Signs: How to read the sea

We all love looking out to the sea. There is something majestic, calming and hypnotic about the sight of breaking waves and floating ships. The sea holds secrets and if we look closely these secrets are revealed. For a navigator, the sea provides signs that can make a difference between sinking and surviving, between setting adrift and finding land.

Colour of water

Aerial view of Tioman: Notice the different hue signifying the depths and objects underneath

There are four main factors that determine the colour of the seawater we see, mainly what is beneath the water, what is in the water, what is on the water and the effects of light. Since I write this article primarily for looking out signs to help navigation, I will focus on the shade of water for now. Items like salt, mud, salinity, particle and organism content will be discussed in a separate forum.

The colour of the sea holds secrets to what is beneath the water, in the water, on the water and light

Because of the reflection (and refraction) of light on the surface, sea water changes shade with depth. When approaching a land mass from open ocean, you will transition from dark blue to lighter shades of blue to turquoise to green and then white (for sandy bottoms). This knowledge is useful when looking for a place to anchor or while navigating our way out of a bay full of reefs and rocks. Be careful as dark patches in turquoise waters normally mean rocks or corals.

"When approaching a land mass from open ocean, you will transition from dark blue to lighter shades of blue to turquoise to green and then white (for sandy bottoms)"

 The ocean looks blue because red, orange and yellow (long wavelength light) are absorbed more strongly by water than is blue (short wavelength light). So when white light from the sun enters the ocean, it is mostly the blue that gets returned. Same reason the sky is blue.


Tides

"Time and tide will wait for no man"

A good navigator must always know what the tide is doing, coming or going at any time. Tides are caused by the gravitational pull of the moon therefore in an unencumbered space like open oceans, the timing of the tide corresponds to the exact location of the moon in the sky. High tide is when the moon is directly overhead. However closer to shores and in places where movement of sea water is hindered by surface contours, there is a lag i.e time difference between the tides and the moon.


Places like Straits of Melaka until Kuantan have two daily tides (semi-diurnal) while those on northern east coast of the Peninsular and Sabah Sarawak have one daily tide cycle (diurnal).  For places with semi-diurnal tides, the tide will rise for 6 hours to reach high tide and then it will fall for six hours to reach low tide. The cycle continues.

Tide table for Tekek showing times and height of High and Low Water

On a personal note, this information was crucial when I found myself grounded on sandbanks off Beaulieu River or Tulai. Lets say I touched bottom as the tide was leaving three hours before low tide. This means that I will need to wait three hours for the tide to leave until it reaches low tide and then another three hours for it to get back to the level I was stranded at. Then maybe add one or two hours when the boat will release itself from the bottom.

Alright guys, put the kettle on. Time for a cigar! or two.

Note: Tide times shift by an additional 50 minutes a day since they are based on a lunar day, which is 24 hours 50 minutes long.

Spring Tide, otherwise known as air pasang besar happens twice a month.

The difference in height between the sea level at high tides and at low tides is called the tidal range. It can go up to 3.6 meters near Matang, Perak among the highest in the Peninsular.  Twice a month, during a new moon and a full moon, the tidal range is at its maximum for all places. This is when the high tides is its highest and the low tide is its lowest. This is called the spring tide in English, or air pasang besar in Malay. Combined with the monsoon rain, this is normally when flash floods happen in the East Coast.

Waves (Ombak) and Swell (Alun)
It is an easy mistake to say that waves set items adrift at sea. Tides move vertically up and down, while currents move water horizontally left or right. What are waves then? Or for that matter what are swells?

"Angin takde dah, ombak pun kurang tapi alun masih kuat"

A wave is generally caused by wind or earthquakes undersea. It transfers energy across a body of water. Think of it like shaking out a bedsheet. You can see the waves moving through it but the bedsheet remains in our hands.

The difference between a wave and a swell is a matter of magnitude and not so much scientific. A wave can be born and die within three or four hours. A wave might have travelled tens of kilometres. The crest between waves is somewhere less than 10 seconds. A swell can last a week or so and the crest between swells is more than 10 seconds. A swell can travel hundred of kilometres, even outrunning the storm that  gave birth to it. Thus a swell is a good indicator of bad weather to follow.

Crests and Trough on a wave. General rule of thumb is anything beyond 10 seconds between trough should be considered a swell

It is also important to note that a wave that is approaching shallow waters will increase its height. Since it is still carrying the same energy, yet it's wavelength is decreased.

There are places like the Bay of Biscay where seasoned sailors prefer to sail further outside the continental shelf to avoid rough conditions inshore

In 1773 Captain Cook sailed close to the treacherous area in the Pacific Ocean called the Tuamoto Archipelago nicknamed the 'Dangerous Archipelago' as too many ships have smashed into the scattered reefs. Although he could not see them, he 'felt' their presence. He was not psychic. The swell that he was expecting from the south, waves that would have been easily felt was absent. He therefore knew that the archipelago must have been in the south and shielding him from the waters!

The same ripple pattern is true for islands in the oceans. See if you can spot it next time you are out at sea.