High performance propeller & gearcase blowouts

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How to fix it!

High-performance boating is a combination of extreme skill and good boat setup. The boater’s skills are honed over many years at the wheel in a variety of conditions, yet we must never discount the value of a boat that has a set up with a high degree of technicality. Gearcase or propeller blowout is often determined by incorrect technical setup. What transpires in this case, is that the boat is restricted in reaching its maximum velocity. When the water and air mixture found around the propeller is disturbed enough, the propeller no longer has the ability to bite into water with the greatest effect, and this is where top-end forward speed is lost and the blowout occurs.

If the motor is positioned too high on the transom, the propeller will not create the desired lift for the hull. In this instance, the driver will need to trim out excessively. This outward trim will determine the propeller is inclined pointing to far downwards as opposed to being positioned in line with the water’s surface. This type of propeller position equates to increased drag and insufficient lift, reducing overall top speed.

Inspect the gear case for any damage that might have occurred from riding over debris. Check that the nose cone is properly installed and that the skeg is not damaged. Any damage to the gear case, nose cone or skeg can have a major bearing on the way the boat handles and hits top speed. Chips, scratches and bends in the skeg can cause the gear case to oscillate from side to side. This oscillation will create a pocket of air around the propeller and this will equate to reduced performance. Sorting out nicks, metal tears, propeller alignment will improve the high speed running of the boat and allow it to reach its desired top speed. Even small imperfections and minimal damage can cause the onset of gear case or propeller blowout.

Some hulls can be more prone to blowout than others and finding the correct balance between your particular hull shape and the positioning of the motor is key. Unfortunately there is no definitive setup and each hull shape needs its own particular motor position. A well designed hull should be able to offer a balanced performance through all phases of acceleration. Hull cleanliness, weight dispersion, motor height and a variety of other factors influence blowout, so it is rarely a simple task to get the very best out of a boat but rather requires a variety of adjustments for blowout to be eradicated.

Gear cases are usually designed to reach an optimum speed for the particular engine type. Once these gear cases are pushed beyond their optimum speed, the water begins to separate when passing around the leading edge of the bullet and this creates turbulence and bubbles which disturb the flow around the propeller, reducing the propeller’s efficiency and inhibiting performance. If a well designed nosecone is added, it can increase speed and delay the onset of the blowout occurring.

Gear case blowout can be eliminated with the correct fine-tuning of the boat’s setup, the correct placement of the motor on the transom and a variety of other factors designed to offer greater top speeds and smoother travel on the water. Just make sure your boat is correctly set up before you crank the throttle open.


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