A Unique Hull Design that Benefits from CFD Analysis
Petestep is the name of an innovative company that designs hulls for various manufacturers. In a typical project, Petestep’s engineers work together with a boatbuilder and a designer to create the best hull possible for a new boat model. Since they use their proprietary patented technology and not a conventional hull geometry, there are no applicable established methods to guide the company in the design of their hulls. Therefore, the computational fluid dynamics (CFD) software Orca3D Marine CFD has become an important analysis tool in their process.
The name Petestep is simple enough, but still needs some explanation. ‘Pete’ is Peter Bjersten, the Swedish race boat driver who invented the basic concept back in the 1990’s on which their technology is based, and ‘step’ could seem obvious since their designs have several jogs in the hull. But Jonas Danielsson, Chief Executive Officer, and Fredrik Wikerman, Chief Technology Officer, are quick to clarify further. To differentiate these jogs from the strakes we are used to seeing on a planing hull, they call them ‘deflectors.’ I guess Petedeflector didn’t have the same marketing ring to it.
To ensure that we understand the function of the deflectors that are so key to the philosophy, we should look at the reason for traditional lifting strakes. Powerboat monohulls (single hulled vessels) are designed in a symmetrical fashion with the port side of the hull matching the starboard. Viewed from aft, the angle of the hull bottom measured from the horizontal is the deadrise angle and the choice of that angle is a designer’s classic trade-off. The lower the deadrise (flatter the bottom) the more efficient the hull, but the more uncomfortable the ride as it slams into each wave or wavelet. In order to increase the lifting ability of a high deadrise hull, small strakes with near horizontal lower surfaces are typically added, which has its clear disadvantages in terms of pounding and noise. The strakes and deflectors both increase lift and shed spray. But Petestep’s patented deflectors are an innovative replacement for strakes and don’t carry the disadvantages that strakes do.
The deflectors in a Petestep hull don’t eliminate the trade-off between high or low deadrise, but they widen the benefit of each choice and will significantly reduce the drag without making the ride more uncomfortable or noisy.
Peter, the inventor of the Petestep, started with his intuition and lots of fiberglass and resin. Watching all the wasted energy that flew out to the sides of his race boat as it planed bothered him, and he wanted to harness that energy to reduce the drag and/or provide a drier ride. It just didn’t seem right to have the horsepower and fuel going to waste in the form of spray. He was a long way from Computational Fluid Dynamics and high-powered computers, but it was the seed that launched the company.
When Peter started making full-scale prototype tests 15 years ago, he also discovered that the boats had a great dynamic stability. Below is Peter in his race boat ‘driving’ with both hands in the air to demonstrate how stable the boat was. But let’s add the disclaimer, “this is a professional driver on a closed course.”
The difference in how the patented deflectors work compared to lifting strakes is significant. In this illustration below, you can see the geometry difference – conventional strakes are added to a hull to divert the water outward, while Petestep deflectors intercept the flow as it hits a more vertical face and deflects the flow down and aft.
Jonas notes, “We want the deflectors as close to the intersection of the hull and the water’s surface as possible, to remove as much of the spray sheet from the hull. They do not make any contribution if they are submerged. The main resistance reduction comes from taking care of the frictional resistance from the spray.”
The illustration below shows the reduction in the wetted surface from spray for various bottom configurations. The conventional hull without strakes is fully wetted by the spray created at the intersection of the water and the hull. Conventional strakes, the middle image, remove some of the water as it is thrown aside by each strake. The Petestep hull is designed so that the deflector is set just outside the water-to-hull intersection, shown in white, to intercept the entire spray sheet and channel it aft and down.
A conventional hull without strakes has a wide swath of wetted surface from spray. Adding strakes reduces that spray (and drag) and provides some lift. Petestep deflectors improve that effect even further, as the vertically protruding deflectors divert the spray aft and down. More lift and less drag are the immediate results.
Much of the early design process was trial and error while Peter was trying to both prove and improve his idea. In 2012 Jonas joined the team and brought the first level of science in a MATLAB application that he wrote, which combines the Savitsky planing performance prediction with the deflector geometry to optimize the size and location of deflectors in an iterative process. It was successful and is still used today as a first step before the CFD work begins with Orca3D Marine CFD.
This output from the MATLAB tool, shows the wetted area (blue) and the appropriate deflector locations (black lines). This is a fish-eye view with the bow to the right.
The addition of CFD to the suite of analysis software in the Petestep office began with a trial license orchestrated by Bruce Hays of Orca3D. It is great to read about software and talk to others that have used it, but unless you can test it on a real project, it is hard to assess its merit. Orca3D Marine CFD, which is based on the SimericsMP RANS code, is integrated with Rhino3D/Orca3D so the meshing of a hull and setting parameters for a CFD run is much simpler than most CFD software. For Jonas and Fredrik, one project was all it took to justify the cost of bringing CFD inhouse. Orca3D Marine CFD provided the efficient, accurate, and cost-effective analysis tool that they needed. It’s integration with Orca3D means that a naval architect can use it productively without the need to become a CFD specialist. Petestep works exclusively with design and simulations of planing hulls and now has one dedicated CFD engineer in Filip Wängelin, who originally comes from the automotive industry. Filip quickly learned Orca3D Marine CFD and has helped Petestep and Orca3D in improving various methods and setups.
CFD permits both a finer level of analysis that doesn’t depend on previous experience, and a high level of confidence in a design. The most basic outputs are speed, drag and trim, but the bonus is being able to assess vertical acceleration in various wave states and turning characteristics. The Petestep goal was to use innovative design to reduce fuel consumption and vertical accelerations, increase speed, reduce pitching, and control the behavior in turns compared to today’s best designed planing hulls – a lofty target, but one they have achieved.
Thanks to these methods, Petestep can provide large savings in R&D and prototyping for its clients, and also reduce time to market on a new model.
Project – HOC 33
The HOC 33 was designed from the ground up with Petestep in mind. It’s an all-round boat with both comfort and efficiency as targets. This photo of identical sisterships illustrates the lack of spray when travelling at speed. This was also one of the first demonstrations of the reduction in noise level – with reduced spray hitting the hull the decibel levels are noticeably less.
Project - Fugu 29
The Fugu 29 was in 2018 the first Petestep hull with twin steps combined with multiple deflectors. Orca3D Marine CFD was used extensively to fine tune the deflectors, measure drag and accelerations, and give the confidence to proceed directly to tooling. With deflectors that divert the spray aft as it moves from centerline out toward the chine there is also a component of thrust produced that reduces the drag of the hull by between 2 and 4 percent. Because the flow is turned down as well as aft, there is also a component of increased lift of about 7-10%. In this project, the client was putting great emphasis on offshore high-speed cruising ability, and CFD analysis is the only way to analyze dynamic behaviour in waves at the design stage. The ability to determine and compare the vertical accelerations of the hull when traveling through waves has proven valuable in many of Petestep’s projects.
A rendered video from Orca3D Marine CFD of the Fugu 29 in waves.
Petestep typically has a 30% reduction in vertical acceleration compared to a conventional vee bottom and therefore a substantially increased comfort level.
This plot of vertical accelerations tells the tale. The higher the speed the greater the benefit of deflectors.
The deflectors on this boat are typical for a Petestep hull combined with transversal steps and are zero height at the keel line and typically around 1-3 inches at the chine. The softer landing of a Petestep hull after launching from a wave does two things – it limits the impact loading and also reduces the changes in pitch angle (bow up or down).
A twin step hull combined with deflectors results in a minimum of wetted surface. This CFD pressure plot shows green for zero pressure (no wetted surface). The high-pressure red areas are the contact surfaces with the water. The effectiveness of the deflector is evident in that the spray ends at the deflector line.
Project - Vanguish 115
You might think a boat of this magnitude needs a change in thinking from the hull form of a 25-footer. But in fact, all the same trade-offs exist. The client request was for efficiency, and therefore fuel economy, and comfort underway. With a 45-knot top speed, the very long hull is approaching the efficient planing zone, but does not reach speeds beyond, so it was difficult to produce a large drag reduction. But even a small efficiency gain in a boat that uses a significant amount of fuel (on the order of 30L/nautical mile / 0.15 MPG) results in a large dollar and emissions saving. Designed with a Petestep hull, this 115-foot boat saw a 6% reduction in resistance.
Vanquish 115 rollout.
Project SAY 42
The SAY 42 is a great example that showcases many of the benefits of CFD analysis. Say Carbon Yachts, a German boat builder, came to Petestep with a request to update their current model with a Petestep hull. But there was a catch – they loved the turning ability of the current hull and wanted it to be maintained. The improvements they were looking for were in top speed, efficiency, and a drier ride.
By analyzing the old and new hulls while turning, they found that by altering deadrise, deflector heights and location, they could match the turn rate, heel angle, and slide rate. It was quite a balancing act between efficiency and agility to finally match the previous hull’s turning characteristics. There were so many factors to adjust the turning response that if it had been attempted full size on the water, it would have been a long and near-impossible task. In CFD it was still a challenge, but the successful conclusion of the Orca3D Marine CFD runs provided the confidence to go straight to tooling without a prototype. The client was ecstatic with the resulting performance.
Say 42 – demonstrating its well-loved turning characteristics.
A clear view of the deflectors on the Say 42 hull
Leveraging Innovation with Computation
One should not get the impression that innovative solutions to marine problems are solved by clever software. It was the insight of Peter, Jonas, Fredrik and their team that led to the development of the Petestep hull. But we also don’t want to underestimate the importance of having the right tools for the job. In this case both CAD (Rhino3D with the Orca3D plug-in) and CFD (Orca3D Marine CFD) were key contributors to this success story.
Sometimes the Orca3D software is a confidence-building confirmation that what you have designed is on the right track, but at other times Jonas and Fredrik have discovered that CFD is a necessity - there is no other practical way to investigate an idea. It is the tool to use when assurance and quantitative results are required to make large gains in efficiency and performance.
A trio of Petestep hulls –the Silver Viper, Fugu 29, and HOC 33 Explorer
Orca3D Marine CFD – Orca3D.com
