The 2020 sulphur-in-fuel-oil cap is expected to drive demand for propeller redesign and retrofits as shipowners and operators seek to improve the match between the propeller and the ship’s main engine to maximise fuel efficiency, Dr Lars Greitsch, managing director & head of research & innovation, MMG Propeller told VPO Global in an interview.
A propeller will initially be fitted to suit the engine and the ship’s operational profile, but according to Dr Greitsch, this is often very different to the engine profile the ship is operating with several years later. Ships may start to slow steam with a propeller that was most likely designed for higher speeds and low cavitation. A propeller not suited to the engine profile will lead to excessive fuel consumption and increase emissions.
Higher production costs of 0.5 per cent sulphur blend and higher use of gas oil could increase the cost of low-sulphur fuel. As consultants Wood Mackenzie predicted earlier this year, global marine fuel prices could rise as much as 25 per cent, or $24bn in 2020. As Reuters reported, if no vessels added scrubbers and all ships complied with the rules, the spike could be as high as $60bn.
This surge in fuel prices combined with concerns around the availability of complaint low-sulphur fuel, means that a fuel-efficient ship, and fleet, is imperative to competitive and compliant operations. An optimised hull and right hull coating, and maintaining engines is important to fuel efficiency, but operating with a propeller that is not suited to the operational profile of the engine and the ship will be detrimental to the fuel and emissions savings that could be achieved.
Dr Greitsch is confident that “the sulphur cap changes a lot. Without sulphur cap, we would expect retrofits to slow down.” He believes that a lot of larger vessels where the fuel savings are significant have already undergone retrofit because the business case is there. Smaller vessels, where the business case is not so strong, undergo redesign and retrofit less frequently. However, higher fuel costs, fuel availability uncertainties, and tougher emissions standards are expected to catalyse further retrofits, particularly on smaller ships, where higher fuel prices make the business case for fuel efficiency through propeller retrofit greater.
Dr Greitsch predicts that there will be a demand for propeller redesign and retrofit as shipping companies begin to acknowledge that their propellers are no longer optimised to the profile of the main engine.
Retrofitting from a technical side
According to MMG Propeller, a significant quantity of fuel, up to 14 per cent, can be saved by operating with a propeller that is geometrically and hydrodynamically optimised to the main engine of the ship. “A constant saving over different speed changes and draughts is possible with an optimised propeller” said Dr Greitsch.
Dr Greitsch explained that the connection between a ship’s operational profile and propeller efficiency is based on keeping the propeller clean and ensuring its diameter, pitch, pitch distribution, and blade number is right for the ship, but this has many challenges.
The blade area should be carefully considered to reduce cavitation risk. This needs to be precise, it cannot be an estimation. “You cannot have an estimation of the propeller diameter because it would change the blade number. If you change the diameter of the propeller you would also have to check all the other areas and probably change them, so it’s a risk and is very complicated. It is therefore difficult to base the propeller design on the operational profile of a ship because you have to consider so many different operational points.”
“There are a lot of influences between the hull and the propeller and these influences depend on the propeller loading, vessel, speed, draught, weather conditions, and more. This makes the analysis process complicated as you have so many influencing parameters and you have to pick out which parameter, for the propeller design, is the most important, and which influencing parameter leads to the highest loss in efficiency.”
In a case described by Dr Greitsch, two propellers on a container vessel, one with an 18m diameter and the other with a diameter of 8.2m found that despite all other design factors remaining the same, there was a propeller efficiency difference of 3 per cent. “Propeller diameter is clearly driving efficiency and the diameter is in conjunction with the aft body shape.”
Dr Greitsch admitted that working with the shipowner from the start is key. “You have to work with the shipowner to begin with to optimise the propeller design for the speed and operational profile of the ship. If the shipowner has some ideas on operational profile then we can pick what will be the best combination between the ship and the propeller configuration to satisfy his operational demands.”
Designing for newbuilds
For a newbuild vessel, it is vital for the propeller for to be designed following simulated computational fluid dynamics (CFD) predictions to ensure the highest quality propeller for the ship’s operational profile and therefore lowest fuel consumption, confirmed Dr Greitsch.
This is all about getting the right geometrical design. “If there are deviations between the theoretical propeller and the full-scale propeller delivered, there could be deviations in the savings. For example, of the propeller fit is not optimised to the main engine then the fuel consumption may increase. If the characteristic curve of the propeller does not fit the engine diagram as it should, then it is not optimal and there will be additional fuel consumption because there is not a good match between the propeller and main engine.”
Today, there is a focus on simulation and comparison of many different propeller designs to find the optimal one.
Dr Gretisch considers digitalisation as a vital enabler to better simulation and comparison of propeller designs for vessel efficiency. It has been noted that there are problems with scaling up a propeller model to full-scale but the Digital Twin concept is expected to change this. The Digital Twin enables simulation of different propeller parameters on the ship’s operation. “This is the largest advantage to the digital, data, and smart data concept on board ships”, he explained. “There is a real clear effort today to get rid of all data problems associated with model testing and use a direct comparison. A Digital Twin in full-scale can compare and shoe physical values with real operational data taken on board a vessel.”
Designing or redesigning an efficient propeller is challenging in an industry where the international standards for manufacturing propellers is very weak, claims Dr Greitsch. There are a lot of margins and high tolerance levels, which can lead to deviations in the geometrical quality of a propeller.
Dr Gretsch believes that a lot of propeller makers will follow this required standard but not go beyond, which will be essential if standards in propeller design are to improve.