Whilst the size and capacity of ships have increased dramatically over the past 50 years their speed across the water has remained the same at around 19 - 22 knots which is similar to the speed of a running man.

We all know about the advancements in other modes of transport during the last century – namely railroads, trucks and aircraft. The container ships remains the weakest link in the supply chain , there are certain types of time sensitive cargo that cannot survive weeks at sea and as a consequence the airfreight industry has burgeoned. The events of September 11^th have revitalised interest in improving shipping and a new array of technologies from NASA and the Americas Cup are helping naval architects design faster, more reliable and safer vessels.

The information regarding new hull design and propulsion systems makes for interesting reading as the sea is one of natures most powerful forces – a gentle breeze can excite small ripples on the water that, given time and distance, build into enormous rolling cylinders of energy. A typical ocean wave is nearly three stories tall, 180 metres long and moves as fast as a galloping horse. Those that prevail on the North Atlantic during the winter can slow a container ship at full power by 20 – 30%, our four to six knots.

Even in fair weather ships battle waves of their own making which are referred to as "captive waves".

Let me explain : As a vessel moves through the sea it displaces the water around it and creates a drag just as the wind does. The faster the ship travels the larger these disturbances become until they merge into a single wave, called a captive wave. That is where the trouble begins because if a vessel increases its speed beyond that of the captive wave, the wave lengthens and the stern (rear) of the ship sinks in the trough between the crests. This is a not a good place to be as the elongated wave then places additional drag on the ship as it tries to climb the wave’s back. Centuries ago Vikings discovered a way to reduce pressure and drag and attain higher speeds . They simply made their ships longer and slimmer – these "longships" made smaller waves and so moved faster. Let’s not forget the similarly long and narrow waka canoes that the Maori used for long distance ocean voyages. The downside was (I know you were waiting for it) these ships were less stable and less able to carry large loads.

Of course designers have crafted narrow hulls for warhips and passenger liners and in doing so they found another speed limit on the high seas; above 30 knots, the propellers on large ships begin to cavitate – that is, the pressure on their forward surfaces becomes low enough to cause the water to boil, which induces powerful, hull-cracking vibrations. Freaky huh?

This phenomenon is why naval architect have simply accepted for many years that container ships are bound to be slow. In fact, to balance large,heavy loads freighters must have hulls that are fairly wide for their length. But wait folks, there are developments afoot to scale up the design of gas turbines and water jets which are already employed in small passenger and car ferries.

So now it appears that the large ships will follow modern aircraft propulsion and switch from propellers to jet engines. And another aeronautical feature is also to be employed to reduce the drag – wings!(underwater of course!).

In a world where everying is increasingly becoming homogonized we may yet see a ship that looks like a plane or will it be a plane that looks like a ship?

To all my readers (that’s you mum) have a safe and enjoyable Xmas & New years !

Previous Diaries

The history of Containerisation. read more ....

Forget the Gold Coast - try Port Chalmers instead. read more ....