Sterndrive Bellows: The Cheapest Part That Sinks Boats

On a sterndrive boat there is a hole in the transom. A fairly big one, below the waterline, with the drive shaft and the exhaust passing through it. The only thing between that opening and the inside of your hull is rubber — the bellows. They are among the cheapest components on the whole installation, and they are the ones that sink boats.

When do sterndrive bellows need replacing?

Rubber does not fail because you used it. It fails because time, salt, ozone and ultraviolet light harden it until it cracks — which means bellows age on a calendar, not on an hour meter. The boat that did fifteen hours last season is on the same clock as the one that did two hundred.

Volvo Penta publishes a replacement interval for the drive, and your engine and drive manual is the number to trust. Alongside that, get them inspected at every haul-out. If the boat lives in the water, if it is more than a few seasons since the last set, or if nobody can tell you when they were last done, treat that as due rather than fine.

The three bellows, and what each one does

Most Aquamatic sterndrives carry a set, not a single item:

• The drive joint bellows (sometimes called the U-joint bellows) seals the universal joint and drive shaft where they pass through the transom. This is the one that floods a boat when it splits, because it opens a direct path to the bilge below the waterline.
• The exhaust bellows carries exhaust gas from the transom shield out through the drive. A perished exhaust bellows lets water in and fumes where they should not be.
• The shift cable bellows seals the shift cable passage. Smaller, less dramatic, and replaced with the others because the drive is already off.

That last point is the economics of the job. Nearly all the labour is in pulling the drive and setting it back up in alignment. Replacing one bellows and leaving its neighbours — the same age, the same rubber, the same conditions — means paying that labour twice. Bellows are replaced as a set.

What to look for

• Cracking in the folds of the rubber, especially the outer bends. Flex a fold gently and look into the crease; that is where it starts.
• A chalky, hard, dried-out feel instead of a supple one.
• Any weeping, staining or bilge water that appears after a run and cannot be explained.
• Splits or tears — at which point the boat should not go back in the water until it is fixed.
• Clamps that are rusted, loose or missing. A perfect bellows on a failed clamp is still a leak.

Which kit fits your drive

Volvo Penta supplies bellows as kits with the clamps and sealing parts needed to do the job once. Which number you need depends on the drive, not the engine:

• For the modern DPH and DPI drives behind D4 and D6 engines, the drive joint bellows kit is 3594509 and the matching exhaust bellows kit is 3588753.
• For the long-running classic drives across the AQ, AD, TMD and BB ranges, one drive joint bellows kit — 876294 — covers a very wide span of installations.
• Saildrives are different. A saildrive has a rubber diaphragm sealing the leg where it passes through the hull, and it is a safety item with its own replacement schedule; the kit is 21389074. If you own a saildrive yacht, this is the single most important piece of rubber on the boat.

The full range sits in the Volvo Penta drives and propulsion parts, and the Engine / Drive filter narrows it to your own series — the D6 list is an example of how far that cuts it down.

Is this a job to do yourself?

Honestly: this is the one where we would say think carefully. Pulling the drive is manageable with the right tools, but the drive joint bellows has to seat correctly on both the transom shield and the drive, the clamps have to be positioned and tightened properly, and the gimbal bearing and alignment want checking while everything is apart. A bellows fitted slightly wrong does not announce itself on the day. It announces itself on the water.

While the drive is off is also the moment to look at everything else in there — the gimbal bearing, the shift cable, the anodes and the drive oil for any sign of water. That is the sort of work our engine and drive unit rebuild service covers, and doing it all in one visit costs far less than doing it in three.

Not sure what drive is on the back of your boat, or when its bellows were last changed? Ring 0412 532 984 with the drive designation off the ID plate and we will tell you which kit it takes.