After publishing a recent study identifying the causes for accidental outboard powerboat sinkings, Seaworthy, recreational boating’s damage avoidance magazine, has combed through the BoatUS insurance claims files to identify the reasons why inboard/outboard (I/O) powerboats sink. Interestingly, while outboard powerboats are similar in design to I/O’s — and often made by the same manufacturer — the study found vastly different reasons why each accidentally sinks.

Outboard powerboats tend to sink at the dock due to poor cockpit designs that trap water, but an I/O’s weakest link is the delicate connection between the inboard engine and the outdrive unit, called bellows.

“The use of bellows or boots — pleated, flexible rubber membranes — that run between the inboard engine and partially submerged outdrive to seal the transom where cables and shafts pass through have one weakness: a limited lifespan,” said Seaworthy Editor Bob Adriance. He says that bellows should be inspected annually — and more often in hot, sunny climates. “Any bellows that is over five years old is living on borrowed time,” said Adriance.

He adds, “Many boats have more than one bellows (driveshaft, shift cable, exhaust, etc.) any one of which has the potential to sink the boat. And if one shows wear, they all should be replaced.”

Here are the top reasons why I/O’s sink: 

At The Dock: 

#1 (44%): Leaking bellows. A surprising number were attributed the small shift cable bellows. 

#2 (23%): Failed below waterline fittings, hose clamps, and melted hoses as a result of overheating. 

#3 (15%): Failed above waterline fittings. Heavy rains overwhelm cockpit scuppers. 

#4 (11%): Poor docking arrangements: Boats or outdrives that snag on docks. 

#5 (6%): Uninstalled drainplug. 

#6 (1%): Miscellaneous such as a cracked engine block. 

While Underway: 

#1 (36%): Struck a submerged object such as rock or logs. 

#2 (24%): Failed below waterline fittings. 

#3 (20%): Leaking bellows. 

#4 (12%): Swamping. 

#5 (8%): Miscellaneous.

