10 Tips To Install An Alternator


Alternators For Live-aboards Are Different

Before we start, the key thing to understand is that an alternator that is adequate for a boat that is used for weekending and occasional cruises of a few weeks’ duration most likely won’t be able to handle the demands placed on it by full-time live-aboard offshore cruising.

So let’s look at what a real cruiser’s alternator is, and how to install it.

#1 Junk the alternator that came with your engine

Yes, I know it’s a lot easier and cheaper to keep the alternator that came with the engine, and perhaps have it modified to use a smart external regulator, rather than buy and mount a new one. But the sad fact is that, even if you have modest power needs that the standard alternator can theoretically supply, it is not designed to supply high current in relation to its rating for extended periods, and if you ask it to, as most liveaboards will, it will give you endless trouble.

#2 You need a truly smart regulator.

More on what to look for in a regulator in the next chapter.

#3 You probably need a bigger alternator than you think

Even with a really good solar and wind installation, your alternator is going to get a hard workout on a regular basis if you have any sort of decently sized battery bank, and particularly if you have a fridge and/or freezer, as most live-aboards do these days.

And the larger the alternator’s output, the longer it will be able to do its job without failing and the less time it will take to charge your batteries.

For most live-aboards, 100 amps output is the absolute minimum and 180 amps is optimal.

My rule of thumb is that the size of the alternator measured in output amps should be 20 to 30% of the house battery size measured in amp hours.

This results in an alternator that will be able to charge the house bank from a 50% state (the most you should discharge your batteries) to 80% charged (the point where the battery acceptance rate starts to drop off and alternator size no longer matters) in 1 to 1.5 hours.

An example of why I picked this percentage is that an alternator so sized will, after a night at anchor, charge your batteries to the point that they will support a day of sailing as you get the anchor up and motor out of the anchorage. And much of your day’s usage will be put back as you reverse the process in the evening. Said sizing will also allow you to recharge at sea relatively quickly.

Finally, a larger alternator will load the engine at a higher level than a smaller one, which is better for the engine if you need to run it in neutral to charge.

#4 Down-rate your alternator

Building on tips #2 and 3, if you oversize your alternator and get a good user configurable regulator, you can down-rate the alternator output. We configure our regulator so that the maximum current (amperage) the alternator will supply is 80% of its rating. We have found that this about doubles the alternator’s life.

#5 Get a large case alternator

Sure, small case alternators are easier to fit—much easier—but they will also fail more quickly because they run hotter at a given output. If there is any way at all to make it work, get a large case alternator and have a good machinist modify the mounts to fit it. And this goes double if you need 120 amps or more.

#6 One alternator is better than two

I know many pundits advise keeping the original alternator to charge the engine start battery and fitting a second larger one for the house batteries, but there really is no benefit in complicating things this way and a lot of downsides including:

  • You will be kludging a mount for the second alternator on a part of the engine that was never intended for that purpose, instead of just modifying and beefing up the original mount.
  • And don’t even think about mounting the second alternator off the engine. You will have endless problems and will probably screw up the alignment of your engine into the bargain.
  • The original alternator will not be powerful enough to act as a viable backup for the house alternator anyway.
  • You are complicating and junking up a mission critical system: your engine. Remember, simple=reliable.

And while I’m on the subject, don’t hang a refrigeration compressor, AC generator or a watermaker pump on your engine. When we bought our McCurdy and Rhodes 56 her engine had two alternators and a refrigeration compressor hung on it. What a complicated unreliable mess that was! A good large case alternator will easily power all that stuff and charge the batteries.

It used to be that if you had one of those engines, more common than not, that uses the same belt that drives the alternator to drive the water pump, you didn’t have a choice about adding a second alternator, at least if you wanted two belts, but now a serpentine belt kit will solve that problem (see below).

With a single alternator, you do need to split the output to the engine and house battery banks so that they remain isolated from each other except when charging. The way to do this is to wire the alternator primarily to feed the house bank and then use one of these automatic charging relays to feed the engine bank after the voltage comes up. We have been wired this way for 20 years with no problems.

By the way, don’t use a battery isolator that relies on diodes. These devices cause a voltage drop that will prevent your batteries ever being properly charged.

#7 Brand doesn’t matter much

Despite the endless debate on the forums about which alternator is best (Balmar, Electromaax, Leece-Neville, etc.), my guess is that there isn’t much to choose between them, particularly if you go large case.

After all, I think I’m right in saying that none of these companies actually make their own alternators. All they do is re-label generic alternators. (I know that Balmar’s used to be made by Lestek, who I think went out of business.) Probably they’re all made in the same plant in China now.

So my advice is: negotiate your best price, make sure you are comfortable with the supplier’s support, and move on.

Having said that, if you want the ultimate alternator, get one of these from Electrodyne. Here is Steve Dashew’s take on them. The problem is that they are too big to fit on most yacht engines.

#8 You need a spare

We have found that a large case alternator, down-rated to 80% of maximum output, lasts 1500-2000 hours of heavy live-aboard use before the coils or bearings fail, or both. This has been a remarkably consistent statistic over about 12,000 engine hours on our boats. I would guess that you will want to plan on about half that for a small case alternator, particularly if it’s not down-rated (see above). The nice thing is alternators usually give you warning, with bearing-squeal or slowly dropping output, before dying completely.

When we detect either symptom, we swap in our spare—less than 30 minutes’ work—and have the old one rebuilt by a reputable alternator professional.

Don’t get Bubba at your local Dufus Everything Electric Repair to do this. Check with the company that sold you the alternator for a good rebuild outfit. Done this way, a rebuilt—which should include complete rewinding and new bearings—alternator really is as-new. Alternators are one of the few things we rebuild, rather than replace.

By the way, don’t buy one of those cute alternator spare parts kits with a couple of brushes and some diodes that the alternator companies love to sell. If you have a spare alternator you don’t need them and, anyway, I have never seen either of those parts fail.

#9 You need two belts

Pretty much any alternator that will be subjected to the loads of live-aboard cruising needs two, preferably 1/2”, belts. Yes, you might get away with one belt for an alternator with an output under 100 amps, but you will be constantly fiddling with it.

The best alternative is a serpentine belt. We had a belt like this on our old Cummins engine and it worked great for over 6000 hours, with only one belt change in all that time and no slipping. I have not tried one of these kits or these, but they look like a good idea.

#10 You need a way to accurately tension the belts

Make sure you and your machinist devise a good simple way to tension the belts accurately. You don’t want to be screwing around using a screwdriver to lever the alternator while trying to get the tension right and tighten the bolts. You will probably get the tension wrong, either too slack or too tight.

The former will cause belt wear and low and erratic charging and the latter will damage the bearings in the alternator or even (I’m guessing here) cause damage to the front end crank shaft bearing on a small engine.


The best solution that we have found for accurate and easy belt tension adjustment is a lug that slides on a threaded rod with a nut and a washer each side. The bottom bolt on the alternator bolts to this lug. With this system, perfect belt tension is a piece of cake.


Doing a proper job of fitting an alternator to the engine of a live-aboard cruising boat can be both expensive and a pain in the neck. It is also a task that is fraught with opportunities to swerve off the road to reliability: one V-belt, small case alternator, half baked mount…If you are tempted by any of these short cuts just say to yourself “do it right, do it once”…repeatedly, if necessary.

And, if you stay on this righteous path, you will be able to lord it over me, since I have made, at one time or another, just about all the mistakes mentioned above.

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  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. The Danger of Voltage Drops From High Current (Amp) Loads
  10. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  12. Battery Bank Separation and Cross-Charging Best Practices
  13. Choosing & Installing Battery Switches
  14. Cross-Bank Battery Charging—Splitters and Relays
  15. Cross-Bank Battery Charging—DC/DC Chargers
  16. 10 Tips To Install An Alternator
  17. Stupid Alternator Regulators Get Smarter…Finally
  18. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  19. Smart Chargers Are Not That Smart
  20. Do You Need A Generator?
  21. Efficient Generator-Based Electrical Systems For Yachts
  22. Battery Bank Size and Generator Run Time, A Case Study
  23. Battery Options, Part 1—Lithium
  24. Battery Options, Part 2—Lead Acid
  25. Why Lithium Battery Load Dumps Matter
  26. 8 Tips To Prevent Lithium Battery Load Dumps
  27. Building a Seamanlike Lithium Battery System
  28. Lithium Ion Batteries Explained
  29. 11 Steps To Better Lead Acid Battery Life
  30. How Hard Can We Charge Our Lead-Acid Batteries?
  31. How Lead Acid Batteries Get Wrecked and What To Do About It
  32. Equalizing Batteries, The Reality
  33. Renewable Power
  34. Wind Generators
  35. Solar Power
  36. Hydro Power
  37. Watt & Sea Hydro Generator Review
  38. Battery Monitors, Part 1—Which Type Is Right For You?
  39. Battery Monitors, Part 2—Recommended Unit
  40. Battery Monitors, Part 3—Calibration and Use
  41. Battery Containment—Part 1
  42. Q&A—Are Battery Desulphators a Good Idea?
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