Propeller Shaft Vibration – Our Story

What's the difference between 0.05 mm and 0.07 mm? In our case, about $2,000.

We own an older boat and recently began noticing a vibration at certain cruising speeds. At first it wasn't dramatic, but it was enough that we knew something wasn't right.

While inspecting the engine room, John noticed saltwater spraying around the shaft area whenever we increased speed. That was the clue we needed to investigate further.

Our diesel mechanics removed the propeller shaft, propeller, and coupling flange and sent everything to a propeller shop for inspection. The verdict? The shaft measured 0.07 mm of runout. While that sounds like an incredibly small amount, many manufacturers, including Volvo Penta, recommend a maximum runout of 0.05 mm (0.002"). That extra 0.02 mm was enough to create noticeable vibration.

After the shaft was straightened, everything was reinstalled and the vibration disappeared.

We can go fast again!

How is a propeller shaft checked?

One of the simplest ways to check for a bent shaft is with a dial indicator, which measures shaft runout as the shaft is slowly rotated by hand.

The basic process is:

• Secure the dial indicator to a solid mounting point.

• Position the indicator tip against a clean section of the shaft.

• Zero the gauge.

• Slowly rotate the shaft one complete revolution.

• The difference between the highest and lowest readings is the shaft's runout.

For most recreational boats, 0.002 inches (0.05 mm) is generally considered the maximum acceptable runout. Anything beyond that can create vibration and should be evaluated for straightening or replacement.

(Credit to JLM Marine for their excellent explanation of shaft runout and vibration diagnosis.)

Our takeaway

Boat vibrations rarely fix themselves. They often become more expensive over time.

In our case, ignoring the vibration would have meant continued saltwater spraying onto the port engine—not something you want happening in your engine room. Left unchecked, shaft vibration can also contribute to premature wear of cutlass bearings, engine mounts, shaft seals, and transmission couplings.

If your boat develops a new vibration after striking debris or if the vibration gradually gets worse, it's worth having the drivetrain inspected before a minor problem turns into a major repair.