Boats

The Fine Art of Making Propellers for Outboards

Yamaha reveals the basics of building stainless-steel props for outboard engines.

By Chris Woodward

Updated: April 14, 2016

Yamaha Prop Making 101 Courtesy Manufacturer

Since buying Precision Propeller, Inc., in 2008, Yamaha has made nearly all of its stainless-steel props at the PPI factory in Indianapolis. Here’s the story of how a wax model becomes a tough, stainless-steel tool that drives your boat. Yamaha also makes aluminum props, but its steel products are the go-to accessory for offshore fishing boats.

The first step in the prop-making process is creating a wax mold — an exact replica of what the finished prop will look like in steel. Wax is melted and injected into dies that are designed on computers. This pattern will be the basis around which a single-use mold is built. It must be exact as it will determine what the steel prop will look like. Courtesy Yamaha

Here, wax props hang, awaiting an important bath. Atop each prop is a pour cup, a specialized wax cup that is used for pouring in the molten hot steel. The barrels are washing stations. Each prop has to go through three dunkings so that there is no oil, residue or dirt. Courtesy Yamaha

Yamaha outboard boat propeller being built

The propeller is coated with a slurry of sorts, to begin building a shell around the wax. Courtesy Yamaha

Multiple layers are applied to build up the shell. Each layer has to be correctly and completely dried before the next layer can be applied. Courtesy Yamaha

There can be no water in this shell. If there is, the shell can crack when the wax is melted out. It takes a lot of time to make the mold for one prop, and then the molds are not reusable. Courtesy Yamaha

The prop molds are put into an autoclave, and using temperature and pressure, the machine rapidly removes the wax. The wax is then sent to a wax supplier to be recycled for other uses. Courtesy Yamaha

A man in a moon suit puts the props into a very large oven. The shells are heated to bake out any possible remaining wax and to add strength to the shell. Courtesy Yamaha

A prop mold getting warmed up for action. Courtesy Yamaha

A worker holds one of the virgin stainless-steel ingots that will be melted in an electric crucible. Courtesy Yamaha

Melting a virgin stainless-steel ingot. Courtesy Yamaha

Here, a worker is dipping out any slag or impurities in the molten steel. Courtesy Yamaha

Two workers control a hand-operated crucible of molten steel, used for making smaller props. Here they have just finished pouring the steel into the prop mold. Courtesy Yamaha

The prop mold glows red hot. Courtesy Yamaha

For bigger props, a larger crucible is used. To pour, one worker tips the fiery box. You can see the molten steel glowing through the lower part of the prop. Courtesy Yamaha

A closer view of the automated crucible and the glowing prop. Safe to say: If it’s orange, don’t touch it! Courtesy Yamaha

After the mold is poured, it’s moved to a cooling rack. After it cools, it goes into a proprietary process by which the shell is completely removed, and the prop is prepped for a trip through the grind shop. Courtesy Yamaha

Next, it’s off to the grinding shop, where the props are prepared for polishing. This is a multi-station, multi-person process using various grits to carefully and precisely grind the multiple prop surfaces. Courtesy Yamaha

Along the way, the prop is balanced to make sure that the grinding is even on all surfaces — that not too much is taken off any particular side. Courtesy Yamaha

After grinding, the props enter a proprietary polishing process, which involves using large machines that utilize progressively finer media on the propeller surfaces to produce a lustrous shine. Afterward, the appropriate edges are put on the prop, and then, depending on what type of hub is used, it’s either pressed in at the factory or installed at a dealership. Courtesy Yamaha

Yamaha outboard boat propeller being polished

The props are polished once more by hand, as they receive a final inspection. Then, they’re boxed and shipped to dealers. Courtesy Yamaha

More: Boat Engines, fishing boats, Yamaha

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