What is it, Why does it happen, and can it
or:........WHAT THE HECK WAS THAT??????
Believe me, blowout is a phenomenon that is better left to
somebody else. But saying that, I've blown out more than my fair
share of boats and have lived to tell about it. Let's investigate
the reasons and results of this sudden change in boating experience.
Blow out occurs when the ration of air to water around the
propeller gets so high that the propeller is no longer grabbing water,
but is trying to propel itself through air or a vacuum. This
causes the prop to lose bite, then a chain of events occurs that can
range from merely a "loose" steering feeling, to a vicious
turn to the right (typically, but not always).
The speed at which this occurs varies with boat design, gearcase
design, and propeller design, so there is no magic formula or
solution. However, the main culprits are:
1. Gearcase inconsistencies: If you have a gearcase that
has been run up on the rocks a few too many times, or has a nosecone
that has epoxy popping out of it or a skeg that is only half there, the
gearcase cannot provide the proper rudder effect, thereby allowing the
gearcase to "crab" or slide sideways through the water too
much, creating a vacuum or air pocket in which the propeller tries to
operate - unsuccessfully. Solution: Be sure to clean up all the
nicks and gouges in the gearcase. Also, be sure the nosecone is
put on straight.
2: Motor is too high: If the motor is too high, the
prop will not be able to lift the bow, causing the drive to use too much
trim causing the propeller to be angled downward, thus causing the force
itself to go sideways through the water - unsuccessfully.
Solution: Be sure to use the proper engine height.
3. Hull Design: Certain brands of boats are more
prone to blowout than others, based on their hull designs and dictate
more care be taken in the setup stage. Solution: Be sure to
have a competent, knowledgeable, experienced person rig your boat.
4. Speed: When you go too fast with a stock gearcase,
the water hits the front of the blunt bullet and actually
"bounces" around the prop. Solution: Add a
nosecone to improve the hydrodynamics of the gearcase.
The problems are typically a combination of these issues.
Gearcase modifications and prop changes can drastically reduce your
chances of blowout, but when you go fast it becomes part of the nature
of the beast, so to speak. Typically a blowout is immediately
preceded by a "loose" steering feeling, an increase in RPM
with no speed increase, and sort of a drop of the nose. What
happens after than can best be described as "WHAT THE HECK WAS
Good Luck and be safe........
Sam Baker, Baker Marine Support, Antioch,