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Date: 11 February 2018. Upsizing the Alternator - My Mistake.


Alert: This article is longer than usual. You might wish to pour yourself a beverage before beginning.

First of all, "everyone" says get a bigger alternator. I did so. Though my reasoning was correct errors were made. As with most things, there was not a single flaw that caused the problems I experienced. Instead a litany of Good Ideas morphed into mistakes. I was fortunate to catch this before a catastrophic failure incurred. Here is what I did wrong.

Everyone says it is a Good Idea to get a bigger/better alternator. So I did. Twice.

The original alternator that came with my 18hp Kubota engine was 15 amps. I wanted a faster charging system. My friend Rich over on the east coast sent me a 55 amp alternator. Thank you Rich.

This is the 55 amp ↓ alternator. I take pictures of the labels then transfer the information into my Log Book.



Spare Parts Inventory


Like most boaters, Rich had a spare alternator "just in case" something went wrong with his original. Usually what happens is we will have a failure. We buy the new part for immediate installation.

Starters, pumps and alternators are often rebuilt or repaired. 

Later when we have time we repair/rebuild the original. That rebuilt item becomes an addition to the Spare Parts inventory. Those of us on limited budgets can amass a decent number of backups over time.


This collection of spare parts can often be
attributed to a bunch of mechanical breakdowns.



In any event, I wanted the 55 amp alternator installed on my 18hp Kubota. I did not realize a 55 amp alternator has TWO BOLTS to adjust the tension. The original mounting used the top bolt only. This upgrade was the beginning of my alternator problems.

The bottom bolt is supposed to be long and attached from front to back of the alternator. Two anchoring points at the bottom secure the alternator in place. Undue stress was placed on that single bottom bolt because it only attached at the front.

The original alternator mounts with a single bolt at the top of the unit on the curved slider bracket/arm.

The 15 amp alternator was mounted to the engine with a single bolt.

The 55 amp was supposed to be mounted with two bolts spanning three attachment points. The one at the top pivots. That adjusts the belt tightness.

For a 15 amp alternator that's all you need. When I upgraded, the 55 amp alternator requires a second longer bolt to hold the unit in place on the engine.


The second bolt is at the bottom. You can see it just above the red oil filter on the right. →

That bolt is 4 1/2" long. It should attach to two places on the engine block. Instead it was bolted through one, and then through to the other side of the alternator. The back side is where the problem incurred.

On a larger engine there would be a place to attach a big alternator properly. That was not true for my Kubota 18hp tractor motor.

Because the initial install was
faulty my upgrade was doomed.


I did not notice that the original was wrong. My alternator was not attached front and rear at the bottom. Thus when I upgraded further to a 110 amp alternator the torque was simply too much.


You get what you inspect, not what you expect.

  • My first mistake was in not noticing the second bolt attachment on the original install was wrong. I should have seen that.

  • Conversely, the original mechanic should have said something too. Just because a client wants something, we depend on wise counsel too.

The Person to blame is often found in the mirror reflection down in my cabin.

Regarding the alternator fiasco, there is plenty of blame to go around. Ultimately however, it is the boat owner who assumes responsibility for any mistakes made. This alternator upsizing was a Good Idea as I understood it. The unintended consequences bit me on the transom.

Almost all failures are not the result of a single misstep. My problems were the culmination of error piled upon error. Regarding the alternator upsizing saga, I made several mistakes. So here is my cautionary tale of woe.

Fitting the alternator bracket was a problem. That was solved by having a new bracket welded.
See the previous article
Alternator Bracket Pattern (how to) for details.

For the record, the above photo was taken during the examination process. I never ran the engine with the alternator mounted like that.

An alternator has to mounted in alignment with the pulleys or belts on your engine. On my Kubota, the fan belt makes the water pump work. That keeps my engine cool. All these parts have to work in coordination. When one part goes belly up, disaster and breakdowns are likely.

This is the front of the engine looking aft. The 55 amp alternator is mounted.

Mistake Number One:
The 55 amp alternator took a lot of power from my small engine. She would bog down (RPM's decreased) and I lost 1.5 mph in speed when underway. That's when Edwin came up with a solution.

This is Edwin:

Edwin's solution was the addition of a switch. I turned off the power generation while underway. At anchor I could use the alternator. Of course running an engine at idle is not ideal.

The thing is this: I wanted to be able to run my air conditioner at anchor without a generator. I do not have enough solar for an a/c unit. It gets hot here in Florida so I spent a lot of time considering options. The upsized alternator seemed like a Good Idea at the time.

There was a catch though. Details follow...


Alternators are rated at their Maximum output. Thus a 55 can briefly put out 55 amps. That said, an alternator will "throttle back" when it gets hot. Generally speaking, figure a 55 amp alternator will roughly provide 25 amps from now until Sunday if your battery voltage is low.

Now this is a Rough Estimate. Of course with external regulators (above my budget range) you can expect better results, i.e. more amps in to your batteries. Nigel Calder knows all this stuff far better than I. He is also better at explaining it. If you have not yet bought Calder's Boatowners Mechanical and Electrical Manual 4the Edition, please do so. His tome is truly the Bible for boaters.

Every live-aboard boat owner I know has Calder's. Indeed, I would not be without my copy.

← Affiliate link.

This was my Theory: Because the 55 amp alternator could not put out consistently 55 amps, the theory was that a larger alternator (the 110) could put out 55 consistently. So if I upsize the alternator I could have air conditioning with just the cost of diesel. My Kubota sips fuel.

I need about 50 amps to run the air conditioner. Thus a 110 amp alternator could theoretically keep up with the power requirements of my Haier 5k btu a/c unit.  Putting out 50 amps would not overheat the larger 110 amp alternator.

So I went shopping!



I got a ride to Will's Starter and Alternator shop in St. Pete where I met Tony. Phone: 727-531-4667

Will's is the type of shop those of us who appreciate tradesmen will enjoy. They have the equipment necessary to test items. The men behind the counter are knowledgeable. I was amazed by the sheer number of starters and alternators in stock. Literally the walls were lined with them.

Will's Starter and Alternator has bunches of parts on the premises.

The facility also tests items. They will tell you if yours is okay or in need of a rebuild.

I was concerned that the 110 amp unit would have the same *Case and bolt pattern as the 55 amp one I was replacing. Tony (the fellow behind the counter the day I visited) assured me external dimensions were identical.

*Case: When changing your alternator you need to verify the outer dimensions of the new alternator matches what you are replacing. This is called the Case or Frame by professionals.

Will's Starter and Alternator does offer a guarantee. They also mark their units.



That is my newest double belted alternator (single belt installed)


Side Note regarding belts and alternators: When you get to 100 amps, you move from single belt to either a double v-belt as I purchased or serpentine style belt.

Serpentine belts seem to be the better regarded of the two styles.


It is absolutely NOT RECOMMENDED that you attach one belt and go with it. I did so anyway. That is because this was intended as a temporary measure until I could retool with the serpentine belt pulleys. Those are flat with ridges whereas my pulleys are v-shaped to accommodate the fan belts I use.

When using just one fan belt ALWAYS attach to the pulley closest to the hub. Please note that experts will tell you this is not ideal. You really do need to use both belts.


The bumpy parts of the fan belt are v-shaped. Thus the name v-belts. For me, and I'm not a mechanic, I simply call this type fan belts. It looks like those I used in my car all those years ago to make the fan blow on the radiator.


The plan was to replace the fan belt with a serpentine belt. Serpentine is a fancy word for a flat rubber belt that is approximately 1.5" (4.5cm) wide. Larger units would have wider serpentine belts.

The Fatal Flaw in my plan was discovered prior to pulley replacement.

Did you notice that every step seemed logical? Each decision made progress toward my goal of having the ability to run an air conditioner at anchor.

There were costs every step of the way. I was of course motivated to make it all work together. A bit of ego was involved too. I wanted to be right.

Boat projects tend to grow exponentially. They become more
expensive and take far more time than you imagined. Always!

The alternator bracket from the original 15 amp charger does not fit properly.

The original bracket could not accommodate the 110 amp alternator. The torque (spin strength) was too much for the smaller brace. That is one of the reasons I had a new stronger bracket made at Lisotte's Welding shop. The new bracket was installed.

So one Sunday I wanted a parfait from McDonald's. I untied and headed down the river. Along the way I flicked the switch to start the alternator charging my battery bank. All was well for about two minutes. Then I heard a sound. Nothing particular, just a "wrong" noise.

A quick glance out the side of my boat showed zero exhaust water. Quickly I pulled off the channel and anchored. I shut down the engine and hoisted my anchor ball.

When anchoring in the daytime you are supposed to display a black ball. That indicates you are anchored.

Very few boats display an anchor ball. It is the right and proper thing to do.


Anchor balls are the same size and shape as the radar reflector balls many sailboats use. I found an old radar reflector at a marine nautical flea market for $2. Then I spent $5 on a can of black plastic spray paint. Several coats of paint later my neighbor had his own anchor ball.

Mine is plastic and folds flat. His, a former radar reflector, will not fold.

Being at anchor just off the channel on Boca Ciega Bay in St. Pete is not the place to be on a weekend.

Every idiot was out there. Including me! For the record I almost never go anywhere on a weekend. The alternator fiasco is yet another reason why not. I knew better. It was the lure of a McDonald's parfait... I love those things.

McDonald's parfait ↑ that I doctored up with extra strawberries last year. A parfait is a real treat.

So I began troubleshooting. That is when I discovered the broken bolt. The bolt at the bottom of my alternator was supposed to be attached to the engine in two places (front and back) had sheared off. When I tried to loosen it I discovered the bolt was broken. A 3/8" stainless bolt sheared in two is not a good thing.

Then things got worse. A boat went right my my transom throwing one heck of a wake. I came up out of the bilge rubbing my sore spots and got on the VHF radio. I called the navy blue Mainship that had passed me so closely and said "Nice wake buddy".

Note to other boaters: do not tick off anyone who is in the bilge. We are already not having a good day and you do not want to annoy us by being stupid. But Mr. Mainship had never heard of that rule.

That's when Mr. Mainship put on his "instruct the woman on the radio attitude". Believe me when I say you will find that type on more than one occasion. He proceeded to tell me if I was at anchor I should put up a black ball so that Real Boaters will know I'm at anchor.

Can you feel my blood pressure rising?!?

Quickly I popped my head out the side and checked. Yep, mine was there. And flying high.

I got back on the radio and suggested he "Look up. I am displaying."

Radio silence. Not a word out of Mr. Expert.

The next boat slowed down as it approached me however
 by then I was back underway. I'd fixed the problem.

So anyway, during the examination to see what had happened, I spotted the reason why. That long bolt was not attached to a mounting point at the rear of the alternator. All of the torque was on that one point in the front. It was a recipe guaranteed to fail.

With a 15 amp alternator, no issues could occur. The 55 was "pushing the envelope" and then I made things worse. I got a Good Idea and upsized to a 110 amp alternator. The process was doomed from that point onward.

Although when testing at the dock the 110 pulled down the engine RPMs, the alternator did put out lots of power. It was the stress of being underway AND running the alternator that created the failure. That combination was too much.

Alternators are important. The alternator belt plays a part in the cooling system of the engine.

The large round hub at the bottom center marked 07220  turns the entire
time the engine is running. That is the crank shaft pulley. Belts run off that.

The belt is tensioned (tightened) by moving the alternator. Another thing to remember is that your belt stretches. According to Calder's, for the first one hundred hours the belt continues to stretch. That's why you have to keep tightening the belt via the alternator bracket when it is new.

On the bright side however, by the time your 100 hours
is finished you'll be an expert at tightening that dang belt.

Having spare belts is Very Important. For details please see the
Expert Advice versus Intuition (spare parts inventory) article.

That day stuck out there on Boca Ciega Bay, well, I found another long bolt and put it in at the bottom. It was Wrong, but it would work long enough for me to move the boat to safety. The good thing is that I discovered a serious issue. I was able to rectify the current problem without help. Betsy (my 18hp Kubota) could have been ruined.

Of course I shut off the alternator using the switch. After adjusting the alternator to tighten the belt, I was on my way. I babied Seaweed back to her dock to rethink the mess.

I had spent a lot of perfectly good money on a
system that simply will not work on my engine.

For the record, there were at least a half dozen experienced mechanics aboard Seaweed between the time the 15 amp alternator was upsized and the failure. No one noticed this small error. It simply slipped by us all. I do not blame anyone except myself though. As soon as I saw the broken bolt I knew what the issue was. This was my fault. Totally!

Any other person would have realized immediately the why as soon as that bolt snapped. It really was that apparent. IF I had not had that Good Idea to upsize my alternator... well, there is no telling.

Often times it is little things that bite us on the transom. I bet y'all will check your alternators before you run your boats again. That is a good thing.

It is always better to learn from others. Frankly that is why I listen and learn from so many far more experienced boaters than I. It is less costly too.

That is my tale of woe. It is not entirely a bad one. I learned something. One of the things I like about boating is that there is always something to learn. I enjoy the challenges. I also like the blissful times at anchor. After consideration I opted to return to the beginning. The original 15amp alternator is back on *Betsy.

*Betsy is the name of my engine.

And life is good afloat.

Did you upsize your alternator when you bought your boat?
And, do you have a spare alternator in case yours quits?

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