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Reasons Why Inboard/Outboard Powerboats Sink
BoatU.S. Goes to the Insurance Claims Files for Answers

Posted Tuesday, January 9, 2007

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After publishing a recent study identifying the causes for accidental outboard
powerboat sinkings, Seaworthy, recreational boating's damage avoidance magazine, has
combed through the BoatU.S. 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.

All BoatU.S. marine insurance policy holders get a free subscription to Seaworthy,
but subscriptions are also available for $10 a year (4 issues) by going to or by calling 800-262-8082, ext. 3276. If you'd like to get a
free insurance quote for your boat, go to or call

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