Ayrton Oneill
Tachometers are indispensable for monitoring engine speed on a boat. They display the RPM (revolutions per minute) of the engine, which is usually in the 10s of hundreds. There are several types of tachometers, including generator tachometers, which input a signal from a mechanically driven generator unit, and inductive tachometers, which have an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head. Tachometers can also be mechanical, with a gear drive that spins a cable and transfers the rotation to the tachometer gauge.
| Characteristics | Values |
|---|---|
| Function | Display the RPM (revolutions per minute) of the engine |
| RPM | Typically in the 10s of hundreds |
| Tachometer types | Generator tachometer, inductive tachometer, mechanical tachometer, automotive tachometer |
| Tachometer signal | Utilises alternator's revolutions to generate a discernable indicator |
| Alternator power | AC (alternating current) that is then rectified to DC (direct current) |
| Mechanical tachometer | Gear drive on the engine spins a cable, transferring rotation to the tachometer gauge |
| Pulse signal | Similar to alternator pulse signal, converted via an electrical gauge to provide RPM information |
| Magnetic inductive pickup | Least common, most commercial, generates a pulse |
| Automotive tachometer | Works off the ignition system and is "cylinder based" |
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What You'll Learn
Tachometers display the RPM (revolutions per minute) of the engine
There are several types of tachometer, based on the sensing system with the meter. The generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading. The inductive tachometer has an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head.
On some engines, the tachometer is mechanical. There will be a gear drive on the engine that will spin a cable, and transfer the rotation to the tachometer gauge, so that it can be read. A slight addition to the mechanical approach is to use the mechanical rotation of the gear drive to generate a pulse signal, similar in nature to the alternator pulse signal, which can then be converted via an electrical gauge to provide RPM information. The least common, and most commercial, is engines that utilize a magnetic inductive pickup, usually on the flywheel, to generate a pulse.
Outboard tachs operate by counting the pulses from the stator (charging system).
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Tachometers are essential for monitoring engine speed
There are several types of tachometer, based on the sensing system with the meter. The generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading. The inductive tachometer has an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head.
On some engines, especially in commercial applications, the tachometer is mechanical. There will be a gear drive on the engine that will spin a cable, and transfer the rotation to the tachometer gauge, so that it can be read. A slight addition to the mechanical approach is to use the mechanical rotation of the gear drive to generate a pulse signal, similar in nature to the alternator pulse signal, which can then be converted via an electrical gauge to provide RPM information. The least common, and most commercial, is engines that utilize a magnetic inductive pickup, usually on the flywheel, to generate a pulse.
Outboard tachs operate by counting the pulses from the stator (charging system). If the auto tach will recognize the very different pulse shape then you can do the math to equate the number of cylinders on an auto engine to 6 pulses per rev from most outboard stator systems.
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There are several types of tachometer, based on the sensing system with the meter
On some engines, especially in commercial applications, the tachometer is mechanical. There will be a gear drive on the engine that will spin a cable, and transfer the rotation to the tachometer gauge, so that it can be read. A slight addition to the mechanical approach is to use the mechanical rotation of the gear drive to generate a pulse signal, similar in nature to the alternator pulse signal, which can then be converted via an electrical gauge to provide RPM information. The least common, and most commercial, is engines that utilize a magnetic inductive pickup, usually on the flywheel, to generate a pulse.
Outboard tachs operate by counting the pulses from the stator (charging system). If the auto tach will recognize the very different pulse shape then you can do the math to equate the number of cylinders on an auto engine to 6 pulses per rev from most outboard stator systems.
By far the most common method employed in providing a tachometer signal is utilizing the alternator’s revolutions to generate a discernable indicator. Alternators generate power fundamentally as AC (alternating current) that is then rectified to DC (direct current) that can then be stored in the batteries.
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Tachometers can be mechanical
The function of a tachometer is to display the RPM (revolutions per minute) of the engine. Typical RPMs will be in the 10s of hundreds, so the method for counting the revolutions must be quick and accurate. By far the most common method employed in providing a tachometer signal is utilizing the alternator’s revolutions to generate a discernable indicator. Alternators generate power fundamentally as AC (alternating current) that is then rectified to DC (direct current) that can then be stored in the batteries.
There are several tachometer types, based on the sensing system with the meter. The generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading. The inductive tachometer has an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head.
Outboard tachs operate by counting the pulses from the stator (charging system). If the auto tach will recognize the very different pulse shape then you can do the math to equate the number of cylinders on an auto engine to 6 pulses per rev from most outboard stator systems.
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Tachometers can also be magnetic inductive
Tachometers are used to display the RPM (revolutions per minute) of the engine. There are several types of tachometers, based on the sensing system with the meter. The most common method for providing a tachometer signal is to utilise the alternator's revolutions to generate a discernable indicator.
The generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading.
On some engines, especially in commercial applications, the tachometer is mechanical. There will be a gear drive on the engine that will spin a cable, and transfer the rotation to the tachometer gauge, so that it can be read.
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Frequently asked questions
A tachometer is a meter that displays the RPM (revolutions per minute) of the engine.
There are several types of tachometer, based on the sensing system with the meter. The most common method for providing a tachometer signal is to utilise the alternator's revolutions to generate a discernable indicator. The alternator generates power as AC (alternating current) that is then rectified to DC (direct current) that can be stored in the batteries.
Automotive tachs work off the ignition system and are "cylinder based". Outboard tachs, however, operate by counting the pulses from the stator (charging system) and do not depend on the number of cylinders the engine has.
A generator tachometer inputs a signal from a mechanically driven generator unit, which outputs an AC voltage proportional in amplitude to the speed, and this is then decoded by the tachometer. Variations in speed give a proportional change in output voltage, and therefore a change in meter reading.
An inductive tachometer has an inductive magnetic sensor that detects changes in magnetic flux as the teeth on a flywheel move past the sensor head.
