|
Twin Disc Arneson Drives are among the most efficient
marine propulsion system on the market.
They have been designed to solve some of the problem related
to conventional shaft systems when installed on vessel with
speed in excess of 35 knots.
Problems are due to installation and physical aspects. Here
after some explanation about how ASD can overcome conventional
shaft line limitation.
Geometric aspects: Installation
Traditional propeller design and selection is almost always
an exercise in trading off diameter against several other
performance limiting parameters, this while the basic momentum
theory tells us that for a given speed and thrust, the larger
the propeller, the higher the efficiency.
Blade tip clearance from the hull, maximum vessel draft, shaft
angle, and engine location all tend to limit the maximum
feasible propeller diameter to something considerably smaller
than the optimal size. The propeller diameter is simply the
maximum that fits, sometimes it can also result in a
considerable sacrifice of propulsive efficiency. Or, if these
geometric limits on propeller diameter are exceeded, the
result can be excessive vibration and damage due to low tip
clearances, or a steep shaft angle with severe loss of
efficiency and additional parasitic drag.
The surface propeller, installed with an articulated surface
propulsion system, frees the designer from these limitations.
There is virtually no technical limit to the size of propeller
that will work. The designer is able to use a much deeper
reduction ratio, and a larger lightly loaded and more
efficient propeller.
 |
Physical aspects: Cavitation
When a submerged propeller blade cavitates, the pressure on
part of the blade becomes so low that a near vacuum is formed.
When these cavities collapse, water impacts on the blade
surface with a local pressure singularity. The effect can
approximate that of hitting the blade with a hammer on each
revolution. Cavitation is a major source of propeller damage,
vibration, noise, and loss of performance. And although high
speed propellers are often designed to operate in partially or
fully cavitating mode, problems associated with cavitation are
frequently a limiting factor in propeller design and
selection. |
|
The surface propeller effectively eliminates cavitation by
replacing it with ventilation. With each stroke, the propeller
blade brings a bubble of air into what would otherwise be the
vacuum cavity region. The water ram effect that occurs when a
vacuum cavity collapses is almost completely suppressed by
this entrained air. While the flow over a superventilating
propeller blade bears a superficial resemblance to that over a
supercavitating blade, the vibration, surface erosion, and
underwater noise are absent.
Appendage drag
Rudders, exposed shafts, struts, and propeller hubs all
contribute to parasitic drag. Inclined exposed shafts not only
produce form and frictional drag, but there is also induced
drag associated with Magnus effect lift caused by their
rotation.
Frictional drag increases with the square of the speed. For
speed in excess of 35 Knots the resistance induced by
appendage drag becomes relevant and a considerable part of the
hull resistance.
For speeds over 35 Knots this becomes a problem if
associated with conventional systems, while surface propellers
virtually eliminate drag from all these sources, as the only
surfaces to contact the water are the propeller blades and a
skeg |
