location. In addition to the sailing chart, star maps were available.
To find depth and to determine what was on the bottom of the body of water traveled, pilots used a lead line. These were very similar to the lead lines used by western sailors.