Polished to a mirror finish and carefully engraved, a gleaming stainless steel propeller is like a piece of lovely jewelry for your boat. When exposed, the blades can dazzle other boaters like Harry Winston facets.
The Investment Casting Process Goes Modern
The jewelry analogy is especially apt considering that, with a few exceptions, the modern stainless steel propeller is created using the investment casting process, a metallurgical art that dates back more than 4,000 years. Also known as the lost wax process, artists in Mesopotamia, China, and Aztec cultures used investment casting to create artwork and jewelry, usually forming and carving a pattern from beeswax. Clay was used to invest, or cover, the pattern with a shell. Molten copper, bronze or gold was then poured through a wax channel into the shell, displacing the wax. Once cooled, the shell was knocked away to reveal a perfect and often very intricate replica of the original wax pattern.
This same process, though influenced by modern technology, is used to create stainless steel marine propellers. We recently toured the Yamaha Precision Propeller Industries (YPPI) facility in Greenfield, Indiana, one of the newest investment casting foundries in North America. The 55,000-square-foot plant began operation in March, 2021 and all Yamaha polished stainless steel props are cast here. Those propellers are finished at a YPPI facility in nearby Indianapolis. Yamaha also casts, but does not finish, loop-blade propellers for Sharrow Marine. With a combined 200 employees, the YPPI facilities in Greenfield and Indianapolis operate three shifts five days a week.
Improved Quality, Increased Safety, and Faster Production
Like any modern manufacturer, Yamaha sought to utilize automation to improve quality, accelerate production and enhance worker comfort and safety when it designed the Greenfield facility. The rate of production has certainly increased, from 60,000 props per year before the Greenfield shop opened to more than 100,000 pieces today, according to Yamaha. This is good news for boaters waiting for a new propeller, which have been in short supply industry-wide for several years. Automation also results in tighter manufacturing tolerances, which reduces the amount of finish work each prop requires, and produces very repeatable performance on the water.
“The ultimate goal is that automation delivers improved consistency in blade geometry from prop to prop,” said Greenfield plant manager Batuhan Ak. “When a Yamaha customer replaces their original prop with a new prop, or installs new props on a multi-engine boat, the performance of each prop should be nearly identical.”
Today, in-plant automation is focused primarily on the most-popular Yamaha propeller lines, the Saltwater Series II and Reliance, and on two aspects of the process–coating of the wax pattern with refractory material and the actual pouring of molten stainless steel.
Steps to Quality
For these high-volume props, the wax pattern is formed in one piece within a steel die. Five patterns are then placed on a fixture and transported by conveyor to a set of robotic cells for the application of refractory material. The first prime coating, which contacts the stainless steel, is fine zircon sand, which creates a smooth finish and preserves details in the pattern, such as the “Saltwater II” logo on the prop barrel.
A robotic arm first dips the five patterns in a zircon slurry and then places them in a cabinet containing the rotating drum of a rainfall sander that sprinkles zircon sand that clings to the slurry. Each rack is next placed by the robotic arm on a conveyor and carried into an adjacent drying room, maintained at 72 degrees with very low humidity. A series of robotic cells coats each pattern five times, with a drying period of one hour to 12 hours between each coat.
Subsequent coatings are coarser silica sand, applied in a fluidized bed, where circulating air suspends the gritty refractory material in a liquid-like state. After each step the robot arm lifts and rotates the rack of molds in a very precise way to drain away excess material. The entire process takes about 24 hours, and when complete the refractory material looks like a coating of white stucco.
Thanks to the robotics and other automation, only three people each shift are required to manage the entire dipping process. Three people also previously handled this task in the Indianapolis facility, but the patterns were hand dipped in slurry and sand. Robotics has made this process much more efficient.
Next the coated patterns are placed in a pressurized steam vessel that causes most of the wax to melt and drain away from the refractory material. The molds then move on a conveyor through an 1,800-degree oven that removes any moisture, melts away last traces of wax and heats the shell of refractory material cherry red. An automated handler moves the hot shell into position for the pour. A tilting crucible furnace holds 300 pounds of 3,000-degree molten stainless steel, and that vessel is supplied by a core-less induction furnace that can hold 4,000 pounds of molten stainless.
The large furnace constantly supplies the crucible furnace, so the pouring step never needs to slow down. Automation controls the crucible so the precise amount of steel is poured for each prop size. The shell then moves down the line and is covered with a lid to control the rate of cooling.
To the side of this operation a team dressed in silver heat-retardant suits is pouring low-volume props by hand—manually removing shells from the oven before two people tilt a ladle and pour the molten steel into the shell, which is then moved by hand to the cooling area. This is hot and somewhat dangerous work. Watch for a while and it’s easy to understand why Yamaha has automated this step for high-volume models, and hopes to automate all pouring in the future. There are also plans to automate other aspects of prop production, including the washing of the wax patterns—now handled one at a time—and the process that knocks the shell off of the casting.
According to Yamaha, in the next three to five years the Greenfield building will be expanded to accommodate a new automated grinding and vibratory polishing process. Today the grinding phase, which can be dirty, nasty work, is done by hand in Indianapolis. The new foundry is ISO 9000 certified, and Yamaha anticipates future opportunities to take on non-marine contract casting such as medical devices which would keep the foundry busy as the marine market fluctuates, because there is a shortage of investment casting production capacity in North America.
But for now, the focus is on cranking out as many pieces of gleaming Yamaha boat jewelry as possible.