What is current?
An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, the Coriolis effect, breaking waves, cabbeling, and temperature and salinity differences. Depth contours, shoreline configurations, and interactions with other currents influence a current’s direction and strength. Ocean currents are primarily horizontal water movements. However ocean current at our dive sites are mostly created by the tide.
The greatest effect on current are tidal movements.
Gravitational pull from the moon and sun is the main cause of tides. As divers we feel strong current at spring tide which happens every 14 days. The weakest current we have at neap tide every 14 days too. A full moon cycle is 28 days The tidal mean range in Phuket is around 3 metres. Compered to the biggest tide range of 13 m in Canada (The Bay of Fundy) we have a moderate spring range.
When the gravitational pull from the moon and the sun are in line, we experience high high waters and low low waters
When the gravitational pull from the moon and the sun are at tight angles to each other, we experience low high watersand high low waters.
If you want to geek-out we recommend taking a drift diver specialty course, dive guide course or one our boating courses. Tide charts are published in Thailand by the Royal Thai Navy. There are as well Apps out showing you the tides. When it comes to choosing a dive site our main source of information is an App showing the tidal movements.
In a perfect world we would only dive at high tide or low tide. We start our scuba diving day trips always at the same time to to avoid confusion. Nevertheless, we have to carefully select the dive sites and time we want to start the dives.
To see how strong the current will be we check how many hours we are in the cycle.
Let’s plan a dive together on 7th of May at 10:20. How strong current can we expect while diving?
This is a typical tide chart. The red circled figure on the chart is the water hight. As bigger the difference to the next hour as stronger the current. Between 8am and 9am is high tide and between 2pm and 3pm is low tide.
1 hour (9am) after high tide the water level lower by 1/12 (0.1 meter) of the tidal hight at that day which is in our example 1.3 metre.
2 hours (10am) after high tide the water level lower by 2/12 (0.21 meter)
3 hours (11am) after high tide the water level lower by 3/12 (0.32 meter)
4 hours (12am) after high tide the water level lower by 3/12 (0.32 meter)
5 hours (1 pm) after high tide the water level lower by 2/12 (0.21 meter)
6 hours (2 pm) after high tide the water level lower by 1/12 (0.1 meter). It is now low tide.
We know now that the water level drops by 0.21 meter at the time planned to dive. The water level will drop faster towards the end of the dive when we assume a dive time of 1 hour.
This will result in a mild to moderate current.
We consider 0.1 – 0.2 meter in tide change mild current, 0.3 – 0.4 meter moderate current and 0.5 – 0.7 meter strong current.
Which direction the current flow can tell us the marine charts.
At Shark Point (Hin Musang), Anemone Reef and King Cruiser ship wreck current flows towards high tide to east north east and towards low tide south south east.
At the Racha Islands current flows towards high tide to east north east and towards low tide west.
Around Koh Mai Thon and Koh Doc Mai current direction is north – south. Towards high tide current flows north and towards low tide it flows south.
Knowing current is crucial for a safe and joyful dive. As divers we try to use ocean current for our advantage by mainly “going with the flow”. Sometimes a swim against current can make sense. How to do that effectively is one topic of a current diver specialty course.
Wind and sea state is as well important to consider when choosing the best dive site available.