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Boat Electrical Power Generation

by James W. Hebert

Most all outboard motors are equipped with electrical generating devices for producing current needed to run the motor itself, to re-charge the motor's starting battery, and to supply current for auxiliary loads. The auxiliary loads include the boat's navigation lamps, electronic accessories, other lighting loads, and auxiliary electrical motors used in pumps, windlasses or trolling motors. Supplying all of these electrical loads places considerable demand on the modern outboard motor. Engineers have responded with several different types of electrical generating systems on outboard motors.


All outboard motors generate electrical power by converting surplus mechanical energy into electrical energy. In an electrical sense, almost all outboard motors operate in the same manner. The engine is used to rotate a source of magnetic flux in close proximity to a series of coils. The rotating magnetic force induces an alternating current flow in the coils. This current is rectified and converted to a pulsating direct current. On smaller motors there may be no regulation of this current, and it is connected to an external battery which absorbs the current charge and at the same time also acts as a crude regulator by holding the voltage down to a reasonable level. On larger motors which have much greater electrical current output, there is normally a voltage regulator included to hold the output voltage below about 14.5 volts maximum.

Generation of electrical energy from the mechanical energy of the motor robs some of the horsepower of the engine from the propeller shaft. Mechanical power (horsepower) and electrical power (watts) are related:

1 horsepower = 750 watts

In a 12-volt electrical system the voltage output of the charging system is approximately 15-volts, and thus 750 watts represents about 50 amperes of current. If the conversion process were perfectly efficient, it would be possible to generate 50 amperes of electrical current with just one horsepower of an outboard motor's mechanical energy. In practice, electrical generation is not that efficient, and consumes more horsepower.

There are two different mechanical arrangements of coils and magnetic flux sources traditionally used in outboard motors. For many decades the most common method of generating electrical energy has been to use permanent magnets attached to the engine flywheel which rotate above a series of coils arranged beneath the flywheel, called a permanent magnet alternator or often called a magneto. More recently, automotive style alternators (which do not use permanent magnets and are) driven by belts and pulleys have become common. They are usually specially adapted for use in outboard motors, but otherwise are very similar to automotive-style alternators.


The most common electrical current source on outboard motors is a magneto with coils and magnets under the flywheel of the outboard motor. A rectifier and regulator are located elsewhere. The under-flywheel magneto is an integral part of the outboard motor itself, and is not a bolted-on accessory. A magneto generates electrical current using permanent magnets rotating on the flywheel which induce current into a series of fixed coils. The design of the components determines the electrical output available. All of the following influence the capacity:

Historically the electrical demands of outboard powered boats were modest, so for many decades the magnetos provided with outboard motors were similarly modest in their electrical generating capacity. One should not infer, however, that because of this there is anything particularly limiting about the magneto approach to electrical power generation. In fact, quite the opposite, it offers a number of advantages.

Most significant, the magneto is truly self-powered. The magnetic force locked into its permanent magnets provides a self-exciting electrical power generating system. Thanks to improvements in magnet technology, it is now possible to create permanent magnets of astonishing strength. (In fact, the magnetic power contained in a modern under-flywheel magneto is so great that precautions must be taken. The flywheel must be carefully lowered onto the top of the engine using special tooling to maintain alignment, otherwise one could easily crush a finger if the flywheel jumped into contact with other engine components due to its extremely strong magnetic attraction.)

In addition, the rotating mass or weight of the magnets can be used as part of the flywheel weight. The magnets need not add any additional weight to the engine, as they can replace weight that otherwise would have been used in the flywheel. Also, the use of an under-flywheel magneto saves valuable space under the cowling.

The rotating part of the magneto is built into the flywheel, and it is driven directly from the motor's crankshaft. Thus there are no losses associated with pulley, belts, and idlers. Note that on a two-stroke engine this is particularly significant, as there generally is no need for a belt and pulley drive system to operate cam shafts, water pumps, oil pumps, or other components of the engine itself.

The rotating magnets pass over and close to a number of non-rotating coils (the "stator"). In a magneto there are no rotating coils or wires, so there is no need for any rotating electrical connections. There can never be a problem with "brushes" or other rotating electrical connections. The fixed coils of a magneto can be arranged in any number of poles (subject to the constraint of the space available under the flywheel), so a multiple pole magneto is possible. The more poles used, the higher the frequency of the pulsating current that is generated.

The rectifiers employed in the magneto do not have to be located within the coil assembly, and in practice they usually never are. In fact, it has been common for many years to locate the rectifiers separately from the coils. This arrangement has many advantages, too. The solid-state rectifiers of any electrical current generating device are more likely to be a source of failure than the actual wires and coils associated with them. By positioning the rectifiers in a more accessible location, they can be easily replaced if necessary, often in a matter of moments and with simple tools. Also, as rectifier current capacity increases, dissipation of heat become more important. It is common now to find that the under-flywheel magneto uses water-cooled rectifiers and regulators, and such cooling is very easily obtained in an outboard motor.

The electrical power produced by an under-flywheel magneto is proportional to the speed of rotation of the flywheel. The number of turns in the stator coils is chosen to produce a suitable output voltage at a certain engine speed, typically enough to produce effective battery charging when the motor is running at idle or fast idle speeds (700-1000 RPM). As engine speed increases the voltage output tends to increase, as well as the current available.

The drawbacks to the magneto are mainly in its location under the flywheel. There is a limit to the space available in most cases. In addition, the flywheel is usually mounted atop the engine, an area of considerable heat. Overheating of the wiring in the coils can be a problem, particularly if they are absorbing additional heat from the engine. Servicing the coils of the magneto can be difficult, as it almost always will require removal of the engine flywheel. In some cases, the charging current coils are packaged with other electrical coils and must be replaced as a unit. These two factors make servicing a marine alternator expensive.

Because the magnetic strength of the permanent magnets is fixed, there is no way to regulate the power generated in the coil. Voltage regulation is usually accomplished with a regulator placed after the rectifier. The regulating technique used is often a "series-regulator" technique, in which excess electrical energy is consumed as heat in the regulator. In larger engines with larger electrical output, this often means the regulator must be water cooled. Modern magneto systems can include more sophisticated regulators which can work more efficiently.


In the past decade or so, automotive-style alternators have been adapted for use in outboard motors. An automotive-style alternator is characterized by being driven by a belt and pulley from the motor crankshaft. They also usually contain the rectifier and regulator components in the same enclosure. Hereafter I refer to these specially adapted alternators as "automotive-style alternators", even though they are intended for marine use. There are some significant electrical and mechanical differences between an under-flywheel magneto and an automotive-style alternator

A belt system for powering accessories from a crankshaft pulley is very common on four-stroke motors. In some motors, the belt may be a crucial component itself, as it also turns camshafts, pumps, or even a supercharger. However, in a two-stroke outboard motor there generally is no need for a belt driven system to drive other components of the motor itself. An exception to this is the Optimax two-stroke engines, in which a pump for boosting air induction pressure is operated from a belt driven pulley.

Most automotive-style alternators also contain the rectifier and regulator components within the alternator itself. Replacement or service of the rectifier or regulator is usually not possible, and, in the case of damage, the whole automotive-style alternator is replaced or rebuilt. Rebuilding of automotive-style alternators for vehicle applications is common, but this service is not as easily obtained in the marine industry. Cooling for the electronics is provided by air flow generated by a small fan attached to the automotive-style alternator drive pulley.

An automotive-style alternator is not self-exciting. Although there may be some residual magnetism in its rotating components, an alternator normally requires some initial current to spur it into operation. Once excited and rotating, it can generate its own exciting current. To start operation, however, a small current is supplied by an associated external battery (which is usually the starting battery for the motor).

Most automotive-style alternators have three non-rotating windings ("stators") which supply the output current. The interconnection of these is fixed by the internal wiring of the automotive-style alternator. In most cases the individual windings are connected directly to their own rectifiers, and the rectifier output is connected in parallel, producing a three-phase pulsating direct current. The automotive-style alternator has a rotating coil ("rotor"). Current to this coil is supplied by a rotating contact mechanism ("brushes"). Output current in the stator is proportional to current in the rotor.

Regulation of the generated voltage and current is usually accomplished by an integral regulator. Regulation is accomplished by controlling the smaller current in the rotor. The automotive alternator can be thought of as a current amplifier, with the output current being proportional to the rotor current, and the mechanical energy of the engine turing the rotator being converted into electrical energy in the stator coils.


An automotive-style alternator regulates its DC output voltage by varying the AC voltage being generated. It does this by varying the current supplied to the rotor, which in turn adjusts the output current in the stator. On the other hand, a magneto does not regulate the AC voltage being generated, as this is fixed by the strength of the magnets and the design of the coils. Instead it controls the DC output voltage downstream of the rectifier. In the past inefficient series regulator circuits wasted excess voltage as heat. Modern regulator circuits can be more efficient.

In the under-flywheel magneto the windings of the stator are chosen to produce sufficient voltage output to be able to provide battery charging current beginning at idle speeds. In the case of an automotive style alternator, the rotational speed of the alternator is often adjusted by choice of pulley diameters so that output begins to be useful at fast idle speeds. Note that it is common for the automotive alternator shaft to rotate faster than the engine crankshaft. As outboard motor engine speeds approach 6,000-RPM, this can lead to some very high rotation speed in the alternator.

With an automotive style alternator, the connection of the internal coils and their rectifiers is not reconfigured as engine speed changes. As a result, the amount of electrical power available may not be optimum at all engine speeds.

In under-flywheel magneto, it is possible to electronically re-configure the wiring arrangement of the multiple coil windings as engine speed varies. At low engine speeds the windings can be connected in series so that they begin to quickly generate voltage at relatively low engine speeds. This allows them to generate enough voltage to allow the outboard to be rope-started without a battery attached. It also provides sufficient voltage output to begin battery charging at relatively low engine speeds. However, as engine speed increases the coils tend to become saturated, and their current output becomes limited. At this point it is possible to electronically change the interconnection of the coils, placing them in parallel. This provides a significant boost in current available at higher engine speeds, producing greater output current from the alternator. Bombardier is using this innovative technique in the E-TEC outboard motor. Below 1800-RPM the alternator windings are connected in series to maximize voltage output. Above 1800-RPM they are shifted to parallel to increase current output.


For many decades almost all automobile electrical systems have been operated at 12-volt DC. Boat electrical systems have followed this practice. As electrical loads increase, it becomes more efficient to raise the system voltage and use lower current. In automotive applications it is anticipated the system voltage will be raised to either 24-volts or 36-volts in the future. Some outboard motors have already changed to a higher voltage electrical system. The E-TEC engines from Bombardier/Evinrude have an under-flywheel magneto with a 55-volt output. This higher voltage is used to directly operate many engine electrical components such as the fuel pump, the oil pump, the fuel injectors, and spark ignition coils. By using pulse-width modulation voltage regulator techniques, the 55-volt source can be converted to a lower average voltage. If the voltage is chopped on and off at a high rate (10,000-times per second), by only providing the full 55-volts in one of every four pulses, the average voltage drops to 1/4 X 55 = 13.7 Volts (i.e., the nominal "12-volt" system voltage at full battery charge). Thus a 12-volt output can be created.


The electrical demands of the outboard motor itself vary considerably. Some cannot be operated without substantial electrical current, while others have much lower requirements.

All outboard motors require electrical current for generation of the ignition spark. In most older two-stroke motors this current is self-generated by a separate winding in the under-flywheel magneto.

Almost all modern motors have electronic controls or electronic ignition timing devices, and these devices require electrical power to operate. Again, in most two-stroke outboard motors this current is self-generated, often from a separate coil under the flywheel.

If the motor uses fuel-injection there will be substantial additional demand for electrical current. Fuel-injected engines almost universally have an electric fuel pump which supplies gasoline under pressure to the fuel rail and injectors. The fuel injectors are also electrical devices, and must be supplied with current to operate. Some engines also have additional air injectors (like the Mercury OptiMax, for example), and these require additional electrical current to operate. (In the Mercury OptiMax engine the air pressure for the air injectors is raised by a mechanical pump, but the injectors themselves operate electrically.)

The degree to which a motor can self-generate the electrical power needed to run can be an important consideration. One test for this desirable characteristic is the ability of the motor to be started and run without a battery attached. In some motors the current needed for starting is very small and can be made available from current stored on a capacitor. This will provide enough energy for the very brief interval until the motor begins to self-generate electrical current. In other motors, the starting current demands may be higher, and the motor may not be able to self-generate all of the electrical energy need to start. In such motors a source of external electrical current (such as a well-charged battery) is needed to permit the motor to operate. Motors of this design may be impossible to start without having a battery attached. Once started, it is desirable that the motor be able to self-generate all the electrical current needed for operation. This makes operation of the motor independent of the state of charge of an attached battery.

If the motor is dependent on an attached battery for starting, there may also be special requirements for sustaining a certain minimum voltage. The load of the electrical starting motor often causes a sag in voltage, and some motors may not be able to start if the battery voltage falls below a certain level, even though the engine is cranking and turning over. Motors of this design have an extreme dependence on an attached battery for starting. In some motors, not only is a battery required for starting, but there are very severe electrical demands placed on the battery, requiring it to have a very large capacity (in some cases a minimum of 1,000 Amperes of cranking current).

Most motors have electro-hydraulic trim and tilt systems. The electric motor of the hydraulic pump will consume significant current (10-20 Amperes) when operating. These devices do not consume electrical energy when not being operated. During engine starting they usually are not consuming any electrical current.

Any additional electrical current generating capacity in excess of that needed to run the motor itself can be made available to supply additional loads. In some cases, the motor must be operating at a particular speed before any excess current is available. Typically, once the motor reaches an idle speed of about 600-700 RPM it will begin to produce surplus electrical power.


Recharging the starting battery is the primary demand for any excess electrical power available in the motor. A battery is only a storage device, so electrical energy used in starting a motor must be quickly replenished by charging current from the operating motor. The amount of surplus current available for battery charging varies with the motor's electrical design and its own loads. The rate at which the battery will accept recharging current varies with the type of battery, its state of charge, and other factors.


The modern boat can have many auxiliary electrical loads. Electrical pumps for clearing water from sumps are very common. In some boats these are the most essential auxiliary load on the boat! Another primary load are the electrical lamps used to provide navigation lights when operating at night. In addition, most boats have VHF Marine Band transceivers for communication and safety. They also typically have electronic accessories like depth sounders, navigation receivers, and even RADAR systems. More electrical pumps for circulating water are also common. In some cases, auxiliary propulsion is provided by electrical motors. These trolling motors can be very high consumers of electrical energy. Typically an electrical trolling motor is used when the main outboard motor is not operating. The electrical trolling motor draws its power from a battery (usually a separate battery from the engine starting battery), and this energy must be quickly replaced once the main engine is started.

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Copyright © 2015 by James W. Hebert. Unauthorized reproduction prohibited!

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