Are you a new boat owner or an owner who wants to take on more maintenance responsibilities instead of hauling your boat to the mechanic every time a tiny problem arises? Boat engine maintenance is not something to skimp on — many tiny problems with a boat can quickly become huge, costly issues without proper care and attention.

One such issue that can arise for some types of boats is engine damage caused by old or faulty bellows on a boat. Without proper care, damage to bellows can cause serious issues for a boat. It could even sink completely if enough water seeps in, and no one wants that!

With some routine maintenance and troubleshooting, however, you can avoid these issues and keep your boat cruising smoothly.

What are Bellows on a Boat?

Think of bellows as a rubber boot on the lower drive of the engine that protect the exhaust system, the U-Joint, and the shift cable. They're located on stern drives (so, on boats that have an inboard engine connected to an outboard drive) but not on inboard or outboard motors.

Bellows have an accordion-like appearance and they function as a gasket to protect the engine by creating a watertight seal. And they're required since boats use open-systems for cooling the engine, which can leave certain parts of the motor vulnerable to damage by water if the bellow seals are broken.

Bellows are also often treated with some form of pesticide to prevent damage that could be caused by marine animals chewing on them when the boat is docked in a wet slip.

Troubleshooting with Your Boat’s Bellows — How To Know If They Are Bad

Have you been having issues with your boat and are now wondering, “are my bellows bad?”

Well, first: bellows on your boat should be checked regularly to catch issues before they arise. If you notice any cracking or leaking, you’ll want to get your boat serviced immediately or replace your bellows. Signs that there might be issues with your boat’s bellows are water in the cabin, issues when shifting, loud noises while driving, and overheating.

A proper inspection is best left to a professional, but if you want to take a look yourself first, you can pin the drives to one side, then the other. Look for cracks and other signs of dry-rotting. Pull apart the folds of the boot so you can clearly see if there are any cracks or leaks within the accordion structure. You should also check for rust on the output shaft and U-joints, as this is a sign that water is getting in and causing damage.

Replacing Boat Bellows

We highly, highly recommended that you work with a professional boat mechanic when making any major repairs to your boat. These steps are meant to be a basic overview for replacing bellows on a boat for those curious about the process, but they're not a substitute for working in-person with a qualified professional.

Materials You’ll Need:

  1. Transom service kit with exhaust bellows
  2. Bellows expander tool
  3. Bellows adhesive
  4. Socket wrench set
  5. Socket wrench set with an extension
  6. Knife (for cutting out old bellows)
  7. Flat-head screwdriver set
  8. Open-end/box wrench set
  9. Clean rags
  10. Marine lubricant spray (for removing bellows if they are stuck)

Before starting anything, make sure you have the service manual for your specific drive. This will help you identify the fasteners, clips and gaskets you need to remove and in what order. It’s also advisable to take on this project with at least one other person, so if you have a friend, or better, a friend who is a boat mechanic, enlist their help so the repair can go smoothly. You don’t want to accidentally drop the drive shaft on the ground and bend it.

If one of the bellows needs replacing, you might as well replace them all. Note, this is a very time consuming process that could easily take half, or even an entire day, so don’t expect it to be a quick process. You’ll want to take your time to ensure you replace your bellows properly.


Phase One: Removing the Drive

  1. Remove the prop and the drive of your boat. To do this, put your boat in the appropriate gear — if your boat is right-hand drive, put the shift lever in reverse. If it’s left-hand drive, put the lever forward.
  2. Remove the toggle at the cable end. Leave the straight-threaded rod so you can pull it through the transom.
  3. Release the trim cylinder and remove the bolts placed on the outdrive. Take off the housing around the exhaust.
  4. Disconnect the trim limit switch on the port side of the upper housing by removing the screws that secure it.
  5. Disconnect the shift cable at the engine to access the bellows.

Don’t force the bellows out when it comes time to remove them — use a lubricant and tools if necessary.


Phase Two: Replacing the Exhaust Bellows

The bellows that cover and protect the exhaust system are the first ones you'll want to replace.

  1. With the drive open, remove the exhaust bellows. You’ll need to loosen the screws that are on the clamps of both ends of the bellows. When the clamps are removed, cut the old bellows with a knife to make them easier to remove. Be super careful to not damage any other part of the system when cutting the bellows.
  2. Clean mounting bosses
  3. Install new bellows on forward mount
  4. Position bellows on aft mount
  5. Tighten aft clamp


Phase Three: Replacing the U-Joint Bellows

The U-joint bellows are the rubber boot that enclose the u-joints, gimbal bearings, and output shaft to shield it from water. You'll also need to replace the o-rings and gasket, in addition to the U-Joint bellows themselves, at this stage.

  1. Remove the hinge pins in the bell housing
  2. Tilt the bell housing out at the top remove the u-joint bellows
  3. Replace the bellows


Phase Four: Replacing the Shift Cable Bellows

This is the easiest step in the process, but it’s still important to do it properly.

  1. Remove the clamp
  2. Clear the housing
  3. Install new shift cable bellows


Maintenance & Preventing Problems with Bellows

With routine maintenance and proper care, you can avoid problems with your boat bellow and catch serious issues before they can cause damage to your boat. As a boat owner, you should always do the following to prevent problems with your bellows.

  1. Have your mechanic check the bellows when your boat is in for other maintenance or routine servicing.
  2. Check for rust on the output shaft and U-joints. If you see rust, this is almost a guarantee water is getting in.
  3. Inspect your bellows annually. It’s recommended to replace these parts of your boat at the first signs of wear or every two years typically.

Always consult your boat’s specific manual and manufacturer’s guidelines before making any repairs, and when in doubt, call the professionals. A mistake when replacing the bellows on your boat could be very costly in the long-run, so it’s best to make sure you are working with someone who really knows what they are doing.

For more information and resources on boat maintenance, check out the How-To section of the GetMyBoat Journal. We have articles on boat deck sealants, how to install and use a bimini, everyday items to help your boat’s engine, and more.