CJR’s managing director, Mark Russell, explains how recent technological developments are driving the propeller industry in the right direction, and how data and an intelligence-led approach to design and manufacture is helping to deliver notable performance and efficiency improvements. 

Continuing its drive to improve performance and quality standards, CJR Propulsion, the Southampton-based propulsion and CFD specialist, recently announced a new £4 million investment in design and manufacturing technology.

What can you tell us about the new opportunities technology is providing the propeller industry?  

It’s important to note that not all the technology is ‘new’ in its truest sense. Some of the tech we are discussing is used extensively within the automotive and aeronautical industries but it’s now proven, so new opportunities are opening up for those in the marine industry with the ambition and willingness to invest.

As an industry, what we have now is the ability to develop a complete end-to-end design and manufacturing solution, combining multiple technologies to produce a fully optimised package of products. Starting with the initial designs, right through to finishing, there is equipment, hardware and related software for each step of the process – which, when used together, provides a best-in-class output.

At CJR, we design and manufacture propulsion systems – a process many people would imagine is largely manual, and it used to be. However, over the past decade, data, automation and even machine learning is playing an ever more important role in our business and across the whole industry.

We started our own journey – more than a decade ago now – looking at how we could leverage data more effectively. That led us to build a computational fluid dynamics (CFD) department to enable us to design a fully-optimised propulsion package unique to each specific project.

I believe CFD is essential for all propeller manufacturers

Today, I believe CFD is essential for all propeller manufacturers. It enables you to consider the factors that genuinely impact performance, fuel efficiency and longevity – in a way that is simply not possible using ‘traditional’ methods and generic patterns.  As always, your output depends on the data you put into the system, but if we know the hull form dimensions and appendages, as well as the preferred operating parameters, we can create a design that is perfectly aligned with the vessel shape and expected operational requirements. However, an incredible design is not much use if you can’t build the thing to match the CFD exactly. That leads nicely on to the manufacturing process, which is where big gains have been made in recent years.

How has the manufacturing process moved forward? Can you run through each step?

The design stage is now more critical than ever but it is also where some traditional prop manufacturers go wrong. The old-school way of designing a prop typically used empirical data and industry standard measurements to ‘best fit’ the propeller to the chosen vessel. Much of this data is decades old and isn’t detailed enough to create accurate designs for every modern vessel type, so you could already be creating inefficiencies and storing up problems for later.

With the approaches now available, any true propulsion specialist can simulate the real vessel’s wake field within its CFD software, using information supplied by the boatbuilder, such as the 3D model of the hull and appendages. The software then automatically performs tens of millions of calculations to predict the flow of water into the propeller in myriad conditions. The propeller design is then analysed in this non-uniform inflow, revealing the expected performance, as well as other aspects of the design, such as cavitation and vibration levels. The design is automatically iterated a number of times, optimising every parameter – sometimes up to 100 designs are analysed until the optimal design is reached, working in conjunction with the whole propulsion package and the hull.

The next step is mould making. Some propeller manufacturers may still use standard, off-the-shelf patterns, which are selected based on previous projects deemed to be similar in some way. If they need a new mould, each blade is shaped by hand from wood or resin, or produced with the use of a 3D printer. The single blade is then packed into sand and resin, and indexed around to create the correct number of blades, before being removed to provide the blade form mould.

The 2019 solution, used by a small but growing number of propulsion specialists around the world, utilises automation and advanced robotics, and takes the design directly from the 3D model. The system then validates the design, performing several essential geometric tests, before machining the exact mould from a single block of fine sand and resin. The density of the resin enables a highly accurate reproduction of the design to be achieved, operating to within minute tolerances.

Once you have the mould, you can cast the prop. Casting is the one area that hasn’t really changed, and still involves the foundry. But, due to the higher levels of accuracy achieved through robotic mould-making, the prop manufacturer is able to cast a propeller to much tighter tolerances. This helps reduce the cost of the finished product and cuts down on waste but is only possible when you have an end-to-end design and manufacturing solution, where every stage is driven by validated data and where hand finishing isn’t required.

Traditionally, once a hand-made propeller had been cast, all the excess material was ground away by hand. If the grinder removed too much, they would have to repeat the process on the other blades – risking the dynamics and performance of the propeller. All this was done by eye and with the use of standard templates, but they only had to meet generous tolerances to reach the ISO standard for lower quality Class 1 propellers, so it wasn’t deemed to be an issue.

In contrast, the equipment now available means your prop supplier should have no problem guaranteeing ‘ISO Class S’ as a minimum quality standard. Class S is a higher standard and requires the manufacturer to adhere to far smaller tolerances, which is only achievable if you can remove the human element. To do that, your chosen supplier will need a five-axis CNC machining centre, which when properly installed, and connected to the other cells in the manufacturing process, can operate autonomously – moving along each propeller blade and carefully removing the excess material to meet the precise dimensions established by the CFD 3D model.

Another optimisation that has benefited the entire manufacturing process, is pick-and-place storage and delivery technology. You can think of it like a production warehouse, with shelves housing your products, at varying stages of manufacture. At each stage, you can collect a prop or bracket from a storage cell and deliver it to the required position to be machined or finished, before being returned to the rack, ready for the next stage – right through to dispatch. This saves a lot of time and effort manually moving heavy items around the facility, and creates a seamless process from the foundry to delivery.  

The final stage is validation. It seems somewhat self-explanatory that you need to know how the propulsion system operates in real-world conditions – out on the water. In order to do that, you need a data acquisition tool that is placed on-board the vessel and captures a wide range of data sets. This typically includes speed, trim, roll, noise and vibration. If the project is a replacement prop or an existing production vessel, this process is especially revealing, quickly demonstrating the difference a fully optimised propulsion package is able to make, compared to taking a traditional approach.

When it comes to the customer, what are the collective benefits of the approach you have described?

Fundamentally, the benefits are financial and performance related. An optimised propulsion system – designed for the vessel in question – will be more efficient, delivering more power to the props, which means you can reduce the engine load for the same speed. This has the knock-on effect that you could trim your fuel consumption by up to 15%. Your props will also last longer, as cavitation has been considered and mitigated at the design stage, extending your replacement cycle. Other notable benefits include massively reducing the potential for noise and vibration, which on a superyacht should be a complete no-brainer.

The relatively small cost of performing the CFD and the advanced manufacturing process is very quickly repaid through reduced operating costs and extended longevity so when you add the on-board benefits in the equation, it is pretty clear that this is the right route for the industry.

The final benefit worth mentioning is speed of delivery. If you’re in the middle of the summer season, replacing a damaged prop could historically leave you on the dock for up to ten weeks – potentially costing you millions in lost charter revenue. With the approach we’ve been discussing today, that could be cut down to two weeks, or even less.

Overall, we want to champion the players that are doing things the right way for their customers. For too long, the status quo has been left unchallenged and that breeds complacency, which is bad for the industry and the end user.    


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