[C38] Another idea on alternators

[tdtron at earthlink.net]

I was doing some thinking today and it's something I don't have a lot of practice with so bear with me.

I was going through some old stuff I had and I came across a picture of a race car the company I worked for had.  In the '70's I worked as an automotive technician (mechanic in those days) and I worked for a multi franchise import store near Atlanta.

We did a lot of work on sports cars that were used in the Sports Car Club of America racing series, the SCCA.

The rules said every car competing had to have a battery and alternator in working condition.  At the start of every race, each car had to crank by starter and battery without any outside assist.  Once the race started, a starter wasn't needed any longer and the battery needed only to supply a very small amount of current to the ignition and brake lights.

One performance trick we did was exactly opposite to what we sailors do, we made our alternators smaller and ran the smallest and lightest battery that would start the cars.  We took the stock factory alternators and had them rewound to produce only a few amps of charge.  This relieved the engine from having to pull the effort of the alternator charging the battery which robbed horsepower.

Where am I going with this?  If we put larger output alternators on our vessels, we can charge batteries faster but at what price?  We know high output marine alternators aren't cheap, and we know they put more mechanical strain on our engines and alternator brackets, water pump, etc.  One thing we have not discussed is the extra cost in fuel by the increased load on the engine.

At any given RPM, a 150 amp alternator will obviously cause more drag on the engine than a smaller alternator.  If you have to add more throttle to power the larger alternator, you are burning more fuel but at a shorter period of time vs. the smaller alternator.

I made a 12v power generator once so we could fast charge cars at the dealership without having to drag an extension cord and charger to the back storage lot.  I put a 90 amp alternator on an old lawnmower deck with a Briggs and Stratton 3hp engine running the alternator.

It didn't work!  We had to put a series resistor in the field line to reduce the charge output to keep the alternator from stalling the engine.  We thought 3hp would be sufficient but evidently not.  It was an eye opener for me to see how much horsepower it takes to run an alternator connected to a discharged battery.

If you are running a Universal 5424, 3 hp is about 12.5% of the 24hp redline throttle, or in another way, it's about 1/8 of your total engine power.  Since my alternator stalled the 3hp engine, the actual load had to be more than 3hp.

Since our original alternators only put out about half of the amperage of my experiment alternator, that 3hp would still be over 6% loss of engine power on a 5424.

My point is, the faster charging large alternators will charge faster but it remains to be seen how much they save on fuel by their shorter run times.  Of course, if that 150 amp alternator destroys your water pump, mounting bracket, belt, etc., you will save a lot of fuel because you will be sailing home without an operable engine.

Any comments, thoughts, etc?

Tom Troncalli
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