Zenoah Ignition Fitted to a CMB 35Rs
A selection of two stroke tuning programs
New CMB 26 Pictures courtesy of Ian's boats
The complete tuning programs from Gordon .P. Blair
Propeller Basics
Diameter
Diameter is two times the distance from the centre of the hub to the tip
of the blade. Also it can be looked at as the distance across the
circle that the propeller would make when rotating. It is the first
number listed when describing a propeller.
Pitch
Pitch is defined as the theoretical forward movement of a propeller
during one revolution -assuming there is no "slippage" between the
propeller blade and the water. For most boats, there is slippage and
therefore the distance advanced is less than the design pitch. The
amount of slippage varies from boat to boat. Pitch is the second number
listed in the propeller description.
Cupping
Many of today's propellers incorporate a cup at the trailing edge of the
propeller blade. This curved lip on the propeller allows it to get a
better bite on the water. This results in reduced ventilation, slipping,
and allows for a better hole shot in many cases. A cupped propeller also
works very well where the motor can be trimmed so that the propeller is
near the surface of the water. The cup will typically result in higher
top end speed on one of these applications.
Rake
Rake is the degree that the blades slant forward
or backwards in relation to the hub. Rake can affect the flow of water
through the propeller, and as implications with respect to boat
performance.
Aft Rake helps to trim the bow of the boat upwards, which often results in less wetted surface area and therefore higher top end speed. Aft rake propellers also typically "bite" better on ventilating type applications.
Forward, or negative rake, helps hold the bow of the boat down. This is more common in workboat type applications. F Blade with no rake Aft Rake Outline of Blade with Rake -F=d (Bow)
Ventilation
Ventilation is a situation where surface air or exhaust gasses are drawn
into the propeller blades. When this situation occurs, boat speed is
lost and engine RPM climbs rapidly. This can result from excessively
tight cornering, a motor that is mounted very high on the transom, or by
over-trimming the engine.
Cavitation
Cavitation, (which is often confused with ventilation), is a phenomena
of water vaporizing or "boiling" due to the extreme reduction of
pressure on the back of the propeller blade. Many propellers partially
cavitate during normal operation, but excessive cavitation can result in
physical damage to the propeller's blade surface due to the collapse of
microscopic bubbles on the blade.
There may be numerous causes of cavitation such as incorrect matching of propeller style to application, incorrect pitch, physical damage to the blade edges, etc...
Be advised disturbances in the water flow forward of the 4 propeller can result in blade damage which appears to be blade cavitation, but is actually due to non-favourable water flow into the propeller.
Copyright Cobra Racing 2008