inevitable increase in bunker cost. Eventually, energy saving may again emerge as a major theme for marine propulsion, and the contra-rotating propeller (CRP; Fig. 1), which has already been developed but is not yet extensively used, may come into the limelight. Another opportunity may be offered by methane, which is the cleanest available natural fuel, but is not used as marine fuel except in the form of boil-off gas in LNG carriers. Using methane as fuel for vessels in general would be no easy proposition because it would require, among other things, storage arrangements on board. Fuel cells are also very promising as an energy conversion device, but there is a long way to go before they become available for common use.
If SOx is to be removed from engine exhaust gas, each vessel will have to be equipped with all expensive anti-SOx system and, more-over, arrangements will also be needed for the disposal of the treating agent, waste water from the system and the separated sulfur. In particular, the method of removal by which a scrubber is used to neutralize the sulfur content with calcium carbonate in seawater would pose the problem of how much of the calcium carbonate in seawater could be consumed without adversely affecting the marine environment.
2) NOx predominantly derives from the nitrogen content of the air and can conceivably be reduced by either controlling its generation from combustion in engines or removing it from exhaust gas by absorption. Marine engine in current use excel in thermal efficiency because combustion takes place at high temperature and high pressure, but lowering the combustion temperature to reduce NOx emissions would invite an increase in fuel consumption. However, an alternative method being studied would have fuel oil and water are sprayed together to reduce the combustion temperature without reducing fuel economy.
While the removal of NOx from combustion exhaust gas is a common practice on shore and catalytic methods are considered effective for that purpose, elimination of NOx from the exhaust gas of large diesel engines for marine use involves many problems including the non-functioning of catalysts and the need for installation of very expensive equipment in each vessel, which would mean an enormous financial burden on shipowners.
2. Modal shift
The volume of cargo traffic tends to increase every year, and the basic requirement for "quicker, cheaper and safer" physical distribution remains unchanged. Especially regarding overland transport, the congestion of highways, manpower shortages and atmospheric pollution with exhaust gas are making it necessary to adequately meet the demand for a modal shift to water transport.
2-1 Development of TSL
The project to develop the Techno-Superliner (TSL), under way since fiscal 1989, has been undertaken to create a new product which would represent a new way to meet this social demand. The target of the technological development is to provide cargo ships capable of carrying 1,000-ton payloads, at a service speed of 50 knots, an edurance of 500 sea miles and riding out waves of up to Class 6. Two alternative new types of hull support were studied, one using an air cushion generated under a catamaran and the other relying on hydrofoils. Performance confirmation tests with two experimental vessels, respectively measuring 70 meters and 17 meters in overall length, have been carried out in Japanese waters since fiscal 1994. Speeds of over 50 knots have been registered, apparently making a big stride toward the commercial application of the TSL concept.
Fig.2 shows the two experimental vessels.
2-2 Development of high-speed craft by individual shipbuilders
Ferries, playing a major role in coastwise shipping, are steadily increasing in speed, and a growing number of such vessels are running at service speeds of more than 26 knots.
Newly built vessels, including passenger vessels deployed on routes across the Seto Inland Sea, within