Many repeated stress's, including
the impact of each blade as the prop rotates (usually do to catching
a little air) subjects this area to fatigue failure. Of course there is
always that one giant wave that sends you to the moon when all you can
think about is holding on to the wheel and BAM - broken gear - that is a
yield failure. The same applies to the vertical and prop shaft. Repeated
loading below the yield point will result in fatigue.
How you drive your boat has a lot to do with the longevity of the
drive (along with the engine and the rest of you expensive parts). The
single biggest cause of drive failure is poor throttling. To put it in
perspective, a factory stock boat today can run in the 90 MPH range.
This is faster than a world champion Open class boat of the 70's and
80's. Boats in Offshore competition today (and for the last 30 years)
have at least a two man crew. With very few exceptions one of them is
responsible for throttling.
Proper control of engine RPM when leaving and entering the water will
do a lot to make your parts live. Timing is everything. The goal is to
keep the engine RPM constant, regardless of whether the prop is in the
water or not. This means that just as the props leave the water the
throttles should be pulled back enough to keep the rpm from surging. It
is not humanly possible to react quickly enough once the boat is up, and
the tachs lag as much as 1500 RPM behind the engines.
Ideally what really happens is that you learn to anticipate and pull
back just enough to maintain rpm. If you pull back too early the boat
will trip (pull the nose down). Too late and you will over rev. All the
way back to idle is not a good idea either. Even though that is ok for
the engines, the props should really be turning close
to the same speed as when they left the water.
Think of the tires on an airliner when it first hits the runway. As
far as the prop and rest of the boat is concerned the water is non
compressible, and the impact on water is as bad (or worse) as if it were
concrete. The key to a graceful reentry is the perfectly timed
application of the throttles to keep the engines at a constant speed.
Too little or too many RPM can both damage the drive.
Most failures in the Bravo are prop shaft, floor of forward gear,
tower in upper gear case, vertical drive shaft, or upper gear tooth
failure. Mercury has finally addressed all these problems with the
The parts described above will break from either fatigue, brittle or
yield failure assuming the drive is assembled properly
An RPM limiter is a must have. No matter how good you are, there is
always that one time when you sneeze or are distracted. If your drive
does blow, it could save your engine from total destruction.