Design, installation and troubleshooting of marine electrical systems            PETER KENNEDY YACHT SERVICES
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Installing a High Power marine alternator on your boat.

This article was written by Peter Kennedy of Peter Kennedy Yacht Services in Annapolis MD and is designed to help you select and install a new high power alternator to replace an existing one on your marine engine. The text editor was Rich Leon.

Arguably, there is no more worthwhile electrical project you can undertake on your boat to improve comfort and convenience than the installation of a high power alternator.  Having a high power alternator will provide you with a substantial improvement in the availability of electrical power on board. This will lead to greater safety, better ability to run electronics, better lighting, colder beer and the ability to run a watermaker, windlass, inverter and other high consumption devices.  It is not the only component that matters in a fully functional marine electrical system, other parts will be addressed in later articles, but in many cases the alternator serves as the foundation of a modern high performance system.

Balmar dual foot mount large case high output alternator with double pulley and external fan.

This drawing courtesy of Balmar shows a typical high output large case alternator.  This model has a dual foot mount and a dual pulley for ½” belts and an external fan.

A typical small to medium marine diesel has a standard alternator with a rating of 55 amps; however this rating is obtained under the most favorable conditions.  The typical sustained output from these stock alternators is 25 – 30 Amps.  It is easy to see how the addition of a 100 Amp high output alternator which has an actual output of 100 Amps could more than triple the amount of power available on board and substantially reduce engine running time. 

Fortunately installing a replacement alternator is not difficult. It does involve some basic electrical and mechanical skills.  I have attempted here to give quite detailed instructions and to cover all of the bases making the text that follows a bit long, but at the end of the day the job is not that difficult.  To get started you first need to identify what kind of alternator you currently have.

Case size

Most smaller 4 and 6 cylinder diesels have what is known as a “small case alternator”.  The diameter of these alternators is about 5 ½” and the length including the pulley is about 6”. They often have a single pulley for one belt although the more powerful ones will have double pulleys or a serpentine belt.  Bigger engines normally have “large case alternators” which tend to be about 7” in diameter and about 7” long and having a double pulley or pulley for a serpentine belt.  There are also “extra large case alternators” available for the biggest engines.  The photo below shows the typical small case alternator found on a Yanmar

Mounting type

The next think to observe is the mounting type.  You need to see how the alternator is attached to the engine at its pivot point.  Does the alternator have a single foot or does it have two?

Dual foot alternators:

Yanmar engines have alternators with two feet, known as a “dual foot mount”.  The standard spacing between the feet is 3.15”, this is also known as a “Yanmar mount”. This standard dual foot mount is also often found on Mercruiser and Mitsubishi engines as well as others.  Many Westerbeke engines have alternators with two feet that are spaced closer together; the spacing between the feet is a bit less than 2”.  Larger alternators may have what is known as a J-180 mount with dual feet that are 4” apart.  These are often used on larger Cummins, Detroit and Caterpillar engines.

Balmar dual foot mount small case alternator with single pulley

A small case dual foot alternator for use on a Yanmar or similar engine.  The spacing between the feet is 3.15”,  This model has a single pulley and has multiple places to attach the adjustment arm. It has an internal cooling fan.  Drawing courtesy of Balmar. Take a look at my Blog Post Installing a Balmar alternator on a Yanmar 3JH3E for a description and photos of an actual project. 


Single foot alternators:

Single foot alternators fall into two categories, 1” foot and 2” foot mounts.  The 1” or 2” refers to the dimension of the foot itself measured fore and aft.  You will just have to look and measure but in general the 1” foot is used on some Caterpillar engines as well as Universal, Crusader, Atomic 4 and some Westerbeke’s.  The 2” foot is found on many Volvo’s, Perkins and Lehman engines. 

Balmar small case alternator with single 2" foot mount, internal fans, and a single pulley

This photo courtesy of Balmar shows a small case alternator with a single pulley and a single 2” mounting foot.  There is no way to be sure of the mount without looking carefully at what you already have, there are just too many variables out there.

Mounting bolt 

The next thing to look at is the mounting bolt that attaches the alternator to the engine at its pivot point.  The larger alternators have ½” diameter mounting bolts while most intermediate alternators have a 3/8” or 10 mm diameter mounting bolt (Note that these diameters are almost identical).  The smallest alternators have a 5/16” or 8 mm mounting bolt. (Again these diameters are almost identical)  It is important to get an alternator with the right size hole for the mounting bolt otherwise it wont fit or will wobble, it is usually possible to order a bushing to adapt to another size bolt, don’t forget to ask when ordering.


Serpentine pulley

Finally you need to look at the pulley:  If your alternator has a single pulley you need to know the width of the belt.  This is measured across the top of the belt, the widest part.  Most single pulley alternators have a ½” belt, smaller ones have a 3/8” belt, and you will occasionally find some wider single belts.  You may also find double belts or a serpentine belt.  These are designed to transfer greater loads and have greater surface area in contact with the pulleys.  If you have a serpentine pulley you need to count the number of grooves in the pulley and the overall width of the belt. The picture to the right shows an alternator with a single 1/2" drive belt, it also shows the alternator pulley perfectly in alignment with the engine pulleys.  If you have a single 1/2" drive belt like this you are limited to an alternator of around 100 Amp output and if the belt is 3/8" diameter you are limited to an alternator of about 70 Amps output.  These are not hard and fast rules and occasionally I have installed a single belt alternator of 120 Amps with a single 1/2" belt but to do that you have to be prepared to monitor wear on the belt more closely and be prepared to replace the belt more frequently.  What about if you need more output than 100 Amps and are stuck with a single pulley system?  One option is to use the Balmar Serpentine Conversion kit to change your engine over to serpentine pulleys.  This clever arrangement allows you to fit new pulleys over the top of your existing crankshaft and water pump pulleys allowing you to install a much larger alternator.  When used in conjunction with the Balmar AT-Series Alternator it allows you to install a 200 amp small case alternator on many small boat engines

Other mounting considerations 

Occasionally there are other things to look at.  Is there sufficient room behind the alternator in case the replacement is slightly longer?  Will the pulley line up with the old pulley?  Will the adjustment arm fit the way the old one did?  These are snags that don’t normally occur and have to be dealt with on a case by case basis.  Before purchasing a replacement alternator it is worth discussing any unusual or unknown aspects of the installation with a professional. The photo below shows the Balmar Belt Buddy adjustment arm with tensioning device.

Electrical considerations 

All of the remaining part of this article refers to “P” type alternators, where a positive voltage is applied to the alternator field coils.  It is possible to have the opposite, an “N” type alternator, with a negative connection to the field coils, but they are extremely rare. 

Apart from noting the alternator voltage you should also look to see if you have an isolated ground engine; these are sometimes found on engines with saildrives or on engines with electronic controls.  To determine if you have an isolated ground engine configuration just check for continuity between the engine block and your battery negative terminal using a multimeter.  If you have continuity then it is not an isolated ground engine and you can use any alternator, otherwise you have to use an special isolated ground alternator. 

Alternator output curves showing output at different RPM

Sample output curves for alternators at various RPM.  Note that the RPM we are talking about is alternator RPM, typically 2.5 times engine RPM.  Typically alternators have their full output rated at 6000 RPM but can continue to spin up to 12,000 RPM or more without any additional increase in output.

Choosing your alternator

Now that you have thoroughly examined what you currently have you are ready to select an alternator. You need to start with an appropriate voltage, case size and mounting foot. Then you have to select an output size based on your electrical demands, battery size, and the pulley type you have.  If you have only a single 3/8” wide belt you will be limited to about 70 amps for a 12 volt alternator.  If you have a single ½” belt you will be limited to about 110 amps for a 12 volt alternator.  Everything more than 110 amps for a 12 volt system will require either double belts or a serpentine belt.  The picture below shows you the wide selection of alternators on sale at shop.pkys.com

When considering an alternator size you can apply a simple rule: the alternator output in amps multiplied by 4 should match the size of the battery banks in amp hours.  So a 70 amp alternator would work well with a 280 amp hour battery bank.  This is a very general rule so don’t worry if you bank rating exactly.  If your alternator is too big it won’t hurt anything but it may not be able to work at its full potential.  If your alternator is too small it will just take a bit longer to charge the bank fully.

Choosing your regulator

Balmar ARS5-H regulator

Most high power alternators are designed to be used with an external regulator.  An external regulator enhances the alternators performance by allowing the user to make adjustments to compensate for battery type, temperature and other important operational parameters. Some regulators have digital displays that will tell the user what is happening, others use more cryptic indicator lights. Some regulators allow for remote voltage sensing at the batteries for increased accuracy and allow for temperature sensors for alternator and batteries.  The alternator temperature sensor prevents the alternator from overheating in the event of sustained high-load use.  The battery temperature sensor modifies the output of the alternator depending on battery temperature; a higher voltage for cooler batteries and vice versa.  Also worth serious consideration, having a battery temperature sensor in the system can prevent a battery meltdown in the event of a bad cell. The video below shows an ARS regulator in action



Installing the alternator

I normally like to start with the physical installation of the alternator on the engine.  It should be a case of just bolting it in, but occasionally there can be small problems.  Often you will have to find a longer mounting bolt than the original one.  It is customary to use high-tensile steel for these bolts.  Occasionally you will have to make some changes to the adjustment arm.  You may also need some spacers or washers to ensure the pulleys line up perfectly.  In most cases though, if you have carefully selected the correct alternator, it should be a straight fit with few complications. Don't forget to read my blog post on the subject of installing an alternator to get a description of an actual project.

Installing the regulator

All regulators basically follow the same wiring scheme with only minor differences between them.  There is a negative wire (sometimes two) that is usually black.  It goes to the alternator negative connection.  There is a positive wire, usually red, that goes to the alternator’s positive output connection and which should have an appropriate fuse in it at the alternator end (normally 10 amps or less on a 12 volt system).  There is a wire that is used to turn the regulator on, usually brown.  This can normally go to whatever wire in the old installation was energized whenever the engine was running.  And finally there is the field wire, normally blue, that is the output of the regulator and goes to the alternator field connection.  This wire provides a variable voltage to the rotating part of the alternator and thus determines how much power it generates.

Wiring the alternator

You will probably need new positive and negative output wires for your alternator because it is unlikely that the original wires were sized appropriately for the new increased output.  If you have an isolated ground alternator there will be no electrical connection to the engine block and the negative wire will have to carry the full output of the alternator.  Even if you do not have an isolated ground alternator the connection between the alternator and the engine block and is not a good one and is not capable of carrying high loads. A dedicated negative output wire needs to be installed.  In the case of non isolated ground alternators this wire can run to the engine negative connection where it connects to the battery.  For isolated ground engines it needs to run to the battery.  The positive wire needs to run to the battery too of course, there are various options for this that will be covered in a future article.  No matter which way the positive output wire is run it needs circuit protection at both ends in order to comply with ABYC standard E11.   This circuit protection should be higher than the alternator output to prevent accidental blowing of the fuse, but no more than the current carrying capacity of the wire.  Follow the alternator manufacturer’s instructions for the output wire size and distance to the batteries.

If your alternator provides a signal to the tachometer then you will have to connect up a wire for that too.  This is usually the white wire leading out of the alternator and comes from the stator.  It provides a varying frequency signal related to the rotation speed of the engine.  You will have to figure out which of the original wires that went to the old alternator to connect this to.  You may also have to calibrate the tachometer.  This requires a handheld optical tachometer for an accurate reading but a rough and ready approach would be to note what speed the engine idles at and what speed the redline is at no load before you start the installation, then adjust the tachometer accordingly on completion. There are normally a limited number of choices and only one will give a reasonable answer.

Final adjustments and testing

When the alternator is physically installed, its output wiring connected and its regulator programmed you are almost ready to start the engine.  First make sure the belt is tensioned adequately, then check at the alternator output terminal that you can read battery voltage, alternators won’t work if they aren’t connected to a battery.  Make sure loose wires, clothing and limbs are clear of moving parts and then take your new alternator for a test drive.  If you have installed a new belt you may need to retention it after about 30 minutes use.  Check belt tension regularly.

Advanced programming options, troubleshooting and maintenance

Many regulators offer advanced programming options apart from the basic ones for different types of battery.  It is possible to “detune” and alternator that is too powerful by adjusting the software.  This is useful if you find that high loads are causing excessive wear on your belts of if you worry that the alternator might take too much power from the engine.

Troubleshooting an externally regulated alternator is easy. First make sure you haven’t blown any fuses by ensuring that you can still see battery voltage at the output terminals of the alternator.  Then test to see if the alternator is working by momentarily connecting a positive voltage to the field terminal. Be careful not to get in contact with any moving parts. There should be a very noticeable slowing down of the engine as the full load of the alternator is applied.

If the alternator works then test the regulator. If it is the type with a digital display it will tell you what it is doing.  Otherwise is it lit up and turned on? If not are you getting voltage on the power wires, the voltage sense wire(if fitted) and the brown wire that turns on the regulator?   If all of these are working you should get a variable voltage on the field wire output (normally blue) of the regulator, otherwise the regulator is defective.

Alternator installations are pretty much maintenance free except for changing the belt annually and keeping correct belt tension.  It helps to keep the alternator clean for better cooling, and a periodic test of its performance will help to spot problems in advance.  Many times what appears to be an alternator problem is caused by bad batteries.  This will have to be the subject of another article.  In the meantime, happy sailing and enjoy that cold beer; you deserve it.

Further reference:

Balmar's 12 volt alternator installation and operation manual

This gives even more detail than my article above and includes the following topics either not covered by me or in more detail than I have given: 

Tach Manufacturer websites:

Belt Manufacturer websites:

Alternator and regulator sales:

Written by Peter Kennedy.  Text editor Rich Leon.  © Copyright Peter Kennedy 2008  Updated December 2014  All rights reserved.


ABYC Certified Marine Electrical Technician


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    Marine Electrical Systems

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