Teignbridge Propellers, near the seaside town of Torquay on England’s south coast, has been designing and manufacturing propellers and stern gear for half a century. Having started out serving commercial ships and workboats, Teignbridge also operates at a higher level with military vessels and large yachts. It’s one of few such companies with its own foundry, and its manufacturing takes place in a historic brick building built by Isambard Kingdom Brunel, the great civil and mechanical engineer. Aptly, Brunel also designed the SS Great Britain, the first propeller-driven, iron-hulled ship to cross the Atlantic, in 1845.

Today, marine propellers come in every shape and size, but they all convert rotational motion into thrust by producing a pressure difference in the surrounding water. Historically, their design has been driven by trial and error, and has been viewed as more art than science. Modern computing power allows engineers to predict a propeller’s performance before its even gets wet. This performance revolves around mechanical strength, hydrodynamic efficiency, noise and vibration, and cavitation.

These four criteria often pull in opposing directions, so the challenge is to find the best balance according to the customer’s requirements.

“If you want low noise and vibration, for example, you may have to sacrifice some efficiency, because to have a quieter propeller you need a bigger blade area ratio, which increases frictional drag across its surface and reduces efficiency, and so on,” says Saeed Javdani, head of innovation and technology at Teignbridge.

It’s a complex process to drive performance in a particular direction while operating in the swirling wake of a ship or boat. Teignbridge utilizes powerful simulation tools to understand the interaction between propellers and other appendages, and the fluid flowing around their surfaces. Its suite of software allows for automated, AI-driven optimization by coupling performance feedback to changes in the geometry parameters.

The design of propellers may be computer driven, but hand techniques are still a fundamental part of their manufacture. Teignbridge works mostly with aluminum bronze, favored for its high fatigue and corrosion resistance, using the sand-casting technique. The sand to make the molds is sourced from a single quarry halfway across the country because it has the perfect level of acidity and varied grain size. (If the grains are too uniform and tightly packed, the gases cannot escape from the mold.) 

To ensure that the sand holds its shape, it is mixed with a binder resin that a catalyst activates just before use. Adding just the right amount of binder and catalyst comes down to experience. Once the mixture is ready, the mold maker has to work quickly to ensure that the sand doesn’t set too early.

Mold cavities are formed by individual spindle “patterns,” which are wooden replicas of the final propeller blades fashioned by hand in various shapes and sizes. Teignbridge has a storeroom containing hundreds of these green- and red-colored patterns that can provide more than 16,000 design combinations by varying the size, number and pitch of the blades.

Patterns are slightly larger than the final article to allow for contraction as the metal solidifies inside the mold, usually being built up into a propeller shape one blade at a time. Shaping the patterns is time-consuming and costly, so full patterns of complete propellers—usually made of aluminum—are more expensive than individual ones, but the molding time is shorter, and they are durable. Teignbridge also makes patterns quickly and cheaply from machined polystyrene, but these are less sturdy and more suitable for urgent one-offs.

“The ingots of aluminum bronze begin to melt at around 1,100 degrees Celsius [more than 2,000 degrees Fahrenheit] but we heat them to a higher temperature to ensure the metal remains molten throughout the casting process,” says metallurgist Ryan Leese, foundry manager at Teignbridge. “The metal has a long way to travel through the runner and filter system before entering the bottom of the mold. That takes time, and all the while it’s losing heat.”

The worst that can happen is that the molten metal starts to solidify and stops moving. Adding heat gives it the “legs” to make sure it reaches the tips of the blades. How much more heat depends on the size of the propeller and is based on the foundryman’s experience.

As the metal turns from liquid to solid, it shrinks by 4 percent in volume, so a reservoir on top of the mold is capped with a “feeder head” containing exothermic powder that generates its own heat. This cap is changed as many as a dozen times to keep feeding the casting with liquid metal and to avoid deformation through shrinkage.

After the casting has been removed from the mold, the finishing process starts with fettling, which involves using a grinder to remove any unwanted metal and rough patches. The final polishing is done by hand to create the trademark scallops or streaks that every propeller manufacturer adds to its products.

How long will these tasks continue to be done by hand? That remains to be seen. Teignbridge recently invested in a robot—nicknamed “Dudley” after the foundry’s longest-serving employee—that’s able to perform some of the molding processes. 

For more information: teignbridge.co.uk 

This article was originally published in the Fall 2024 issue.