Prop Nuts & Thrust Washers

What Are Prop Nuts & Thrust Washers?

Beneath the waterline, your boat's propeller relies on a small but critical assembly of hardware to stay secure, transfer power efficiently, and protect your lower unit from costly damage. Prop nuts and thrust washers are the unsung heroes of that assembly — simple components that play an outsized role in keeping your drivetrain safe on every run.

Prop nuts and thrust washers work together as part of the propeller mounting system on outboard motors, stern drives, and inboard boats. Acme Automotive offers prop hardware designed to complement inboard propeller installations, making them a go-to source for boaters who demand precision-matched components.

How Thrust Washers Work

The thrust washer sits on the propshaft, positioned between the lower unit and the propeller itself. Its job is deceptively important: it prevents the propeller from sliding forward along the shaft while running in forward gear, which would otherwise cause the rotating prop to contact and damage the gear housing. The thrust washer also helps keep the propeller centered on the shaft, especially under high-torque conditions that can stress the inner prop hub.

Many thrust washers are tapered or splined to match the geometry of the propshaft, locking them in position so they can reliably absorb and transfer thrust loads. Tab washers — a close relative — serve a different but equally vital function: they key into the spacer behind the prop and prevent the prop nut itself from rotating loose over time.

The Role of Prop Nuts

The prop nut's primary purpose is to hold the propeller firmly seated against the shaft taper. In normal forward operation, engine thrust actually keeps the propeller pressed onto the shaft, so the nut's main job is to prevent the prop from backing off when the engine is in reverse or during rapid throttle changes. Many modern applications use a two-nut setup — a thinner jam nut installed first against the prop hub, followed by a thicker nut — which, when tightened in sequence, locks both nuts together against the shaft threads for a vibration-resistant hold.

Common prop nut styles include standard hex nuts, nylon lock (nylock) nuts, and castle nuts paired with a cotter pin. Each design has specific applications depending on the engine manufacturer, shaft size, and intended use. Always consult your engine's owner's manual or manufacturer specifications to confirm the correct nut style, thread pitch, and torque value for your setup.

Shopping Tips for Prop Nuts & Thrust Washers

• Match to your engine and drive system: Prop hardware is highly application-specific. Outboard, stern drive, and inboard boats each use different hardware profiles.
• Check shaft size: Shaft diameter (commonly 1", 1-1/8", or 1-1/4" on inboard boats) determines the correct nut thread and washer bore.
• Inspect hardware at every prop change: When removing a propeller, always check that the thrust washer hasn't remained stuck inside the old prop. Also inspect for fishing line wrapped around the shaft, which can damage propshaft seals and contaminate gear oil.
• Replace single-use fasteners: Nylon lock nuts are designed for one-time use. Once removed, the nylon locking feature is compromised — always install a fresh nylock nut rather than re-using the old one.
• Material matters: In a saltwater or mixed-metal environment, galvanic corrosion is a real concern. Look for brass or stainless steel hardware rated for marine use.

Top Brands in Prop Nuts & Thrust Washers

Acme Automotive brings the same commitment to precision found in their renowned inboard propeller lineup to prop hardware and accessories. Their components are engineered to work seamlessly with inboard propeller installations on popular wake, ski, and recreational boats.