11 Signs & Symptoms of Bad Bellows In A Boat

Bellows aren’t what you think they do. They don’t move air when it comes to boats.

Bellows in boats are pliable nozzles protecting the engine from water. This rubber-made tight seal is designed to protect some of the parts of the engine from water.

This is particularly useful as the watercrafts use water to cool the engine. Bellows work in compartmentalizing parts that need protection. They also function in providing flexibility.

In this article, I will talk about the symptoms of bad bellows. So, you will be able to save your boat before it’s too late.

What Sign You Should Look for To Detect Bad Bellows?

The typical sign of bad bellows is water collecting inside the cabin. With that, loud noise may be heard from the engine while driving.

What Are the Symptoms of Bad Bellows?

Symptoms of bad bellows are given below:

1. Water Leaking In the Boat

When you pull out the hatch, you will see water in the bilge. The water may be coming from some kind of crack or damage. Check the surrounding, for example, the bow wall of the bilge. Water may be leaking from there.

Water especially enters the hull if you take it for sailing. Clean the bilge completely, take the boat in the water and open the engine hatch. If you see water, this may be due to bad bellows. 

2. Leakage also occurs due to a loose gasket. 

In the case of the U-joint bellow, water is seen in the gear lube. The  U-joint usually splits in folds inside the stern drives and leaks water into the bilge.

3. Engine Stops

The engine may hesitate in few cases and hit hard. It depends on the amount of water damage it endured.

Running the boat may become a nuance.

4. Loud Noise

If your bellow is bad, you may hear noises coming from the back of the boat. This is especially apparent when U-joint bellows go bad. In this case, noises become louder when the boat is turning hard.

Loud noises from the exhaust are another sign. The sound may be loud or like hissing or tapping.

5. Overheating

As bellows are used to compartmentalize the engine, a problem in them allows water to the wrong places. As a result, the cooling system fails. This causes overheating of the engine.

6. Rusting

When the bellows get damaged, water leaks. The presence of moisture and air creates rusting on the iron parts.

One such part is the bearing. In case of a leak, water seeps through the bearing grease and causes rusting.

Water also soaks the output shaft and the gimbal bearing.

7. Dry Rotting

Similar to rusting, the water affects other parts of the boat too. Check the boot.

If you find signs for dry rotting, consider changing your bellows.

There’s also wetness below the U-Joint bellow.

8. Boat Sinks

You may have docked the boat for the day, left it in the evening throughout the night. In the morning you may find the boat inches down in the water.

Water will seep through the cracks in the boot. It enters the bilge and collects there. Meanwhile, it floods the gimbal shaft while doing that.

This is a big sign that your bellows need changing as soon as possible.

9. Decreased Engine Efficiency

You may notice your engine not putting up. Reduced acceleration is a sign. The problem with exhaust joints results in a lack of RPMs.

10. Unpleasant Odors

Bad exhaust bellows usually cause leakages. As a result, exhaust leaks can occur and fuel can seep through. It is the smell of the escaping fuel that you smell in the exhaust chambers.

11. Exhaust Pipe Comes Off

If there is damage to the bellows around the exhaust, the exhaust pipe usually hangs down.

It looks like the pipe is drooping and comes off the prop area.

How Should You React If You Detect Bad Bellows?

The first thing to do when you detect bad bellows is to replace them.

Consult the manufacturer’s advice before replacing the bellows. Follow them when replacing the bellows.

Usually, a professional mechanic is needed to replace the bellow. But let me give you some guidelines in case you choose to do it yourself.

Step-1: Remove the Exhaust Bellows

Remove the bell housing with a drive unit. It should be blocked in a full up position. Loosen the aft clamp underneath.

A quarter-inch extension should be inserted with a socket wrench while doing this.

Insert it through the access hole on the port side of the transom housing. Now slitting bellows with a sharp knife, remove them.

Step-2: Install New Bellows

On clean mounting bosses, apply adhesive. Keep the clamp screw on top of the bellows. A hose clamp over the end of the bellows should be kept for this.

Now, push the bellows by sliding the ground clip to the lower edge of the bellows.

Step-3: Aft Mount

Put the bellows on aft mount position. Apply adhesive and install a clamp on the aft side of the bellows. Fit the ground clip under the bellows and slide the bellow expander tool.

When the retaining pin holes are aligning, put the pin through the holes.

Step-4: Tighten the Aft Clamp

After tightening the clamp to the specs, reinstall the shift shaft. Shift crank and the Teflon washer should also be shifted.

Make sure you are replacing all the bellows at a time.

If the bellows have already caused water damage, then fix it. This can include lubricating the gear, bearing, etc. Replace all the rusted parts. If you find rotting, replace those parts too, or at least furnish them again.

After you have fixed the bellows, make sure you take care of the sterndrive. Measures include not putting too much pressure on the bellows.

Is Replacing the Bad Bellows Typically Expensive?

Yes, replacing bad bellows is expensive. Although these are very small parts of the boat, the lower unit needs to be removed to reach the bellows.

This service is typically expensive. You cannot replace the bellows by yourself. So, a mechanic has to be involved whose charges are expensive.

Moreover, it’s best to replace all the bellows at the same time. Special tools are required for the job. Expenses are added to rent or buy those tools as well.

The shift cables cost anything under $50 and up to it. The service charges are around $100. The total costs can start from $200 and go up to $1500.

How Often Should You Check or Service Your Bellows?

Bellows should be replaced every two years. It really depends on the usage of your boat.

A Mercruiser bellow lasts for about 6 years. But regular checks for leaks should be done.

Bellows are made of rubber which deteriorates over time. Therefore, replacing them in a few years is necessary.


Bad bellows can cause a boat to sink while docking! It can make you look at new boat prices. Moreover, it can cause accidents as well.

To avoid long-term damage to the engine, it is better to replace the bellows. Although the process is expensive, it will ultimately save you more money.

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