Alternators

Te c h Tips RPM the alternator is only turning at 6,000 RPM, just above the speed
it needs to produce its rated output. Higher RPM certainly won’t hurt
by Old_Salt it. They can safely operate at rotor speeds in the 10,000 RPM range.
During recent meetings and gatherings we’ve been asking your Gasoline fueled engines normally operate in higher RPM ranges
opinion on ways to broaden the appeal for this newsletter. Several than diesels. If you have diesel power, the alternator pulley’s size is
folks have suggested that articles on various marine systems or boat More critical than in a gasoline engine installation. Because of the
maintenance may be beneficial. Hopefully, this column will be a lower engine RPM range involved the pulley on an alternator used
forum for sharing the vast range of practical experience accumulated __With a diesel engine will normally be smaller than one used on a gas
by our membership. It is also intended to be the place where you, the engine. If we use the same pulley sizes as in the previous example, 8″
reader, can submit technical questions and either obtain answers or crankshaft and 4″ alternator rotor pulleys, and use a normal diesel’s
direction to appropriate reference material cruising speed of 2,000 RPM, the alternator is only spinning at 4,000
Since the flood of letters and e-mail has been so magnificently RPM. Well below the speed needed to reach its rated output. In
underwhelming in response to previous articles I can safely assume __ diesel applications size really matters! And unlike most things in life,
either of two scenarios. One, you loved it but are too embarrassed to _smaller is better
ask questions; or two, you haven’t understood a word I wrote. In either One of the problems I see on older boats directly relates to pulley
case | can safely continue my assault on the six chosen topics without size. _In an attempt to maintain our reputation as frugal Yankees we
fear of criticism or revocation of my membership. In case you don’t replace the ancient, rusty, burned-out alternator off the beloved
remember, the topics are; electrical wiring, alternators and regulators, behemoth below decks with one from the old Chevy out behind the
batteries, grounding and bonding, radios and antennas, and instrumen- barn. The wiring isn’t that tough to figure out and the “idiot light” on
tation (depth, speed, GPS, LORAN, etc.). the engine panel goes out when the engine is up to speed. The voltme-
‘The previous articles covered wire and connections that comprise the _ter even reads 12 VDC. But the darn thing just won’t keep the batter-
electrical power delivery system of a boat. But what produces the ies charged. The reason is that the altemator isn’t spinning fast
Power? Since Friendships spend most of their time away from a pier __ enough to produce the current needed to recharge the batteries. The
and power receptacle, it’s a safe bet that most of our electrical power _boat’s engine, being a diesel, won’t tum the Chevy’s (gasoline powered
production comes from the engine driven alternator. I don’t know if engine) alternator fast enough because no one thought to change the
there are any boats out there with 12vdc generators, but for the sake of pulley.
brevity Im not going to get into them. If you have one and have any Here’s a good way to check the alternator installation. Before you
questions give me a call, or e-mail me at old_salt@ix.netcom.com. start it up, measure the voltage across the starting battery with a
Most of the problems I’ve encountered with marine alternators were digital meter. You can use an analog meter but be aware that the
caused by replacement of the original alternator with the wrong part, _change in the voltage reading is fairly small and may not be easy to
or gradual “consumption creep” that overpowers the existing one. The ead on an analog scale. A new, fully charged battery should read
scenarios are quite different, but the results are the same….an unsatis- about 13.7 VDC, but the actual value isn’t that important for this test
factory and potentially unsafe electrical system just remember what the value was. While holding the meter leads
As delivered from the factory and installed on a boat the alternator across the battery terminals, have some one start the engine. The
installation was probably well done. However, years of operation ina _Voltage will drop while the engine is being cranked. This is normal
marine environment eventually take their toll forcing its replacement When the engine starts and you release the starter motor the voltage
or addition of power hungry navigation equipment (ie. the 12-volt across the battery should increase, up to the original value that you
blender for the Margaritas) gradually forces recharging the batteries read before you started it, then slowly beyond that value to about 14.2
more often. In the either case adding a high output alternator to the VDC or greater. At an idle (600 – 800 RPM) the alternator should be
electrical system is a good idea, The trick is to do the installation able to maintain an output voltage of about 14.2 VDC across the
correctly battery
If you have a gasoline engine you have to make sure that the replace- If your results are somewhat less than these, you more than likely
ment unit is certified for “MARINE” use. Units so certified have have a problem somewhere in the charging system. A good place to
internal screens and shielded brush assemblies that greatly reduce the start troubleshooting it would be to measure the diameter of the
potential for ignition of gasoline vapors. Additionally, sealed bearings _ pulleys and calculate the RPM of the alternator. If it isn’t in the 1800 –
are normally employed to keep salt moisture out. But buyer beware! 2400 RPM range while the engine is idling it may not have the right
Even though you have the original part number it may be wrong and _size pulley. The best way to determine the minimum RPM for the
not for a MARINE grade item. The engines used in boats are usually alternator is to check out the manufacturer’s technical data sheet (if
available in several configurations, many of which are industrial… not You can get a copy). The next best thing is to install an ammeter in
marine. Another problem is that an alternate or substitute part number —_series_ with the battery output and actually measure the charging
for the original may be called out in the parts catalog and it may not be current going to the battery under varying engine speeds. The
“MARINE” grade alternator. Check it out carefully or have a marine ammeter should always read slightly above zero or a greater positive
mechanic/electrician inspect it for you value. It should never read a negative (discharge) value with a good
Replacement alternators usually don’t have pulleys installed on the alternator and regulator.
shaft when taken out of the box. This is good! It forces you to use the There are several sources of alternators if you need to purchase one
original pulley that is more than likely sized correctly. Since the Boat US, West Marine and J.C. Whitney are good mail order sources.
pulley on the engine that drives the altemator is bigger than the alter-_ The West Marine Master Catalog also has a very informative Advisor
nator pulley, the alternator rotor spins faster than the engine. For section on how to choose alternators and regulators. The last J.C
instance, if the engine crankshaft pulley is 8″ in diameter and it is Whitney catalog | received listed a marine altemator in their marine
turning at 1,000 RPM, a 4” pulley on the alternator will turn the rotor _ Section that was very reasonably priced when compared with West and
at 2,000 RPM. The speed ratio is inversely proportional to the pulley Boat US. If you are thinking of upgrading your alternator I highly
diameter. A moder alternator needs to spin at 5,000 RPM to produce Tecommend getting an externally regulated model. But more on that
its rated output. If your engine RPM for normal cruising is 3,000 subject in the next article