Military vessels at the environmental cutting edge

It sounds like a contradiction: an environmentally friendly warship. Warships have tended to be designed for specific roles without much regard for their ‘green’ credentials, and effectiveness rather than efficiency has been the design yardstick. Now a team known as Naval Design Partnering that embraces both the UK Ministry of Defence (MoD) and personnel from Babcock Future Projects has been working on a six-month project to look at technologies and design concepts that might be used for future warships. The aim of this study has been to create a concept that might provide a model for future warship design where the accent is more on patrol and enforcement rather than aggressive tactics.

In this development it has been assumed that the role of the Royal Navy will be much as it is today with fishery protection, counter terrorism duties and peacekeeping being the major roles that a warship might have to undertake. The future warship concept study, which is aimed at the year 2035 when it is anticipated that fuel costs will have risen dramatically and environmental issues will have risen much higher up the design priority scale, aims to develop a concept that will embrace ‘green issues’.

With fuel consumption being a major factor in warship operation the study focussed on hull design and propulsion. After detailed study, which ruled out multihull configurations, the design focussed on a wave-piercing monohull with a very fine entry and a transom stern. The fine entry reduces pitching motions in head seas and allows the bow to partially cut through waves, but the high length/beam ratios that this concept demands would induce rolling motions. These motions have to be controlled by active stabilisers. The transom stern allows for a sensible helicopter platform and the incorporation of a stern garage from which assault boats and other small craft can be deployed.

The superstructure is angled to reduce the radar returns from the structure, much in the way of the latest current warships, and composites are proposed for the construction of some above-deck structures.

One of the major changes foreseen for the future is the use of fuel cells for the propulsion plant. In a study of all the alternatives and future technology, solid oxide fuel cells came out at the top of the list because of their high efficiency. In the study, fuel cell technology is forecast to be on a level with diesel engines and gas turbines by the mid 2020s, and the forecast of obtaining an 85% thermal efficiency from these fuel cells scored heavily in their favour. The solid oxide cells can use conventional hydrocarbon fuels, and the exhaust heat that is generated can be used to drive a turbine to further enhance efficiency. This use of fuel cells points the way to electric propulsion being the most efficient form of power utilisation, and German company Siemens already has power plants of this type as demonstration units so the technology is not too far away.

To match this generation of electricity the propulsion units proposed for this warship of the future are rim drives thrusters, where the rim of the drive pod forms the stator of the electric motor, and the propeller the rotating part. Again these propulsion units are already under development, thus reducing the risk element of the proposals.

Such high efficiency, self-contained, propulsion units can be mounted so that they can rotate around a horizontal axis to provide steering as well as propulsion and they offer a high level of manoeuvrability through joystick control. Another benefit of using rim drives is that they do not demand modification of the hull shape to maximise efficiency. This means that the hull can retain its optimum shape for efficiency with no requirement for propeller ducts or apertures. The rim drive and the fuel cell power generation proposed are compatible technologies.

With naval ships spending much of their time on patrol at modest speeds, consideration was given to the addition of wind power for propulsion. Towing kites that can be deployed and retracted under bridge control were the selected system partly because of their efficiency and partly because they can be installed as a simple bolt-on unit. The currently available systems made by SkySails of Germany can generate a pull of 8tonnes which corresponds to 1,000kW of installed conventional power, and future models are projected to have a pull of up to 130tonnes which could generate significant performance. Of course the use of these sail propulsion systems is dependent on wind strength and direction, but the study suggested that the use of sails could generate significant fuel savings.

Perhaps the biggest revolution in this future warship design came from a study of the various hull and superstructure construction materials that might be available in 2035. Composite materials were looked at in some detail and a variety of biodegradable natural fibres were compared with conventional e-glass for composite hull construction material. Materials such as hemp, jute, cotton and flax were all considered as the core material combined with a natural polymer matrix material. These naturally occurring materials had the advantage of reducing the emissions generated in their construction and were bio-degradable, but the team was not convinced that their development would be far enough advanced to make them for suitable for warship construction at the chosen date.

These natural fibre composites were considered to be much better that current e-glass and Kevlar/carbon composites in terms of recycling possibilities and emissions emitted during construction. However, steel and aluminium construction too offers the possibility of recycling, and so the team came to no firm conclusions about construction materials and pointed out that the use of natural fibres could incur a weight penalty that could lead to higher fuel consumption that would negate many of the other advantages of the materials.  

Looking at longer term developments, the use of the wave piercing hull form could induce waves to come over the bow, indeed it almost invites them to do so and there were thoughts that some of the energy to be found in this moving water could be harvested by installing water turbines at suitable points in the bow area that would be turned by the moving water. The team sees this as an area for future development, but many of the ideas and concepts proposed for this warship of the future are already under development with, in many cases, prototypes running.

The incentive for development along the lines suggested is most likely to come from an increase in fuel prices and the need for Navies to be seen to be operating in an environmentally friendly way. The chosen hull design may not prove suitable for commercial cargo carrying applications (though it has promise for passenger vessels) but the propulsion systems such as the fuel cells and rim drives and the use of wind power could have significant potential. Studies of future commercial vessels such as those carried out by Wärtsilä show designs with wind propulsion, but the accent on commercial ships of the future is more on smaller changes to make hulls and machinery more efficient and thus greener. What does seem certain from studies like this naval ship of the future is that there will be pressure to meet new environmental targets, and there is significant technology out there to make a difference.