October 27, 2020

Waves and Islands

This from Andrei Piriutko:

Throw a stone into the water and what happens--the stone sinks of course, but circles of waves are made centred on where the stone fell.
In much the same way, waves in the sea hit an island and are reflected back.
The mattang is a tool showing all the basic patterns that waves can form when they bounce off land.
An experienced Polynesian sailor would be able to read the wave patterns and tell which direction to go to find land.

When oceanographers began to study ocean swells it was found that their action conformed to the laws of wave theory in the same ways as do light and sound. For example, when an ocean swell strikes a shore, part of it ts energy, that is reflected at an angle equal to the angle of its incidence. And when a swell approaches, strikes, and part of it moves part a small island, such as one of the Marshall Islands atolls, its line of movement is changed according to the angle of shoreline toward which it is advancing. The crest line of a swell approaching the shore of an island is bent and curved toward conformity with the shoreline. This occurs because the inshore position of the wave is slowed down as it encounters shallow water, its energy is expended by breaking or peaking up the wave, thus slowing down the forward motion while the offshore portion in deep water continues advancing at a constant rate of speed. This is swell refraction. Finally, a turbulent shadow of a special kind, resembling a penumbra, is to be found extending out from the lee side of an island for several miles.

The art of reading the waves was taught to Polynesian boys with the aid of the mattang, a stick chart, a web of interlocking sticks which demonstrated all the basic patterns that waves can form when they are deflected by land. The adult navigator gauged these wave patterns entirely by his sense of touch. He would crouch in the bow of his canoe and literally feel every motion of the vessel. In this way the original colonizers of Easter Island might have ‘felt their way’ across thousands of miles of open sea to their remote new home.

Polynesian navigators used their senses and memory to guide them on voyages by crouching down or lying prone in the canoe to feel how the canoe was being pitched and rolled by underlying swells.

The Marshallese recognized four main ocean swells. Navigators focused on effects of islands in blocking swells and generating counterswells to some degree, but they mainly concentrated on refraction of swells as they came in contact with undersea slopes of islands and the bending of swells around islands as they interacted with swells coming from opposite directions. The four types of ocean swells were represented in many stick charts by curved sticks and threads.

The rilib swell is the strongest of the four ocean swells and was referred to as the "backbone” swell. It is generated by the northeast trade winds and is present during the entire year, even when they do not penetrate as far south as the Marshall Islands. Marshallese considered the rilib swells to come from the east, even though the angle of the winds as well as the impact of the ocean currents varied the swell direction.
The kaelib swell is weaker than the rilib and could only be detected by knowledgeable persons, but it is also present year round.
The bungdockerik is present year round as well and arises in the southwest. This swell is often as strong as the rilib in the southern islands.
The bundockeing swell is the weakest of the four swells, and is mainly felt in the northern islands.
Stick charts were not made and used by all Marshall Islanders. Nor would every one in the community have access to the knowledge of making them. They were closely guarded secrets of the family of the chief, and knowledge was passed down father to son. So that others could utilize the expertise of the navigator, fifteen or more canoes sailed together in a squadron, accompanied by a leader pilot skilled in use of the charts.

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