Designing efficient hulls using CFD

2015-01-21By Richard Broughton

Superyacht buyers are becoming increasingly keen to ensure their new purchases are as efficient as possible – they simply cannot afford not to, as the cost of fuel and meeting various environmental regulations increase. Computational fluid dynamics (CFD) is one method how owners are ensuring their yachts achieving high levels of efficiency.

CFD has been used in the large commercial sector since the 1980s. But its until now its application in the superyacht arena was underutilised, limited to designing underwater appendages rather than designing hull forms.

Sergio Cutolo, founder of naval engineering company Hydro Tec, understands the difference CFD can make but takes a practical approach to its use for underwater appendages.

‘Computational fluid dynamics is still very expensive compared with traditional tank testing, due to the processing power and software required,’ he explains. ‘We use tank testing for hull design, although very often the testing facility is performing some CFD prior to model construction.

‘We then use CFD for the details, such as propeller tunnels, shaft lines, rudders, and interaction between stabilisers and hull.’

Established in 1995, Hydro Tec, based in Varazze, Italy, embraces a wide range of naval architecture projects but has always held the use of computer technology to advance yacht design in high regard.

‘CFD is still very demanding in terms of software and in terms of training,’ says Cuttolo. ‘Our services include hull lines design, but they are limited to this kind of activity. Every year we have to develop five to ten new hull forms and, in my experience, it is not really cost effective to have an in-house team for CFD with the work we are doing.

‘However, being able to utilise external organisations that can perform CFD is key to a deeper, more comprehensive understanding of the flow around the hull of large yachts. It is certainly the way the market is going; it has to.’

CFD produced streamline velocities used to identify the flow into the propeller plane

CFD’s wider application

One such organisation that is developing these resources in-house is CJR Propulsion. The Southampton-based company has been designing and building precision propellers and sterngear for decades and has invested heavily in the latest research and technology, including robotic manufacturing and ‘true 5-axis’ CNC machinery.

This aspiration led CJR’s managing director, Mark Russell, to look to the industry to investigate how CFD could be used to improve performance and efficiency. The culmination of which was the formation of a partnership with Southampton University’s Fluid Structure Interactions Research Group (FSIRG), the UK’s leading academic institution for marine technology.

Working together to develop accurate design tools to optimise propeller and sterngear technology for improved performance, fuel efficiency, longevity and reduced vibration, CJR’s in-house R&D team was joined by FSIRG’s Simon Lewis, a post-doctoral research engineer, who specialises in CFD.

The partnership also allows CJR to utilise the university’s resources, such as Iridis 3, its advanced supercomputer, which would be otherwise unobtainable for CJR.

‘In the 12 months since the partnership started we’ve seen some tremendous results,’ says Russell. ‘Working with a range of boatbuilding customers on a number of projects, we can show results that demonstrate an increase in speed of up to three or four knots compared to the manufacturer’s original expectations.

‘But for us at least, it’s certainly not just about speed. We already have one customer who wasn’t interested in going faster but wanted to improve efficiency by limiting engine RPM,’ continues Russell. ‘We heard from them recently and they have predicted that the increased efficiency they have gained will save them around £50,000 a year in running and fuel costs. That sort of saving is hard to find anywhere else.’

At CJD Propulsion propeller shafts are precision engineered using an integrated mill/turn CNC machine

Russell is right to be enthusiastic. The results so far appear to represent a considerable return on the added cost of performing the analysis.

When the scheme started, Russell envisaged they would only be looking at the propeller, enabling a better understanding of the inflow. However, the project’s scope has changed dramatically since then and CJR is already looking at the whole underwater package for several manufacturers, in the UK and beyond. The CFD team can now help customers improve their designs in areas such as props, rudders and hull form, as well as propeller tunnels, spray analysis, superstructure airflow and drag calculation.

But how did the project get started and what made CJR invest in an area none of its direct competitors were involved?

‘We have always strived to use technology to improve the performance and efficiency of our products and our robotic manufacturing tool offers us complete repeatability to the design – something which is difficult to achieve with hand finishing,’ explains Russell.

‘There was also a shift in the work we were doing and we saw more and more customers from the semi-production market. In this arena, it is a no-brainer to optimise the propellers and sterngear, because the cost is absorbed with every boat produced. CFD was simply the next step.

‘For the last four years we were using a bespoke in-house program which was developed by our head of design, Marek Skrzynski. It utilised a vortex lattice and lifting surface methods to design the propellers,’ says Russell. ‘Although was an advanced program, it still came with some limitations: the program has the ability to calculate the propeller performance in a non-uniform inflow, but it assumes that it is uniform over the entire propeller disk, because we generally just didn’t have the data to accurately prove otherwise. In reality, that is never the case and the flow encounters various obstacles before it reaches the propeller. Each obstacle has an effect and will cause flow differences across the propeller plane.

‘This meant our first objective was to simulate flow around the hull using CFD; this allowed us to have a far more accurate and detailed input for the propeller design program,’ he continues. ‘The success of this meant we quickly moved on to including the flow around the sterngear appendages to allow design optimisation, as well as fully automating the process so we can generate the data needed for propeller design within two days of receiving a hull geometry.’

Inspection of P-brackets to ensure manufacture reflects design to a tenth of a millimeter

Where to start?

The CFD process always begins with the hull. Usually supplied by the boat’s manufacturer, the 3D computer-generated image enables a mesh of co-ordinates to be created, resulting in a virtual 3D environment around the entire hull. Once this is complete, the hull is positioned within it, initially with a fixed trim and heave. The other input is a set of ‘boundary’ conditions, which comprise fluid parameters such as flow rate, pressure and turbulence; these are then applied at each boundary of the virtual mesh.

In order to gain the required information relating to the area of interest – be that the propeller, the rudder or the hull itself – the flow conditions are calculated throughout the virtual environment, and this information can be used to gain insight into the performance of the hull, propeller or shaft. This is achieved by solving a set of equations governing water flow.

This repetitive process of obtaining the fluid properties throughout every part of the virtual environment is repeated in incremental time steps to allow unsteady flow to be accurately obtained. The time step sizes tend to be in the order of 0.1ms to 0.5ms, and this value decreases as the number of cells in the mesh increase.

Typically, mesh sizes are in the order of 20 million cells. To get a simulation time of two seconds, over 4,000 equations must be carried out, with each repeated action requiring its own internal repetitive loop to calculate the flow through every mesh cell.

The manufacture of a large shaft coupling via integrated mill/turn CNC machine

Measureable success

For CJR, the project started in February last year but it quickly became apparent that a lot could be achieved.

‘It was May before we were receiving estimates of the drag and running trim at cruise speed, using 3D CFD predictions of a hull,’ says Russell. ‘The R&D team created a matrix of nine CFD simulations, with each representing the hull fixed in different heave and trim positions.

‘With that in place we could extract the real values we needed, with the drag broken down into hydrodynamic and aerodynamic components, enabling us to optimise the hull design, reducing drag from the superstructure above the water.

‘Last June, our first propeller was designed using CFD to predict the inflow to determine its performance. That was a significant milestone in the project’s development and a great moment where we realised the true possibilities.’

In July, the project took another major step and progressed to modifying, rather than just understanding, the propeller inflow. A study of the P-bracket followed, showing its design had a significant effect on propeller performance and could reduce its noise and vibration output, which is a considerable proportion of the total experienced on the yacht.

‘Altering the P-bracket design to change the wake into the propeller can have a considerable impact on the pressure pulses felt by the hull in the propeller region. We found we could reduce the pulses by up to 50 per cent and it had a knock-on effect of improving propeller efficiency, which can also contribute to performance.’

By September last year, CJR was receiving positive feedback from customers and when the CFD method was used for rudder analysis, by simulating several different designs, it prompted a major yacht manufacturer to immediately adopt a lower drag rudder.

Work on rudder design optimisation continued with CFD modelling of the swirling flow behind the propeller. This allows the rudder design to be optimised for the propeller race, with induced drag minimised by reducing rudder side force as the boat travels in a straight line. This in turn reduces the overall yacht drag, and can increase boat speed by an average of one knot across its RPM range.

JR’s robotic finishing tools ensure that the geometry of its finished castings perfectly replicate the designs of the 3D models

The results

One of the first on-the-water trials CJR completed was for a high-performance 28.9m boat, comparing the existing, non-optimised rudder with one that had been through the full CFD process and which CJR believed would have a significant impact on the boat’s performance.

CJR recorded straight-line speed at different engine RPMs, and according to Russell, the results were encouraging. ‘Most interesting for us were where the biggest gains came from. Overall the boat’s top speed increased by 0.4 knots but we found that at midrange, between 1,600rpm and 1,800rpm, the improvement was far greater and we were recording increases above two knots. Also, her turning speed had increased significantly, by up to three knots.

‘These trials only analysed the rudder performance, so if we were addressing the whole package, there is a lot more we can do. Just looking at this trial, from the results we gained, we know we can still find ways to optimise further and deliver more uniform speed increases across the RPM range.’

At top speed, the maximum RPM reached by the engine increased from 2,373.75 for rudder B (the existing rudder) to 2,382.25 with the CFD optimised rudder A. This means the engine is operating above the optimum design RPM and as a result will be delivering less power to the propellers. Russell believes this would potentially account for the new rudder having less impact at the top speed compared with at lower speeds.

‘Knowing this means we can make further tweaks,’ he says. ‘For example, increasing the pitch of the propeller would reduce the maximum RPM of the engines and bring an estimated increase in top-speed of around one knot. Beyond that, the CFD optimisation could even be tailored to the yacht’s owner’s own preferred specification, with elements of the process, such as the toe in angle, set for the speed that the yacht will most often be operated at.’

So what this would mean for larger boats? ‘We’ve already done an extensive amount of work on several superyachts, up to around 40m,’ says Russell, ‘but always with efficiency, vibration and ride comfort as the central driving forces. Although there is usually a desire to be faster, it hasn’t been the primary motivator for our customers.

‘We’ve worked recently on a 37m Turkish-built superyacht and a British-built 40m and both have shown there is just as much scope for improvement on superyachts as there is for smaller boats: the process remains fairly constant.’

One example of a larger boat was a 40m superyacht, which underwent CFD analysis of its rudder, and again compared the manufacturer’s existing model with a fully CFD optimised, CJR designed rudder. CJR found that straight-on, the drag of the existing rudder was four times higher than drag of the new one. That translates to a one knot difference in boat speed at full power. The reduction in drag was also evident at a 35° angle, indicating that the boat fitted with the new rudder would retain greater speed while turning, compared with the existing rudder.

Streamlines within the CFD program used to demonstrate and analyse the flow into the propeller

The next step?

There is still a lot CJR wants to achieve. ‘We’ve been delighted with the results so far but as with any service which is centred around advanced technology, things are progressing all the time,’ says Russell. ‘We’re commissioning our own super computer and that will help improve the time it takes to get results and to enable us to offer our services outside our existing customer base.

‘We see the scope of CFD as almost endless and we’re always looking for ways we can extend the work we are doing to other manufacturers, potentially purely as a consulting service, working with yacht designers from the outset.

‘We are often limited by the information we receive from the customer and the 3D geometries supplied by different boat manufacturers can require a significant amount of work before it can be imported into the CFD program for analysis. If we can standardise this process so we always receive the same information, we believe we can reduce the time between receiving the 3D geometry and producing preliminary results considerably. Combine that with the constant refinement of the CFD process and we will have a truly unique product offering, one which none of our direct rivals can compete with.’

CJR hopes to further extend the scope of the CFD process and has ambitions of soon being able to model the entire sterngear with the propeller rotating, as well as allowing the hull to find its own running trim during the CFD simulation. This currently isn’t economically viable but should become more realistic with further advances in CFD, computer power and industry experience.

In their own words, Russell and the CJR team are only just getting started but they appear to be on the right track. And, as boat builders and naval architects increasingly sign up for the service, it might not be too long before the idea of specifying an unoptimised propeller on your motor yacht seems as strange as opting for rope rigging for your new 40m ketch.

June 2009

Photography: Courtesy of CJR Propulsion and Hydro Tec

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