Range and Speed of Torqeedo Motors

Propulsive power and overall efficiency  

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 ineffi ciency. 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:

Drive system and overall effi ciency

Drive train components
Type of energy	DC	DC	AC	Rotation	Rotation	Propulsion
Power
	Current times voltage Taken from the battery	Current times voltage Taken from cable	Temporal division of current times voltage	Torque at the motor shaft times the angular velocity of the motor shaft	Torque at the propeller shaft times the angular velocity of the propeller shaft	Power (thrust) at the boat times the speed of the boat
	INPUT POWER Traditional power specs of electric motors (all system losses not considered)				SHAFT POWER Traditional power specs of combustion engines (high effi ciency losses at the propeller not considered)	PROPULSIVE POWER Power specs of Torqeedo Together with the input power results in the overall effi - ciency (propulsive power divided by the input power)

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 effi ciency 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 effi cient 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 inf uence on overall efficiency. For this reason, Torqeedo takes great care in the comprehensive optimization and interaction of all components.

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.

Background information on electric motors  

There are four criteria for differentiating among electric motors: the frequency response, the generation of the alternating field (commutation), the excitation of the magnetic field, and the structural shape.

Induction motors: the ratio between the engine speed and the frequency of the supply voltage is not constant: it depends on the loading condition of the machine. The higher the load, the higher the speed difference - the so-called “slip”, i.e. a specified propeller speed is not maintained at higher flow resistances. Hence, thrust is not available at the very time it is required.

Synchronous motors: with this type of motor, the ratio between the supply voltage frequency and the engine speed is constant. As a rule, synchronous engines are torque controlled. This means that they always draw as much power as they need in order to provide the necessary torque at the desired speed. For this reason, they are the preferred motor in areas with particularly demanding torque requirements. Should the motor require more power in order to maintain a specified propeller speed, the motor automatically draws more power.

Mechanically-commutated motors: The brush-complemented motors generate the alternating fi eld necessary for the motor to operate by means of sliding contacts. Based on their geometric organization, these “brushes” convert the power depending on the rotor position. A shortcoming in these motors is the wear-and-tear of the brushes, hence making the motors maintenanceintensive. The contact resistance also causes so-called brush losses, impairing the degree of effectiveness of the motor.

Electronically-commutated motors: they generate the alternating field necessary for the motor to operate by means of an electronic circuit - the “frequency converter”. This prevents the occurrence of brush losses, and the motors are maintenance-free. The enormous progress that has been made in the area of electronic power components and circuit design has only made it possible in recent times for high-power motors to be manufactured at a marketable price.

Electromagnetic-excited motors: this type provides the necessary magnetic field by means of a second loading section. This makes this option more economical: however, it is considerably bulkier and heavier than the permanent magnet-excited motor. Further, it is also considerably less advantageous with regard to power consumption and degree of effectiveness.

Permanent magnet-excited motors: in this case, the permanent magnets generate the necessary magnetic field. Hence, there are no performance losses in the field coils.

Internal rotor motor: in this classical model of electrical motor, the rotor is surrounded by the stator. The rotor is a revolving motor component attached to the motor shaft: it is also known as a “rotor motor” or “armature”. Since the coils of the internal rotor motor are located on the outside, the motor has advantages when it comes to cooling. Compared to other structural shapes, however, it is relatively low-torque.

Disc armature motor: it generates the torque (= force times lever) by arranging the axle of the magnetic field parallel to the shaft instead of radial to the shaft. This enables the realization of geometries in which the location of the electromagnetic generation of power is a good distance from the axle. Hence, a higher torque is achieved at the same power. The disc armature geometry is disadvantageous to outboard motors with direct water cooling. Due to its extremely large diameter, it isn’t possible to build disc armature motors directly into a pylon.

External rotor motor: this is the most modern type of motor: the coils are arranged inside. The rotating magnets are located on an externally-running bell. With the same structural shape, external rotor motors hence have a signifi cantly higher torque than internal rotor motors.

Compact power - the Travel 801 motor

The electronic commutation of electric motors described above can generally be either analog or digital. While most providers of electric motors continue to work mechanically using carbon brushes for commutation, Torqeedo has gone two steps further and uses digital power electronics in its new motor models. In contrast to analog-based electronic commutation, digital electronics has a more intelligent power control and handling. This provides more power, more stability and more comfort.

The intelligence of the power control is in its combination of propeller-speed control and control of the power intake. The propeller-speed control regulates the rpm of the propeller, i.e. the motor keeps tightly within the speed specifications and draws whatever power it requires to reach the defi ned propeller-speed. If, on the other hand, the power consumption is controlled then the drive processes the power made available to it as good as it can and the resultant force is then the speed of the propeller.

What is the concrete significance of intelligently combining these types of control logic? For example, the Torqeedo electronics functions speed-controlled within low power ranges in order to allow slow maneuvering that is absolutely precise to within a centimeter. In other cases, the electronics controls the motor via the power intake: e.g. to provide a very light boat with a higher final speed or to provide the driver with a defi ned power level to maximize the range. Additionally, intelligent control logics allows the motor control to adapt itself to the use of alternative propellers, e.g. when optimizing speed or thrust by using alternative propellers.

Propeller technology and hydrodynamics  

Basically, propellers which slowly turn in the water and have a high pitch and a large diameter, have the highest degree of effectiveness. A large propeller diameter results in a high propellant fl ow, while a high propeller pitch has a positive effect on the additional speed induced by the propeller. Multiplied by each other, the propellant flow and the induced additional speed result in the propulsive power of the propeller. On the other hand, an increasing circulation speed of the propeller results in an increasing loss of efficiency.

Conventional outboards in the low-power range fail at using highly-efficient propellers: Either they do not have enough torque to move large sloped propellers or they do not have enough elasticity (availability of torque over a large speed range). Combustion engines are particularly susceptible to a lack of elasticity. This is because they only have an extremely low torque at small speeds. Propellers that would normally have a good rate of efficiency within the efficient range of the motor stall the motor when within low speed ranges. The rates of efficiency for propellers that can be used for low-power class combustion engines are therefore limited to 20-30%.

To ensure that the Torqeedo motor can fully exploit its strengths in the maximum torque and in elasticity, and then covert these into superior efficiency, the Torqeedo propeller has been carefully adapted to the torque characteristic of the motor.

The majority of propellers used in recreational activities are based on series tests that were carried out in the 40’s to 60’s of the 20th century in the Wageningen test facility in The Netherlands as well as by the US Navy. The results of these tests have been concretized in general construction principles and are used by rule of thumb.

On the other hand, the most modern large ships have been equipped for some years now with propellers that are the result of multi-dimensional optimization calculations. In contrast to standard propellers, the pitch and camber of the propeller are not kept (almost) constant across all segments of the propeller. Instead, the pitch and camber are optimized based on a vortex grid calculation for each single segment of the propeller in a stepwise optimization over many thousand iterations. The additional scope for design resulting from this allows the additional speed to be induced by the propeller at the highest rate of effi ciency. Due to these characteristics, the corresponding propeller is designated as a Variable-Pitch-Variable-Camber (VPVC) Propeller.

Torqeedo outboards are uncompromisingly trimmed to efficiency. This also applies to all fluidic-sensitive components such as the shaft and the pylon.

Lattice structure for calculation of the propeller characteristics of the Variable-Pitch-Variable-Camber (VPVC) Propeller from Torqeedo

Section of the calculated propeller jet (red for high speeds, blue of low speeds)

In addition to the important parameters such as the diameter of the propeller and the number of blades (wing vamps), propellers can also be described by the radial course of the following parameters: Pitch, chordlength, skew, rake as well as the profi le parameters of thickness and camber.

“Pitch” describes the distance covered by a propeller during each complete turn without any slip. Since this idealized size cannot be established on a moving boat (in practice, slip always occurs), the slip of a propeller is determined with the aid of the tilt angle of its wing vamps. For propellers in which the pitch varies along the wing (Variable-Pitch Propeller), the pitch is measured on a circle that is drawn around the middle of the propeller at 70% of the propeller diameter.

Cavitations are the phenomena caused by the formation and closing of cavities within fl uids. Cavitations are caused in particular by fast moving objects within the water such as, e.g. propellers. Due to the fast movement, underpressures result in which the water starts to boil and evaporate at normal temperatures. The energy used for this is not converted into propulsive power and is lost as ineffiLTS Ltdciency. Depending on the quality of the drive system and its propeller, cavitations of various severities may occur. The two pictures taken with a high-speed camera at a shutter speed of 1/8,000 second show the difference between the Torqeedo VPVC-Propeller and a standard propeller at comparable operating points:

The standard propeller shows signs of fluctuating cavitations on the "suction“ side of the wing tip.	On the other hand, the Torqeedo VPVC-Propeller only shows signs of light Tipp vortex.

Battery technology

A 300 Wh LIMA high-performance battery is integrated into the Torqeedo Travel models

Lithium-based battery systems are by far the most powerful energy carriers currently available. On the one hand they are characterized by a high specific energy density. This means that they are able to store a large amount of energy per kilogram of battery weight. In addition, lithium batteries can withstand high current: in other words, they are able to deliver their capacity even under high loads. Both of these characteristics are of great importance for applications in boat drives: on the one hand, the battery weight and volume on board is reduced. On the other hand, the lithium-based battery systems ensure that the power supply does not collapse even when the boat's electro-motors temporarily draw high currents from the batteries.

An additional advantage: lithium batteries do not display a memory effect and are cyclically stable. Even when stored for many months, almost no charge is lost, in contrast to conventional lead batteries.

Torqeedo batteries are also extremely robust and are secured against incorrect handling. In addition, they are protected against short-circuiting, overvoltage and excessive discharge.

The high energy density of lithium cells demands effective safety technology. For this reason, Torqeedo uses exclusively lithiummanganese safety cells. These so-called LIMA cells offer the highest safety standard of all lithium-based batteries: Only LIMA cells are able to master the necessary safety tests even when the safety electronics are switched off. These safety tests include in particular:

• Crash tests: a fully-charged battery is crushed with a ten-ton hydraulic press
• Nail tests: a nail is hammered right through the case of a charged battery until it comes out of the other side. This leads to the maximum possible number of battery cells being short-circuited.
• Overcharge tests: the battery packs are overcharged with 42 V at 7.5 A until the cells are destroyed.
• Short-circuit tests: the main contacts are short-circuited without the safety circuit.
• High-temperature tests: the aforementioned tests are undertaken on a 150° C pre-heated battery.

Alternative lithium battery concepts in round cell format or in lithium polymer packs ("Li poly") do not fulfi ll these criteria due to the chemical composition of the battery (cobalt or nickel cathode). Additionally, in case of a fault such as, e.g. shortcircuit, overload, mechanical damage, lithium polymer packs react in a highly critical manner due to the missing safety mechanisms immanent within the cells.

The stored energy quantity - in other words, the capacity of a battery - is measured in watt-hours (Wh). For historical reasons, the battery capacity is sometimes indicated by the nominal voltage in volts (V) and the stored charge in ampere-hours (Ah). In this case, the stored energy is calculated as voltage times charge.

1 Rated capacities: Lithium with 3-4 times the energy density of standard lead batteries

2 The typical working range of boat drives: Lithium with 6-8 times the energy density of lead batteries

3 Working range of light-weight high-power applications such as Torqeedo Travel 801: LIMA cells leave all competitors behind

Source: 2007 Torqeedo Catalogue