It is interesting to look at how far modern shipping has come with respect to navigation. Initially, maritime enthusiasts would ask poignant question such as: How do you figure out in which direction to steer your ship, especially when you can no longer see the shoreline? Or how do you locate the position of your ship on a nautical chart (map)? Asking such questions led to trials and errors and it was over time that skilled navigators helped to make safe travel by water possible, and as a result, maritime trade prospered.
How Do Ships Navigate
The essentials of modern navigation had been well established during the 18th century. The sextant, a practical tool for celestial (sky) navigation, was in common use. The chronometer (an accurate clock) had been invented but had not yet become widely used. Nautical charts had become more accurate. Navigators find their position on the earth’s surface by observing the location of the stars. They need several more utilities to locate themselves and their destination on the globe.
Ship Motions on Sea
A ship at sea moves in six degrees of motion: heave, sway, surge, roll, pitch and yaw. The first three are linear motions. Heaving is the linear motion along the vertical Z-axis, swaying is the motion along the transverse Y-axis, and surging is the motion along the longitudinal X-axis.
A yaw motion is a side-to side movement of the bow and stern of the ship. Pitch motion is an up-or-down movement of the bow and stern of the ship and the roll motion is a side-to-side or port-starboard tilting motion of the superstructure around this axis.
Two Scenarios which make a Vessel Vulnerable
- Parametric Rolling: Is a factor which has led to numerous mishaps. Ship Masters lay due importance on this aspect while the vessel is on high seas. The stability moment of a ship is the product of the righting lever and the total weight. ‘Righting Lever’ is the horizontal spacing between centre of gravity (G) and centre of buoyancy (B). On waters, the righting lever varies periodically due to the changing wave elevation around the ship and her pitch motion. This, in turn, causes the stability moment to vary, which can trigger rolling. The phenomenon is known as parametric rolling because its source is the time variation of a parameter. This resonance can cause the ship to roll to very large angles in a moderate sea, leading to cargo damage and in extreme cases, capsizing of the ship.
- Risks with Containerised Cargo: Risks associated with containerised cargo are high if the carrying vessel encounters heavy weather conditions or if the vessel routing heightens the impact of the weather upon the ship and cargo. The stoppage of the main engine amidst heavy weather conditions could result in the violent rolling, pitching and heaving motion of the vessel. Such movements exert a load on the cargo-securing equipment on deck.
This may result in failure of the cargo securing equipment and consequent loss of deck cargo containers overboard. If the vessel is excessively stiff she may be subject to a short rolling period, creating greater loads on all equipment and fittings. Also, it has a great influence on “Lashing strength” for containers on deck.
Maintain Safe Stability Onboard
The meta-centre has a direct relationship with a ship’s rolling period. A ship with a small metacentric height (GM) will have a long roll period. An excessively low or negative GM increases the risk of a ship capsizing in rough weather.
Going by the nature and surface of ship operations on high seas, the vessels will tend to lose stability, owing to the heavy load. It is the responsibility of the Master to keep a close watch on the aforementioned factors apart from the ‘Big Angle Rudder’ and ‘towing by tugs’ (Berthing / Un-berthing), which often makes the ship vulnerable and unstable.
The Current Scenario
Today, all large deep sea ships are required to carry the latest in electronic navigation equipment. In this age of GPS, sonar, other high-tech navigational devices are the key components for navigating large vessels. The modern navigational system, which is called an ECDIS, an electronic chart display and information system, really gives great improvement in navigational safety because it gives great improvement in situational awareness. It automatically plots the position of the ship with GPS.
All ships operating on the high seas are equipped with AIS or the automatic identification system. Operating in the VHF maritime band, the AIS enables the wireless exchange of navigation statuses between vessels and shore-side traffic monitoring centers.
Today, systems allow for voyage planning, where the navigator sets out the exact track that they want to follow at each course change based upon the electronic navigational chart. If the navigator sets a course which is predictably hazardous, the active system will transmit alerts beforehand. Subsequently, the seafarer can change the voyage plan to a safer track.
Commercial vessel traffic on the high seas has been on a rise and will keep on increasing. No single measure will address all risks, but a robust safety-framework and mindset, amplifies the probability of incident-free voyage.
Sea News Feature, February 6