Blow Out: What is it, Why does it happen, and can it be stopped?....
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 THAT?"
Originally Posted by mdtritn21
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