[C38] Another idea on alternators

[Larry Malmberg]

Just a couple of thoughts TomT, we have sail boats and don't really run the
engines that much or shouldn't anyway.  Given the amount of electronics one
can now put on a boat, instruments, radar, chart plotters, sat phones,
macerator, bilge pumps, water heaters and so on we have become very
dependent upon our in house electrical systems and in specific the batteries
and charging systems.
 
For instance, I have four 110 amp AGL batteries for the house, in order to
charge these with a low amp alternator would take significant time and I
really don't want to run the engine that much, BTW I have a M-25XPB.  I do
have a Balmar regulator and it can be set to charge all the time, part of
the time and so on and to sense the battery charge level and if they are
full not to charge at all.  I would rather run the engine an hour and charge
everything up than run it 6 hours to charge everything up.
 





Best regards,

Larry

Team Hassle

 

  _____  

From: listserve-bounces at catalina38.org
[mailto:listserve-bounces at catalina38.org] On Behalf Of tdtron at earthlink.net
Sent: Sunday, September 18, 2011 3:54 PM
To: Catalina 38 forum
Subject: [C38] Another idea on alternators




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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