What is Cutlass Bearing?
A cutlass bearing is a water-lubricated bearing that supports the propeller shaft where it passes through the hull. It typically sits inside a bronze or stainless steel strut for exposed shafts or within the stern tube for inboard shafts.
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What it is
A cutlass bearing is a water-lubricated bearing that supports the propeller shaft where it passes through the hull. It typically sits inside a bronze or stainless steel strut for exposed shafts or within the stern tube for inboard shafts. The bearing has a metal outer shell with a rubber or composite inner sleeve featuring grooves that allow seawater to flow through, providing lubrication and cooling while the shaft rotates.
What it does
The cutlass bearing keeps the propeller shaft aligned while allowing smooth rotation, preventing wobble that can cause vibration, noise, and premature wear on the shaft, couplings, and transmission. Water flows through the bearing grooves, providing lubrication and cooling, while flushing out sand, grit, and other debris that could accelerate wear. When functioning properly, the bearing allows the shaft to spin freely with minimal friction while maintaining tight tolerances, usually around 0.001 to 0.003 inches, to prevent excessive movement that could damage surrounding components.
Why it matters
A worn cutlass bearing creates vibration that can damage the transmission, engine mounts, and alignment and may even loosen through-hull fittings or stress fiberglass, increasing the risk of flooding. Excessive wear allows the shaft to wobble, accelerating damage to shaft seals or packing and creating vibrations that can fatigue or fracture the propeller shaft. In severe cases, the bearing can seize while underway, causing an immediate loss of propulsion and potential damage to the shaft or transmission, leaving the boat unable to maneuver in critical situations such as docking or navigating tight channels.
General Maintenance
Inspect the cutlass bearing and strut annually during haulout by grasping the propeller and gently moving the shaft up, down, and side-to-side. Any movement greater than 1/16 inch indicates a worn bearing that should be replaced. Annually inspect the strut backing plate inside the vessel for water ingress and corrosion. The strut and backing plate should be tight, any movement should be addressed immediately. Listen for unusual vibration or rumbling noises during engine operation, especially when shifting between forward and reverse, as these can signal early bearing wear. Replace the bearing every 5-7 years as preventive maintenance, even if it appears sound, because rubber deteriorates over time. Check strut, shaft and engine alignment during bearing replacement. After any grounding or propeller strike, verify that water flows freely through the bearing grooves, as collapsed grooves can block lubrication, causing the bearing to overheat and fail rapidly.
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