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Meaningful performance indicators
Torqeedo provides
meaningful performance indicators for boat drives. Basically, the power of a
drive is measured according to how much propulsion it actually delivers for
moving the boat.
This power is referred to as the output or propulsive power. It is
calculated by the power times the speed and can be expressed in watts or
horsepower.
Although the propulsive power forms an extremely informative index for boat
drives, most manufacturers don’t provide it. Suppliers of electric motors
generally use the input power of their motors as the power rating. However,
only a fraction of this value is actually available to the boat drive as
propulsive power.
The rest is lost – for example in the motor or propeller – as inefficiency.
Manufacturers of combustion engines, on the other hand, usually give the
power at the motor shaft as their index.
Also here, only a fraction of this value is available as propulsive power:
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Overall efficiency
Propellers on combustion
engines of a lower power class cause some 70% of the motor shaft power to be
lost as inefficiency that cannot be converted to propulsion.
The second important index is the overall efficiency. It describes the
efficiency with which the drive system converts the available energy sources
into output power and is calculated by dividing the propulsive power by the
input power.
Torqeedo builds the most efficient outboards on the market by far, by means
of consistent orientation towards propulsive power and overall efficiency.
As the overall efficiency of a drive system is given by the product of the
partial efficiency levels of all components, a single poor partial-efficiency
level can have a
significant negative influence on overall efficiency. For this reason,
Torqeedo takes great care in the comprehensive optimization and interaction
of all components.
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Static thrust
Another commonly used motor
index is the static thrust expressed in kilogram force (kf) or Newton (N).
In comparison to the propulsive power and overall efficiency, this index is
less meaningful, as it only measures propulsion in association with the
static bollard pull experiment. In this case, as the speed is zero, the
effective power (power times speed) is also zero. As a consequence to this,
the static thrust does not provide any information on the propulsive power
that can be actually achieved and is therefore not a meaningful index when
taken in isolation. It merely serves as an indication for the maximum boat
size that the motor can be used on.
Before purchasing a motor, it is recommendable to test the static thrust
specifications personally. In Torqeedo’s tests of competitor products, the
manufacturers’ specified static thrust values could generally not be
reproduced.
legal mentions - Copyright
Jean-Pierre Masquelier
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