New Rolls-Royce waterjets: efficiency for sub-25m workboats
Rolls-Royce has elevated larger waterjets to new heights marrying steel design processes with the new aluminium range writes Stevie Knight.
The original FF series, though popular, has been around for 30 years and while there were some elements worth keeping such as the installation concept, “it was high time for a redesign” said Tomas Renlund of Rolls-Royce.
Recently launched at the New Orleans International Work Boat Show, the Kamewa A5 represents a major reworking of the series while retaining the light aluminium body. Further, the seven models that step up in an orderly fashion from 100kW to 1,230kW demonstrate a harmonised approach “which results in more accurate performance predictions”, said Renlund’s colleague Fredrik Appel. This was lacking in the original FF units, which as Appel admitted were not inherently a ‘series product’ using co-ordinated components across the range, but a line that had grown in a more organic fashion.
The changes begin with the pump: this has an entirely new, upgraded stainless steel impeller designed for coherence with the water inlet channel. Overall it has resulted in an improvement of about 3% on the corresponding FF series, while steering and reversing have both gained around 8%, achieved partially “through a complete remodelling of the hydrodynamic flow” including a focus on both the split bucket and the steering nozzle, Appel told MJ.
All together this translates into greater thrust at low to medium speeds and an impressive 13% higher bollard pull along with better cavitation margins, these features holding particular advantages for workboats which typically spend a disproportionate amount of time at reduced speeds.
For some installations, the benefits could be even more significant: Rolls-Royce has been carrying out in-service trials of the new waterjet onboard its own Finnish crew transfer/harbour patrol vessel. Appel explained that while the returning values have been measured against the nearest FF model, he added that in some cases he expects the actual figures to be far higher than those quoted for the generic comparison.
It’s not an incremental development but rather a jump away from the original, into the ‘new’ underlined both Appel and Renlund. So, although the A5 has ‘borrowed’ some elements of RR’s proven S4 and S3/CA steel series, crossover opportunities were limited as a higher rate mixed flow has “different characteristics” to the slower axial flow of the aluminium versions, explained Appel.
However, the A5 series utilised the company’s Hydrodynamic Research Centre (HRC): this has allowed Rolls-Royce’s steel models to gain a cumulative advantage over the years “and so we put the new aluminium range through exactly the same design process” he added. The HRC features two computer-controlled test tunnels which simulate a wide range of blue and green water conditions, replicating different temperatures, water flows and salt levels (interestingly, it was one of the first research centres to investigate cavitation effects).
Therefore, a CAD-designed aluminium model optimised for the 25 to 40-knot range was put through its paces on the HRC test bed.
The overall efficiencies gained through this approach means that, for example, the A29-5 (the first to hit the market) can be swapped for the FF31, “and in general we can say you’ll be able to change down a size for the same power” said Appel.
Importantly, this has all been achieved while reducing the footprint both outside and inside the hull: despite an inlet channel geometry that supports the propulsion improvements, the inboard space savings still add up to around 20%.
Finally, Renlund concluded that there should be a short lead time for this cost-effective propulsion system, and added: “We are looking at starting deliveries in January 2019”.
Given the new efficiencies, the workboat segment may well become as wedded to the new Kamewa A5s as it has been to the older FF line.
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