Benefits brought on the wind

A project led by Danish consulting naval architects Pelmatic Knud E Hansen could breathe new life into the wind ship concept.

In the right circumstances – wind-assisted commercial vessels could be significantly more fuel efficient, and produce less pollution than mechanically driven ships. This are the findings of the Phase 2 report of the Windship project, which has just been published.

The Danish Windship project started in 1995 when the Ministry of Environment and Energy in Denmark awarded funding to Danish naval consulting firm Knud E Hansen A/S to investigate the feasibility of using sail power on modern commercial ships.

Although not well suited to all types of trades – such as the spot market, which could require a wind ship to operate anywhere on the globe, or transporting time sensitive cargoes – the report believes that some ship types could make ideal subjects for wind powered vessels.

Low value, low-density cargoes could utilise the increased cubic volume of wind ships, says the Danish firm, highlighting forest products, and bulk products, such as wood chips as examples.

In, Phase 3 of the project, the consortium is seeking input from owners interested in the environmentally friendly wind ship concept, highlighting the fact that, on the right routes, fuel savings of 50 per cent could be achieved.

This corresponds to annual fuel savings of around 5,000t, and significantly reducing CO2, NOx and SOx emissions.

In the first phase of the windship project the history of wind ships was summarised, and a proposal for a new type of wind ship was developed.

The report from this described the possibilities of a 50,000 dwt ship with a length of about 200m. Comparing this design with that of conventional ships, the report concluded that a wind ship was feasible, although overall transportation costs might be a little higher.

The findings were so encouraging that Danish Environmental Protection Agency released funding for Phase 2, the work started in October 1998, carried out under the auspices of a Steering Committee that included the Environmental Protection Agency, the Danish Maritime Authority, Danish Maritime Institute (DMI), BIMCO, and a number of other parties.

Phase 2 focussed on detailed rig design, wind tunnel measurements, towing tests and realistic simulation, and an economic feasibility study. A new rig concept was developed, using high lift profiles with moveable flaps. Rotating high tensile steel mast sandwich profiles acting as slats and flaps were hinged on horizontal axes.

Measurements were undertaken in a wind tunnel and in a towing tank, and simulations were performed using a Velocity prediction program (VPP) developed specifically for the project. Weather routing based on real weather statistics was performed, as well as Finite Element Modelling (FEM).

The commercial analysis compared modern product carriers with the performance of the wind ship product carrier, using existing trade routes and patterns, in order to calculate the fuel consumption of a modern wind ship with a high level of precision.

Product carrier

A 50,000dwt product carrier was selected as the basis of the study, although originally the intention was to make an economic comparison between two bulk carriers. As Pelmatic Knud E Hansen points out, gear-less bulk carriers could well be ideally suited to the application of wind power.

In fact, according to the report of Phase 2 of the project, choosing a products carrier rather than a bulker tended to highlight the limitations of the wind ship concept, in terms of service speed, rather than highlight its better cubic capacity.

Two main trade patterns were chosen which were representative of product carrier traffic, one mainly in the Atlantic, the other in the Indian Ocean-Pacific region, and it was assumed that the crew costs of a wind ship would be the same as a conventional vessel.

Rig/mast design

Modern attempts to design a rig for a wind ship have either used modern versions of the square or gaff rig, or more modern solutions, such as aerofoils, foldable hard panels or rotors.

The ship studied in Phase 1 of the Danish project had a modern version of the so-called ?lugger? rig. Six self-staying pole masts were fitted in the port side of the ship and each mast carried a hoistable crane-type arm reaching to the ship?s centreline.

This rig had advantages and disadvantages, one of the main problems being stretching the lugger sail in an efficient way. Another problem was the wind resistance of the round pole mast when the sails were reefed, and to overcome the problems it became clear that the steel mast should be an active part of the sailplane. After much effort and analysis of alternatives, a broad, symmetrical self-staying, rotating steel mast was selected.

For the sails, the design and make-up of an aircraft wing was examined. On an aircraft, the lift provided by the wings needs to be increased during take off and landing, so a slat is protruded from the leading edge of the wings and one or two flaps from the trailing edge, and small air slots are thereby created between the wing and the slat/flaps.

The idea is to prevent separation by directing air from the lower side of the wing to the upper, through the slots, thus adding kinetic energy to the boundary layer on the upper side of the wing.

The slot in front of the flap delays turbulent separation over the flap. The slot aft of the slat prevents permanent laminar separations over the forward portion of the wing. and by studying this process in detail, a new glass fibre sail was developed ? the high-lift wing mast.

Choosing a rig

From among the alternatives, two rigs were wind tunnel testing. The first was a combination of a steel mast and sailcloth (known as the sail mast), the second being the high-lift wing mast.

The wind tunnel tests indicated that the high-lift wing mast had a much higher lift coefficient than the sail mast, but also had higher values of drag, but it was less sensitive to changes in the angle of incidence of the wind, and even at upwind courses the wing mast was considerably more effective than the sail mast.

The only drawback of the wing mast is that it is appreciably more expensive than the sail mast ? having more moving parts and hydraulic components.

However, when the cost of fabricating the sails for each type of mast was taken into account, along with the fact that the wing mast was a relatively lightweight structure requiring little maintenance, the sail mast became more expensive, and based on wind tunnel tests, and cost estimation, Pelmatic Knud E Hansen chose to proceed with wing masts for the remainder of Phase 2.

Economics

Although much progress was made during Phase 2 of the project, the study also highlighted some of the commercial limitations of the wind ship.

In fact, it proved uneconomic to use wind ships on typical product carrier routes, and a cost increase of approximately 10 per cent was calculated when comparing a wind ship with a conventional product carrier of the same size.

The economic analysis carried out by Maersk Brokers demonstrated that, in certain circumstances, there would actually be a fuel consumption penalty, particularly when winds were not strong enough to drive the ship at the required speed.

However, by lowering the required service speed a reduction in fuel consumption of 25 per cent could be achieved.

The project partners concluded, therefore, that a product carrier would not, in fact, be the preferred choice for a modern wind ship, but the potential of the concept is demonstrated by the fact that on certain routes, reductions in fuel consumption of up to 50 per cent were possible.

Phase 3

Pelmatic Knud E Hansen says there is still much post-processing work that needs to be done to determine fuel consumption for specific routes at specific times of the year, and much work still needs to be done identifying potential customers, with the right markets, trade routes and goods to transport.

Apart from identifying possible wind ship customers (cargo owners), the wind ship consortium hopes to optimise the hull and rig further in the next phase of the project.

Wind ships could provide economic and environmental benefits, claims Pelmatic Knud E Hansen.