Installing a High Power marine alternator on your boat.

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.

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.  If you are having problems identifying yours this article will help you figure out which case size you have.  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.

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. 

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.



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 above and 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. 

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

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