Until very recently, designing a marine propulsion system for a ship had been an analog process. In an oversimplification: a hull design was sketched out to meet the needs of the application and a power versus speed curve informed the selection of a diesel engine of appropriate horsepower. The naval architect would “assemble all the pieces” on a drawing and calculated engineering ensured the vessel had the right hull design, engine torque and maneuverability for the task and environment at hand. It was a tried and true methodology; there are still many ships afloat today that were made by this process.

Yet, when it came to propeller design — specifically in areas such as diameter, number of blades, blade pitch and more — the design process was as much art as science. The manufacturing processes for making propellers were also imprecise, which meant that even “identical” ships might behave differently. Fuel consumption and even difficulty holding to a specific heading could be the result.

Source: Denys Yelmanov/Adobe Stock

Prop design 70 years ago

The propeller usually started out as a near net shape casting. Getting from this raw part to a perfectly shaped prop usually took a few hundred hours of grinding by experienced machinists with heavy, hand-held, pneumatic grinders. The finished result was a smooth hydrodynamic shape — very much a skilled artisan task. Usually, wooden templates were used as a model to guide the machinists to smooth the propeller blades so that the thrust would be generated as evenly as possible.

Often propellers had an odd number of blades. This was a very practical consideration. Each time the face of the blade rotated past the stern, the flow around that blade would be disrupted. With an even number of blades, this interruption would take place on both the top and bottom blade simultaneously. This in turn created a pulse in the power train along with stress on the blade root (where the blade meets the hub). An odd number of blades reduced this effect. Over time, either three or five blades became preferred. Three blades meant more speed at the top end and five blades meant less slip during slower movement when fully loaded. In this context slip refers to how much thrust the propeller could develop during a rotation.

Commercial ships were the impetus for improving propulsion systems and hull design. This led to large diesel engines, chosen for their fuel efficiency and high torque. In the quest for speed, ship propellers were driven to higher RPM and higher torque. This resulted in an undesirable effect, eventually diagnosed as cavitation.

Cavitation is the condition where the low-pressure side of the propeller blade creates a fluid vacuum that is filled with gas currently dissolved in the water. When this bubble violently collapses from the fluid pressure, it eats away at the edge of the blade, turning it into a sponge-like texture. So, the lesson here is to be careful about speed just for speed’s sake.

There is a counterintuitive counter example for high-speed ships. In that case, it can be smarter to design the propeller to be fully cavitating. In this case the cavitation bubble covers the whole backside of the propeller and collapses downstream of the prop without scouring the propeller. This type of design is used with ships, like navy ships, that are expected to spend most of their operating time near their full design speed. This is only really possible to do reliably with modern design and fabricating techniques. It's a fine line to maximize thrust without damaging the propeller due to cavitation.

Engine and rudder

Now that we’ve established the baseline for propulsion design, it’s worth a look at the main driver, often a diesel engine. Diesel engines are much preferred due to their high torque and better fuel economy. About 90% of today’s modern cargo vessels use large bore, slow speed diesel engines. Propellers are usually driven directly by a shaft coming off the aft end of the engine. Most commercial propellers and diesel engines converged on an operating speed of around 100 RPM and that became the de facto standard for the diesel driving the prop. These are big heavy ponderous machines with large piston/crankshafts as much as 80 ft long or more.

The last element of a vessel’s locomotion systems is the rudder, whose role is to put unequal drag on the ship to adjust the direction of travel. However, as the ship slows down, the rudder no longer has enough fluid running past it to generate the required drag to direct the vessel. That’s why large ships need to be tethered to tug boats to help them get docked. There is just no guarantee that the ship can steer itself safely all the way up to the dock.

The digital revolution arrives

Two of the biggest impacts in marine engineering came from computer aided design (CAD) combined with computer aided machining (CAM). This gave propeller shops a means to exactly reproduce specific designs, down to the millimeter, and made it a lot easier to experiment with different scaled model designs. This also helped to diversify the offerings from different manufacturers and apply these tools to different types of ships: fishing boats, workboats, tugs and ferries as well as custom ships for research.

At the same time, the quality and uniformity of diesel engines were improving with these new machining techniques. This allowed for better matching between propellers and engines. This also could be applied to steam turbines and even diesel electric engines, though these were losing favor as the prime mover. This was no overnight transformation, taking a generation to take hold. The changes were driven by the suitability, and the acceptance of, these new digital tools and how well they were adapted to the specifics of ship design.

The ultimate tool for fluid flow, especially appropriate to propellers, is computational fluid dynamics (CFD). This is the use of Finite Element Analysis computational techniques applied to fluids. Finally, designers could build and run simulated outcomes for situations where the water flow is highly disrupted. Examples include cavitation, running in reverse, interactions of the streamline at the stern where the streamlines dip down into the propeller blade entry, and blade stresses during a storm just as a few examples. During this time propellers generally made gains of 5% to 6% in their efficiency. CFD tools are not restricted to use with propellers only. They can also be used to optimize the hull shape as well. Taking into account both propeller and hull redesigns, the use of CFD has helped increase the overall efficiency of commercial significantly, by as much as 15%.

Azimuth designs

These tools (CAD, CAM and CFD) allowed designers a great deal of latitude for experimentation. One great example is rudders. Let’s face it, rudders are awkward, heavy, crude devices, and limited in their range of use. What if they could be removed altogether? This question led to the development of azimuth or “Z” drives, which pivot the whole propeller assembly through a vertical axis. The thrust could be easily diverted through 360°, making for easy and quick “vector” steering.

Though these designs had been around for a while, digital tools allowed for rapid improvements to those designs. Like most designs, when first developed, they were heavy and not efficient. Application of the three design tools made it possible to produce more compact designs that were more efficient and which could be applied to very specific types of boats. These systems could use multiple, smaller vector controlled thrusters, sometimes using electric motors and with gear sets for better load matching. These techniques were especially useful for workboats, tugs, cruise ships and ferries. only real down side was that with the presence of a gear box down into the water, it could be limited by the available draft in shallow waters.

It had long been known that encasing a propeller in a tube could streamline the flow and reduce the fluid thrust losses within the propeller area. This improvement, along with optimized CFD, resulted in a variety of enclosed (ducted) propellers both with and without Z-Drives. One unique design that appeared during this time was an enclosed five-bladed propeller with a three bladed aligned rudder. This was optimized for use in tugs and workboats and was very successful due to its shallow draft, highly maneuverable design and high thrust. Designers are still experimenting with these new design toys: push-pull propeller designs, “tunnel” designs, vertical axis thruster designs and so on. At least now they are starting with a full basket of potential solutions and simulations that have been well tested.

Summary

Ever since the very first ships were built to haul cargo, there has been a constant push to hit the right balance between cargo capacity and speed. Fortunes were made (or lost) investing in sailing ships. When diesel engines replaced sailcloth and created new, reliable trade routes based on machinery it was a watershed event. This demanded closer attention to design and the development of new tools. Up until now, designs had been trial and error involving a lot of testing, until the digital generation arrived and brought the design tools of CAD, CAM and CFD.

Basically, within the span of a single generation, a hundred years or more of wind-driven sailing ships has been completely transformed. Ships can now be optimized from the start; complex fluid dynamics, propeller designs and the design of everything from mammoth cargo ships, to hard working tugboats in busy ports are better designed and more efficient than once thought possible.