In its earliest days, composite boatbuilding was open to most anyone who could cut fiberglass fabric and spread resin. By and large, builders learned the trade in a repair shop or boatyard. Their expectations and preferences were shaped by the boats they knew. When it came to launching their own brands, they took the hullforms they had an affinity for and added their own secret formulas. Look at any distinctive designs of the ’70s, ’80s, and ’90s, and it’s not difficult to trace how they evolved. Most basic designs, especially the running surfaces, rarely deviated from those of their immediate forebears.
That’s how high-speed powerboats were built and sold—from the bottom up. Boat manufacturers created a brand stereotype from sophisticated charts, pictures, and sales aids promoting the attributes of their particular hull. Some promoted the smooth ride in rough water that characterized variations on a conventional high-deadrise V-shaped bottom. Others pitched the agile, responsive ride of a ventilated hull, while a few promoted the virtues of the very fast but tricky-to-handle pad bottom. Boat buyers who bought the pitch and liked the price, the salesman, and the service shop soon became fans of that hullform and brand.
The recession changed everything as boat sales slumped dramatically in 2008–09. Consumer financing all but disappeared. Dealer and OEM floor planning became a thing of the past. Plus, the BP oil spill killed sales on the entire Gulf Coast for more than a year.
The market contraction led to a frenzy of corporate consolidation, often driven by investment companies with no background in boatbuilding. Soon hundreds of iconic designs and molds were reduced to line-item entries on a balance sheet. Many new owners of these businesses believed that the tooling and molds that came with an acquisition were worth their weight in gold and had a long shelf life. That was not the case. Without the experienced designers and builders who had built many of these brands, bottom-up boat design and refinement became things of the past. “Let’s bring fresh ideas to the boat business” was a refrain echoed throughout the industry as new managers tried to make a virtue of the experience deficit in their ranks. In lieu of updating hull designs, they added accessories, new upholstery, snazzy, colorful graphics, and cup holders in place of rod holders. Such were the changes that defined the new models being offered to the postrecession boat market.
The New Power Dynamic
About the same time, outboard motor manufacturers made the jump from the 250-hp range to 350 hp and 400 hp or more, and consumers’ appetite for speed pushed builders to install triples and quads. Soon a new problem reared its ugly head—legacy hull designs had not been updated, and many were unable to handle this new outboard power and higher speed.
The old bottom-up solution to creating a new model to accommodate additional horsepower was to flip over a bare hull, gather the team on a Monday morning, and decide what to do. A typical recipe was to lengthen the running surface, add a few lifting strakes, provide a place for built-in trim tabs, and you were good to go. That worked well until we bolted 1,800 hp on the transom, at which point many existing hullforms became untenable and downright scary at the helm.
At 45 mph most late-20th-century high-speed designs perform well in moderate sea conditions. But at 60 mph, unfavorable characteristics become evident to the operator, especially when traversing oncoming wakes or navigating power-on tight turns. At 90+ mph the hull is clearly either able or unable to handle the increased power. There is no cover-up for an unruly, poor-handling boat at that speed. See also our story on the patented Petestep hullform that uses CFD to capture the energy of spray.
Because of their soft, almost cushy ride, hulls with 24° deadrise all the way to the stern are still offered by many builders. But, as three, four, and even five 450-hp outboards are pitched to the consumer, modifications including ventilated hulls and additional lifting strakes are required for safe handling at the speeds necessary to justify the investment in power. In short, as engine choices become more specific to desired high-speed applications, so must hull designs.
As recently as the early 2000s, builders cost-effectively offered different models intended for fishing or for high speeds that were built on the same hull or running surface. That’s no longer possible in the face of today’s market expectations for horsepower and speed.
Let’s take two 42‘ (12.8m) boats as examples. One is designed for fishing with a capacity of 10 people, and sports triple helm stations including a “tuna tower.” The other is a high-performance vessel with seating for six and not a rod holder to be found. Both have four 400-plus-hp outboard engines on the transom. Should the hull designs be the same? Absolutely not and for good reason. The same 1,600+ hp serves two totally different applications on these two boats. Load, center of gravity, beam, length of running surface, and other factors must come into play.
Remember Newton’s Third Law from high school physics? “For every action there is an equal and opposite reaction.” The faster the boat, the greater the forces it is subjected to by the water and air it encounters, and the less forgiving it is of design or handling missteps. At high speed the stakes are higher, the tolerances tighter, and the old bottom-up design process must be tweaked. The need for design refinement is clear, but the risk and expense of building a running prototype of a new model based on an existing hullform are too great.
Builders of high-speed powerboats must now turn to new techniques to accurately measure the dynamics of their hulls in action and to predictively model performance changes in response to alterations of the running surface and hull shape.
In 2019, working with Scott Porta of Porta Performance (New Smyrna Beach, Florida), I oversaw just such a refinement project of an existing hull designed by Doug Wright Designs (Melbourne, Florida). The goal for the already winning model was to improve speed and handling with increased horsepower. With an overall length of 32‘ (9.75m) and beam of about 9‘ (2.74m), it was designed and built by Wright in 2013 as a true wide-tunnel performance catamaran with maximum running surface for the OAL (as defined by the class rules for “Super Stock” in the Offshore Powerboat Association). Porta had raced several canopy versions of the boat successfully in competition.
“The demand during a race on a 4½- mile offshore closed course requires the boat to accelerate from a 45-mph ‘rolling’ green flag start to nearly 120 mph on the back straightaway,” Porta said. “Then the boat must turn sharply around a corner buoy at 90–100 mph and accelerate through rough seas and the wash from other boats gathering up in the turns. Catamarans are tricky, as the inside hull can catch on a wake or rogue wave and cause a spinout or capsize.” Based on his experience, Porta had some modifications he thought would make the boat run faster and safer, but we wanted to test them efficiently and safely before suggesting any changes to the hull to improve the competition and recreational versions of the boat.
We first explored hydrodynamic model testing. Because this is a 100-mph boat, where aerodynamics is at least as important as how the hull runs on the water, we knew we would need to test in a facility like the 250+ mph wind tunnel at Embry-Riddle Aeronautical University. For a subsonic model and high-speed wind tunnel testing, the costs would be excessive and would take more than six months to complete, so we needed another option.
I had heard of aerodynamic and hydrodynamic testing of boats, cars, and airplanes being done on a computer. I also knew that aerospace, Formula One, and NASCAR were using computers for complex simulation, but I had never used it to model boat performance.
After an initial inquiry into the viability of computerized hydrodynamic modeling, a potential obstacle arose. Before we could input a boat hull design into a computer for simulation, we would need an extremely accurate and precise CAD file of the boat itself. To complete the project and predictively test the hull changes that Porta had in mind using computerized computational fluid dynamics (CFD) modeling, we first needed a very precise 3D scan of the hull and CAD file preparation based on the scan data. Each step would require outside expertise and contractors, but if we got it all right, we could perform the equivalent of the old bottom-up method of modifying lifting strakes and running surface without cutting a single stick, grinding an ounce of fiberglass, or running an actual boat at high speed (and risk) to determine the performance for each change.
3D Scanning: Metrology
Because our case-study catamaran was of a vintage where a relevant CAD file no longer existed, the first step was to 3D scan the boat we had. The technology has been used for years on propellers, engine parts, accessories, and many small parts when there is a need for high precision and replication. But I’d never seen it done on a large, complex shape like a boat.
Typical scanning equipment that can cope with a boat-sized object is either meant for construction applications, which typically have much lower accuracy and resolution over a much bigger volume, or it’s designed for much smaller objects, where the resolution is sufficient but accuracy degrades as the volume increases.
We chose FARO (originally Frasier and Raab Orthopedics) Technologies, a manufacturer of ultraprecise 3D-scanning equipment based in Lake Mary, Florida. Les Baker, the senior applications engineer at FARO, described their technology for 3D-scanning (metrology) a large object: “With the FARO Super 6DoF TrackArm 3D Scanning System, objects of this size can be scanned with high accuracy and superior resolution to capture critical forms with sufficient fidelity.”
FARO’s system basically marries two measurement devices: the laser tracker, designed for very accurate single-point measurements over distances up to more than 500‘ (152.4m); and the scan arm, designed for high resolution, contact measurement, and noncontact 3D scanning over a distance of up to 13‘ (3.96m).
The laser tracker tells the arm where it is in space and allows it to move around the boat and capture scans at a much higher accuracy than it could on its own. To keep measurements as accurate as possible, the system also monitors temperature, pressure, and humidity in the scanning environment to calculate air density and then compensate for diffraction of the laser beam.
Scanning requires an action similar to spray painting the boat with a 6“ (152mm) paint band, with each 6“ pass capturing 2,000 data points, 350 times per second. FARO’s software can reduce the density of the data based on the curvature of the surfaces scanned, thus preserving great detail around complex shapes.
Baker brought a FARO crew to scan the catamaran on-site at Porta Performance. We had to provide ample working room around the hull. Both sponsons were exposed for scanning from the top, bottom, sides, and in the tunnel between them. We positioned the hull above a concrete floor on solid stanchions that were absolutely rigid. The target accuracy requirement for the scan was ±0.004“ (0.1mm).
The laser tracker and scan arms also required solid mounting with the same rigidity as the boat. Each track arm had a reach of about 6.5‘ (2m), which translated to a 13‘ working span if fully extended fore-and-aft. With a 32‘ boat the arms had to be relocated multiple times. The key was to recalibrate and connect the scans with each new position after the arms were moved, allowing the FARO Laser Tracker to assemble the data from different arm locations into one seamless CAD file. The entire 3D scan with three technicians, over two days, yielded our desired results within the expected tolerance of 0.004“ or better.
Computer File Preparation
The data from FARO’s hull scan were incredibly accurate but not in a format that could be used for the CFD analysis we had in mind or for a CNC router if we had wanted to cut a direct mold of the scan. Our next step was file compilation and homologation, converting the scan into files that could be read by a 3D printer, 5-axis mill, or CFD program. For this project we turned to Dimensional Engineering Inc. (Houston, Texas) for computer file homologation to create the “watertight STL” (Standard Triangle Language) file we needed.
William Bonner, founder of Dimensional Engineering, said it’s important to start with good scan data. “The use of multiple devices is crucial. For example, using an arm scanner alone on an object 32‘ in length is nowhere near as accurate as combining it with a laser tracker,” he said. “And a handheld scanner without scale bars and photogrammetry can distort the object. These technologies used without one another can cause major issues in the integrity of the data.”
Once all the data are collected and accuracy is verified, it must be converted into a watertight STL model, which means any holes in the virtual structure must be filled. According to Bonner, most CAD software packages can automatically fill holes, but in some circumstances autofilling can take too much liberty and cause false undulations in the surface, so the results must be analyzed to ensure that the extrapolation of the filled hole is correct.
“The project for the 32‘ Doug Wright catamaran was to reverse-engineer the scan data into a CAD model,” Bonner said. This is typical if a new tool needs to be machined. If the scan data are not pristine enough, they will always have imperfections either from the scanning technology, the object itself, or a combination of both. If the model is not perfect, neither will be the results from CFD or a new tool cut from it. With the data, it took Bonner about 10 hours of computer work to create a contiguous and faired watertight STL file (a full computer model) of the 32‘ x 9‘ offshore high-performance catamaran suitable for printing an accurate 3D model or for CFD modeling.
Testing Hydrodynamics by CFD
With the CAD files in hand, we were ready for the CFD computer modeling of the boat’s performance and Porta’s proposed changes we’d originally set out to assess.
Using CFD to improve hydrodynamics is still far from standard in conventional boat design. Conversely, automotive designers live by it because of the fuel mileage requirements, and auto racing teams will put a new body or part into CFD to measure the drag imposed by a change. In our case, we were taking apart a catamaran that was designed for racing and putting it back together on the computer as a totally new boat, essentially an exercise in reverse engineering.
For our CFD work we turned to another expert contractor, Naethan Eagles, technical director at TotalSim US in Dublin, Ohio.
With his help we would look at the existing hull and create a digital twin engineered on the computer with the design speeds and conditions set. The parameters of this case study were: running at 100–130 mph in 3‘–5‘ (0.91m–1.52m) chop with little or no crosswind, while keeping the running surface as close to parallel with the surface of the water as possible. The CFD yielded some surprising outcomes.
In a compression tunnel catamaran, the airflow between the hulls is altered and affected by oncoming water conditions—hydrodynamics and aerodynamics collide. We found that the air/water mixture in the tunnel on our boat was reversing direction, creating a backflow on the top of the tunnel moving forward. At 100 mph the forward-moving backflow was leaking out the front of the tunnel and mixing with the airflow over the deck. The intended design was to release tunnel pressure only when the boat launched and left the water. The leaking backflow meant tunnel pressure was significantly reduced from its design parameter.
CFD revealed that the running surface was typical of a ventilated bottom. Hull ventilation was normal and performed as designed, but the lifting strakes created sufficient hydrodynamic drag. We saw that the combination of lifting strakes and chine angle scrubbed off 3–5 mph at a design speed of 130 mph. The combination of confused tunnel flow and disturbed water on the running surface created drag and “dirty” water leaving the transom. Having confirmed what elements were slowing the boat, Porta could work with Eagles to make changes in the CAD files and test performance in CFD as he searched for remedies.
With this design project, we put the days of risky and expensive on-the-water trial and error behind us. Improved performance, safety, and speed were the desired outcomes from the CFD analysis. We attained them efficiently and safely by using CFD to confirm and build on Porta’s real-world experience running the boat in competition.
Many boatbuilders have a difficult time abandoning what they have been building and selling for years. We found the same to be true in this case. Our study results prompted us to make beneficial changes that yielded championship-winning results. Later findings from the case study have informed more recent performance-boat builds, and some have been integrated into production. Often, seeing is believing, and winning races often becomes a convincing tool.
Through our testing we learned lessons for current and future applications to new designs, such as the integration of hydrodynamic and aerodynamic forces, and lifting strakes causing drag. While the boat designer and builder should never consider technology a replacement for experience, it was clear to us that scanning and CFD tools represent huge leaps forward for the industry. As a practical matter, future modifications to existing models can be efficiently tested on the computer before any tooling is cut or a new mold is created.
Arguably, we have come full circle to a point where recreational boats can again be designed, built, and sold from the bottom-up based on changes to the running surfaces that additional power options require. Safe, efficient, and brisk-handling vessels are essential to the discerning boat buyer. Our experience has convinced me that computer modeling applied to somewhat unconventional design ideas will allow high-speed boat design to move forward in directions that were simply not possible before.
In Part 2 we’ll apply CFD aerodynamic analysis to high-speed boat design refinement and performance analysis.
About the Author: Clay Ratcliffe is a 45+ year veteran of high-performance industry technical design and marketing. After converting from auto to offshore powerboat racing, he has been a catalyst for boatbuilders to bring well established design methods into performance-boat manufacturing. He was project lead on this case study.