Tides, Currents & Coastal Hydrology: How Ancient Chinese Sailors Read the Sea

What Did Ancient Chinese Sailors Know About Tides?

Chinese understanding of tidal periodicity is documented from at least the Han dynasty (206 BCE–220 CE), when the philosopher Wang Chong noted in his Lunheng (論衡, c. 80 CE) that tidal cycles follow the moon. By the Northern Song period, Shen Kuo's Mengxi Bitan (c. 1088 CE) offered a more precise account, linking the twice-daily tidal cycle to lunar position and noting that the interval between high tides shifts by approximately fifty minutes each day — an observation consistent with modern tidal science. This was not purely theoretical: coastal pilots along the Zhejiang and Fujian coasts maintained practical tidal tables, passed within pilot families, that specified safe entry and departure windows for major harbours.

The tidal environment along China's eastern seaboard varies considerably by region. The Yellow Sea experiences semi-diurnal tides with moderate range; Hangzhou Bay compresses tidal energy into a funnel shape, producing the Qiantang bore and ranges that can exceed eight metres. Fujian's rocky inlets and the Taiwan Strait present strong tidal currents that run counter to prevailing winds at certain states of tide — a combination that required precise timing rather than simply waiting for fair weather. Song-dynasty port records from Mingzhou (modern Ningbo, Zhejiang) document scheduled departure windows tied explicitly to tidal state, suggesting that tidal knowledge was institutionalised at major ports by the 10th–11th century CE.

How Did Sailors Read Ocean Currents Without Instruments?

The Kuroshio Current — a warm, fast-moving body of water running northeast along the eastern coast of China, past Taiwan and the Ryukyu Islands toward Japan — was identified by Chinese coastal pilots through visual and tactile cues long before it appeared in formal hydrographic charts. Its deep blue-black colour contrasted with the greener, sediment-laden inshore waters, giving pilots a visible boundary. Gang Deng's Chinese Maritime Activities and Socioeconomic Development (Greenwood Press, 1997) notes that Tang-dynasty (618–907 CE) accounts of the sea route to Japan describe current conditions consistent with Kuroshio navigation.

Beyond the Kuroshio, pilots working the South China Sea used the contrast between coastal upwelling zones — cooler, greener, often fish-rich — and the deeper blue of open-ocean water to gauge their offshore distance. Water temperature, assessed by hand, provided a secondary indicator. The Shunfeng Xiangsong (順風相送, c. 1430 CE), a Ming-dynasty sailing manual, records current warnings for specific passages in the Taiwan Strait and the approaches to the Paracel Islands, phrased as practical cautions rather than theoretical descriptions.

What Was Depth Sounding and How Was It Used?

Depth sounding — lowering a weighted line to measure water depth and retrieve a sample of the seabed — is documented in Chinese sources from at least the 10th century CE. The technique, known as tuo sheng (拖繩), used a lead weight sometimes coated with tallow or grease to collect bottom sediment. The sediment type — fine mud indicating deep-water anchorage, coarse sand suggesting a shoaling approach, shell fragments pointing to specific known grounds — gave pilots positional information in fog, rain, or at night when visual landmarks were unavailable. Joseph Needham's Science and Civilisation in China, Vol. 4, Part 3 (Cambridge University Press, 1971) discusses sounding techniques in the context of Chinese nautical instrumentation.

This method was particularly valuable along the shallow approaches to the Yangtze Delta and the Yellow Sea, where visibility could be poor for days at a time and the bottom topography changed with seasonal sediment movement. Pilots who knew the bottom well enough to navigate by sediment type alone held knowledge that was difficult to codify and easy to lose — which may explain why so little of it appears in official texts. The record is incomplete, and scholars including Pierre-Yves Manguin have noted that much practical hydrological knowledge in Chinese maritime culture was held by specialist pilot families rather than recorded in state documents.

How Did Tidal Knowledge Shape Port Design and Trade Routes?

Tidal constraints shaped not only individual voyages but the location and design of major ports. Quanzhou (泉州), the great Song and Yuan dynasty entrepôt, sits at the mouth of the Jin River in a tidal estuary that allowed large vessels to enter only on the flood tide — a constraint that organised the rhythm of the port's commercial life. The Zhufan Zhi (諸蕃志, c. 1225 CE) by Zhao Rugua describes departure seasons and implicitly tidal conditions for routes to Southeast Asia and the Persian Gulf. Merchants and pilots coordinated departure timing not just with monsoon seasons but with the spring tides that provided maximum water depth over harbour bars.

The Taiwan Strait, one of the most heavily trafficked passages in pre-modern Asian maritime trade, presented a particular challenge: tidal currents run strongly north–south through the strait, and their interaction with the prevailing northeast monsoon in winter created conditions that required pilots to choose between riding the current or the wind — rarely both simultaneously. Fujian pilots developed a detailed working knowledge of these interactions, documented in part in the Hailu (海錄, 1820 CE) by Xie Qinggao, which preserves navigational observations that likely reflect much older practice.

How Is This Knowledge Reflected in the Zhoushan Workshop Tradition?

The Zhoushan archipelago — where Ocean Relic Studio's workshop has operated since 1980 — sits at the confluence of the Yangtze outflow and the East China Sea, in waters where tidal range, current patterns, and seasonal sediment plumes have shaped local maritime culture for centuries. Craftsmen in the Zhoushan tradition build models of vessels whose hull forms were developed in direct response to these hydrological conditions: the flat-bottomed sand junk (沙船) suited to the shallow, tide-swept approaches of the Yangtze Delta; the deeper-drafted ocean-going junk designed to hold course across the Kuroshio. The proportions and construction details of these hull forms encode, in physical form, the accumulated hydrological knowledge of the coast.

This is not a claim that individual craftsmen today hold navigational expertise — it is an observation that the vessel types they reproduce were shaped by the same tidal and current environment that produced the navigational knowledge described in this article. The connection is material and historical rather than metaphorical.

Ocean-Going Chinese Junk Ship Model — Built in the Zhoushan workshop tradition, this model reproduces the deep-drafted hull form developed for open-sea passages where tidal currents and the Kuroshio demanded a vessel capable of holding course across strong lateral flows.