Promas comprises of a twisted full-spade rudder with bulb that is smoothly connected to the propeller hub by a hubcap, and is adapted and optimised to the propeller design.
A well-designed twist adapts the rudder to the rotation of the propeller slipstream and reduces the local angle of attack on the rudders leading edge.
This gives a more efficient rudder with lower drag and better recovery of rotational energy from the propeller slipstream.
Conventional design and construction together with simple installation means the payback time for the Promas concept can be surprisingly short. Other benefits include:
- Propeller and rudder are designed together as a single unit for optimum propulsive efficiency.
- Propulsive efficiency is increased by typically 3-6%.
- Improved low speed manoeuvrability.
- Improved possibility for low pressure pulse / low noise propeller designs.
- Simple and robust design.
- Almost as easy to install as a conventional propeller-rudder system.
The best results are achieved on blunt single screw vessels with a block coefficient of 0.75-0.85 and a design speed in the 14 to 16-knot range. Here the efficiency gain can be as much as 6-9% compared with conventional solutions.
For faster and slenderer single or twin screw vessels such as car carriers, efficiency improvements of 2-5% can be expected.
Real efficiency gains
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The diagram shows a test comparison for a medium size chemical tanker. A conventionally designed propeller and rudder is compared to the Promas system. The rudder blades are of the same size in both cases and the propellers are designed to be comparable from a pressure pulse point of view.
The reduction in power consumption recorded was 4-5% for a ship speed range of 14-16 knots. A small investment to improve propulsive efficiency during design will save money through out the vessels life.
Promas Lite – the ultimate upgrade to improve vessel efficiency
Promas Lite, a simplified version of Promas is ideal for vessel upgrading. The vessel’s existing rudder is retained, but is fitted with a prefabricated bulb, while the propeller is equipped with a special hubcap and new blades.
The propeller is designed to utilise the fitted bulb and is matched to the vessels current operational profile, which may have changed since the vessel was built. Installation of a complete Promas Lite upgrade kit can normally be undertaken within a normal 7-10 day docking period.
Recent Promas Lite installations on twin screw cruise vessels demonstrate efficiency improvements of between 5-15 per cent can be achieved, giving a payback period of well under two years. For these vessels the propeller designs were not only adapted to the Promas Lite system but also for the vessels current operational profile.
DFDS Pearl Seaway
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The picture shows a recent upgrade of the twin screw RoPax ferry DFDS Pearl Seaway. Extensive modelling work was carried out at the Rolls-Royce Hydrodynamic Research Centre to refine the design and provide a tailored solution matched to the hull and operating profile.
A twin 4.8m Promas Lite propulsion system was installed during the vessels regular dry-docking in early 2011. The existing CPP blades, originally designed for a higher operational speed were replaced by blades specifically designed to maximise the efficiency of the Promas Lite installation.
Image courtesy DFDS Seaways
Prefabricated bulbs were welded to the existing rudders and stainless steel hubcaps fitted to the propeller hubs. Following a period of analysis, the DFDS Technical team calculated the efficiency improvement to be up to 12.5 per cent, representing a payback time of about 1.5 years.
The biggest factor in vessel operating costs is the cost of fuel. A vessel’s propulsive efficiency is determined by a number of factors, the main component being the effective integration of the propulsion system and rudder with the hull. All Promas Lite designs take these factors into account to derive the most energy efficient solution.
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