Study of local Singapore tides
G.F. Leechman, writing in 1937, produced a practical navigational analysis of tidal streams in the Port of Singapore, arguing that the moon’s declination effect is disproportionately important at equatorial latitudes and that a simple two-curve method can predict stream strength and direction with sufficient accuracy for operational planning. The article combines theoretical tidal mechanics with empirical observations to serve mariners operating in the strait.
Summary
Leechman’s central argument is that Singapore’s proximity to the equator makes lunar declination the dominant factor shaping local tidal behaviour, more so than at higher latitudes. Because the moon’s monthly path carries it between 28° N and 28° S, the tidal peak passes directly over Singapore twice a month, producing a pronounced diurnal inequality that is absent when the moon crosses the equator. This diurnal component, operating on a 24-hour cycle, superimposes on the familiar semi-diurnal (12-hour) tidal stream, and the interaction between the two explains the peculiar prolonged westward sets and exceptionally strong ebb tides observed locally.
The practical contribution is a pair of prediction curves: Curve A gives the semi-diurnal stream strength according to the age of the moon (spring to neap), while Curve B gives the diurnal declination effect as a function of the moon’s declination. By interpolating between the two curves using the time from Superior High Water and the moon’s declination taken 2½ days prior (accounting for the travel time of the derived tidal wave from the Southern Ocean), a navigator can estimate both the speed and direction of the stream at any given hour. Leechman works through a detailed example for 5 a.m. on 1 February 1930, arriving at a predicted westward stream of 0.86 knots in the Main Strait.
The article closes with an analysis of mean sea level, showing a bimodal annual curve with maxima in February, August, and September (10 ft. 7 in.) and minima in April, May, June, and November. Leechman attributes the roughly ten-inch seasonal variation to monsoon wind pressure, the North East Monsoon raising levels in the first quarter of the year and the South West Monsoon doing so in the second half, with the water partially trapped by the landmasses of Sumatra, Java, and Borneo.
Key Findings
- Maximum tidal stream in the Singapore Main Strait: 2.1 knots Eastward, 1.8 knots Westward, occurring at spring tides coinciding with maximum lunar declination near the solstices (p. 151).
- At the S.H.B. wharves in Keppel Harbour, the funnel-shaped entrance approximately doubles the Main Strait current, yielding a maximum of 4¼ knots on the ebb and 3½ knots on the flood (p. 151).
- The derived tidal wave from the Southern Ocean takes approximately two and a half days to reach Singapore, explaining why spring tides lag new and full moon by two to three days (pp. 150, 154).
- Mean sea level varies by approximately ten inches between the monsoon maxima (January, August, December at ~5 ft. 11 in.) and the minima (April, May at ~5 ft. 1 in.), a variation Leechman attributes to monsoon wind pressure rather than astronomical causes (p. 152).
- The highest and lowest tides do not occur in the same months; highest levels fall in February, August, and September, while lowest levels occur in April, May, June, and November (p. 152).
- The maximum interval of priming or lagging of the combined solar-lunar tidal crest is 51 minutes (p. 154).
Conclusion
Leechman’s definitive takeaway is that the local tidal regime at Singapore is governed by the interplay of a semi-diurnal stream and a diurnal declination stream, and that this interplay can be reduced to a simple graphical prediction method adequate for operational navigation. The seasonal variation in mean sea level, he concludes, is a meteorological (monsoon) rather than an astronomical phenomenon, and the highest and lowest tides should be expected at monsoon peaks coinciding with spring tides of great declination.
Context
- The author, Captain G.F. Leechman, was a naval officer writing from Marazion, Cornwall, and the article is framed as a practical aid for mariners rather than a theoretical treatise; it draws on the Admiralty Tide Tables, the Nautical Almanac, and local Sailing Directions as its primary reference works (p. 153).
- The study contributes to the small body of early twentieth-century empirical tidal analyses for the Straits of Malacca, complementing the standard harmonic analysis methods of the Admiralty with a locally adapted graphical approach.