Check Those Exhaust Flapper Valves

Recently, a buddy of mine called me with a problem he had with a new-to-him boat. He had removed the prop to send it out for repair, but when he did he found big chunks of rubber just inside where the prop mates to the lower gear case on his Bravo One drive. He lives several states away, but for some reason he still calls me about this kind of thing.

The red arrow indicates the flapper valve in one side of the exhaust Y-pipe. These valves deteriorate over time, and quickly in engine-overheat conditions.

The red arrow indicates the flapper valve in one side of the exhaust Y-pipe. These valves can deteriorate over time, and quickly in engine-overheat conditions.

“Well, there are a lot of rubber parts in that power plant,” I told him, “but there are a couple of things you should look at first. Like the flapper valves.”

On any engine with through-the-prop exhaust, the tops of the exhaust “Y” pipes on both banks have rubber flappers at the top where they connect with the rubber exhaust hoses that clamp to the exhaust manifolds. The flappers act as one-way valves, to let exhaust flow out of the engine, into the Y pipe, and out the propeller. They’re important because they also prevent water from back-flowing into the exhaust stream and ultimately the engine, where it can cause hydraulic lock and do some real damage.

These flappers wear out eventually anyway, but particularly if the engine is running hot or low on water. (Overheating or low water pressure can signal a faulty water pump impeller, which is driven by a belt off the engine on Bravo-drive-equipped boats. I told my friend to check the impeller, too. You can read more about those here.)

Also, when the flapper valve is burned, it’s possible some of the exhaust hoses also have burned, so I told him to inspect those, too.

It’s too bad he wasn’t aware of the rubber bits in the exhaust system before he bought the boat. That way he could have negotiated for a better price. But I guess that’s the price of not doing a thorough inspection before you buy.

 

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