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Nosecones and Gearcases

"Blow out"


One of the most controversial, least understood areas of marine high performance is that of the Nosecone and Gearcase.  I guess people grow up learning about engines and how to make them more powerful in order to go faster, but often times when it comes to a boat, the hydrodynamic friction of the gearcase is the number one determinant of ultimate speed.

Gearcases have changed dramatically through the years as manufacturers came to this realization.  Now it has reached a point where whole model changes are dictated by the design of the gearcase.  Several years, ago, few, if any performance boaters would have cared whether a Mercury 2.5 EFI motor came with a CLE vs a SportMaster gearcase, let alone then taken the time to actually have that CLE or SportMaster sent out for IMPROVEMENTS!

Basically, the gearcase should function in two ways: 1: To provide a water intake for cooling the motor and 2: To provide proper leverage to carry the bow of the boat.  If your gearcase design is limiting either of these functions, you're not going to reach your boat's design speed capabilities.

Water Intake:  The advent of Nosecones has greatly increased the interest of water intake location.  Bob's Machine Shop, Mazco Propellers, Land 'n Sea, and HydroMotive are some of the largest manufacturers of nosecones.  A nosecone's main goal it to reduce hydrodynamic friction while providing proper - if not improved - water intake capabilities.  They also can serve an additional purpose in higher speed boats - reducing "blow out".  Blow out occurs when the water hitting the gearcase travels at such a speed that it actually deflects off the front of the gearcase and bypasses the propeller, causing the propeller to lose its bit and causing the motor to rev up, the boat to fall and sometimes turn out of control.  Boats travelling under 80 mph will not be affected by this phenomenon.  The biggest mistake most nosecone purchasers make is to buy a low-water pickup nosecone and put it on a moderately heavy to heavy bassboat, etc.  These boats can not run the motor high enough to take advantage of the low water pickups and therefore more often than not lose speed.  If you don't need a low water pickup nosecone, but would like to reduce your hydrodynamic resistance, try our "shorty" nosecone.  This nosecone maintains the side water intake, but gives the nosecone a more hydrodynamic profile.

Carrying the Bow:  Each individual boat has a maximum engine height at which the boat will still be able to get "loose".  Any higher than that and the prop will not have enough leverage to carry the bow of the boat and the MPH will be scrubbed off the top end.  Heavier boats and boats without a lot of airlift designed into them typically require lower engine heights than lighter boats that are designed with a lot of airlift, i.e., Allisons, Mirages, STVs or any other Mod VP style hull.  These boats work best with low water pickup nosecones.

Skeg DesignThe skeg is often overlooked by performance boaters.  On today's high performance boats, torque steer is a major issue.  This if often offset by a riveted on triangular piece or as in the case of Mercury Hi-Perf motors, a thickly cast in anti-torque profile at the rear of the skeg.  These both help to reduce torque, but unfortunately, they also reduce speed because the boat is having to drav that thickness through the water.  A better alternative is to cut that thickness back and weld in a Baker Anti-Torque tab.  This tab maintains the standard skeg thickness but curves to the starboard side of the gearcase reducing torque.  Gearcases with this modification vs the thick skeg can pick up as much as 4-5 mph.  Cheap performance indeed.

Sam Baker, Baker Marine Support, Antioch, Illinois.                        www.scaryfast.com

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?"

Good Luck and be safe........

Sam Baker, Baker Marine Support, Antioch, Illinois.                  www.scaryfast.com