Outboard Motor Making Knocking Sound

This past weekend I while I was waiting to launch my boat, I could hear a loud knocking noise coming from someone else’s boat that was tied next to the dock. Several people were standing around, trying to figure out what was causing the knocking noise.

Before anyone could figure out what was causing it, the owner loaded up his boat and left. Once I got home, I tried to look up what might have been causing the problem, and I found two possible reasons for a knocking noise coming from an outboard engine.

The first possibility is that the impellor housing had corroded or cracked. When that happens, it could cause a knocking noise as the propeller spins. You would be able to fix the problem and stop the noise by replacing the housing and impellor.

Another possible reason for a knocking sound could be because of missing teeth on both the drive bevel gear and the prop shaft bevel gear. As the gears turn, the two gears would align with one another where the teeth are missing, which could cause the two to slip and make a repetitive knocking noise.

There is one more thing that could cause a knocking noise, and that is if the pistons are loose or don’t sit in there snuggly. To check for this, you can remove the housing cover on the engine and try to wiggle the piston in place. If it can move back and forth, then this could be causing a knock.

One thing is for sure; the knocking sound will not go away on its own. Being able to narrow down possible causes will help in eliminating the knock.

While researching this problem, I found out that there are other types of noises that your outboard motor could make and how to go about making them quieter.

How to Resolve And Quiet AnvOutboard Motor That is Making Noise

You might have experienced it—one minute your outboard motor is working fine, then all of a sudden it begins to make a knocking or rattling sound emanating from the motor. An outboard motor noise often comes from the housing of the engine. Such a sound can take a toll on your boat performance, and not to mention a little bit annoying.

Fortunately, various solutions can help you reduce the knocking noise. Among the solutions include:

  • Line the inner side of the engine box

    Using a high-density cushion material like acoustic foam, you can reduce the sound coming from your outboard motor engine by lining the inner side of the engine box.

    You can find the foam from your local marine and automotive stores. When buying one, ensure that it is designed specifically for use in the gas engine housing. The trick works pretty well but only if you are sure that the racketing is not a problem from your boat.
  • Check the exhaust system

    In other cases, a loud engine can signal a malfunctioning exhaust. The exhaust system of outboard motors usually come with a vertical passage that runs through the structure of the engine to minimize noise by blowing it into the water.

    Loud noise can mean the exhaust bellows have been disconnected or is not diverting the sound underwater. Try checking your motors exhaust system to see if this will help remedy the problem.  
  • Ensure that your boat RPM is within the required range

    If you use the wrong prop on your boat, then it will mean the engine will be revving at a way higher rate than it is required. What you get as your boat’s Wide Open Throttle (WOT) RPM along with the propeller pitch and size are the most critical aspects to consider.

    Your boat’s manufacturer often determines the WOT RMP of your boat, and in case yours is not within the required range, then it can affect your boat’s performance. Any engine that doesn’t reach the standard WOT is considered over-propped and can result in lugging.

    That could also put excessive load on the crankshaft, bearings, and pistons. Depending on the activity you do, it pays to ensure that your engine is correctly propped.

    In case your boat’s engine is revving at 4800 RPM or even higher, then you might want to replace it with one with a higher pitch.
  • Lower the engine to reduce exposure

    If the prop is exposed, then the noise could be loud while operating at high speed. You may need to cut down the transom at the back of your boat.

    It is easy to do, and you can begin by using the mobile engine lifting and hoist rings in case of a large outboard motor. 
  • Use a shroud in the engine

    Some manufacturers have come up with cowling that helps to minimize engine noise. Most of them contain foaming that helps to dampen the noise.

    You may remove the foam then use an improved sound dampener like those in-car audio stores. 


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