Can new propellers really reduce fuel costs by 10%?

As the price of fuel continues to increase, combined with new carbon emissions reduction targets set by the IMO, fuel efficiency and the resultant cost-savings remain key deliverables in the industry. With this in mind, analysis of all aspects of a boat build must be considered, including assessing the potentially dramatic effect propellers and sterngear can have on day-to-day operational costs and performance.

This was precisely the challenge facing Mainprize Offshore, who had concerns regarding fuel efficiency and vibration levels on board their windfarm support vessels, and turned to CJR to investigate.

At the time, the vessels were fitted with propellers that were causing excessive vibration under load and were not efficient, resulting in the engines burning too much fuel. The brief to CJR was simple: design, manufacture and supply a new set of propellers for its M01 support boat that would reduce vibration and save money in the long term.

In response, CJR Propulsion set about designing a bespoke set of propellers using its in-house Computational Fluid Dynamic (CFD) department to define the optimal set-up for this specific vessel’s hull form. Once the data was input into the software, the resulting CAD designs were tested using over 20 million data points to understand the flow of water around the propeller and sterngear, whilst also analysing the pressure variations they induced to accurately predict cavitation and provide a realistic indication of the noise and vibration levels expected.

CJR Propulsion’s Managing Director Mark Russell said: “Our ability to recreate real-world conditions in a digital world, and to predict performance with incredible accuracy, dramatically impacts how our propulsion systems function. This ‘appliance of science’ does away with the traditional ‘made with experience’ approach so typical of the industry and ensures that the products we produce match our designs to the millimetre. Unfortunately, many in the industry still don’t understand the long-term benefits CFD delivers, but as more and more owners and yards get on board, it’s only a matter of time before this approach is standard across the industry.”

The vessel is now saving a significant volume of fuel each day – equating to approximately 8–10% of annual usage.

Following on from the design process, the propellers went into production, with moulds created using a 3D printer, before being cast. The props were finished using a computer-driven five-axis CNC machine, which is programmed to remove precisely the required amount of material to match the original design.

The propellers were dynamically balanced and MRI scanned to achieve the Class S standard required for optimum performance and smooth running. Mark Russell added: “The International Standards Organisation has four classes of propeller tolerances:

1) Class S, best, very high tolerance;

2) Class I, very good, high tolerance;

3) Class II, average tolerance; and

4) Class III, low tolerance.

It is worth noting that not all propellers, even when brand new, are Class S. The previous propellers on the M01 were hand-finished Class I and found to be out of balance – a major contributor to the vibration on the boat.”

On the completion of the project, Bob Mainprize of Mainprize Offshore, said: “Working with CJR has been a revelation. As they have demonstrated, technology can dramatically improve the achievable standard for performance and vibration levels. We have learnt that fully-machined Class S propellers are the only option for minimising vibration, putting less strain on the sterngear system and extending the lifespan of the vessel, plus improving crew comfort. The vessel is now saving a significant volume of fuel each day – equating to approximately 8–10% of annual usage. For us, this is an incredible saving and demonstrates the value CJR provides its customers.”