Originally Posted by svHyLyte
Considering prop walk, you're likely to just pinwheel around your anchor/anchor
Engine RPM is not always alternator RPM. Let's consider the optional factory 75A alt off a Yanmar 3YM-30. This alt develops it's rated output of 75A at 5000 RPM but that is the ALTERNATOR shaft RPM, not engine. This engine runs roughly a 2.7: 1 pulley ratio (based on some measurements I took a while ago and from memory). So a 1200 RPM idle yields an alternator shaft speed of 3240 or roughly 63 amps. Bump the engine up to just 1850 and you are at the alts max rated output.
Even at an engine idle speed of just 1000 RPM this factory alternator can put out around 58 amps, only 17 amps less than max rated of 75 amps, or 77.3% of total alternator output, at just idle speed!!!
I have a good friend who will remain nameless. After cruise with them for a few days the outrageous engine speeds in the anchorage, in the evening, were getting overbearing. "what are you doing?" "I'm charging my batteries." . "Do you actually need to run your motor at 2200 RPM to do that?", "Yep.", Do you mind if I come take a look?", "Sure come on aboard."
Long and short was his 70A Balmar reached max rated output at just 2000 alt shaft RPM, it was right in the literature that came with the alt but he never looked or just did not get it and was told the old wives tale that you always need a high motor speed to charge your batteries..
I showed him how to measure his pulley ration and figure out the alt shaft speed, simple enough. What we found was that even his engines idle speed of 900 put his alt shaft speed at 2250 or 250 RPM more than he needed to reach max cold and hot rated outputs of the 70A alternator. Moral of the story is we could barely hear him charging at idle for the rest of the trip...
2200 wasted RPM vs. max rated performance at just 900 RPM.. Wooo hooo...
Of course his alt was only putting about 45 amps into the bank so this then lead to a looooong discussion over drinks about battery acceptance and why a battery monitor is a good idea. He was certain that if we raised the RPM, more current would flow, it did not, trust me he tried it several times before finally "getting it"....
If an engine runs a 3:1 pulley ratio then the engine RPM can obviously be lower.
We can't forget that your bank may not even be able to "accept" the charge current you have available at a given RPM. If the state of charge is high enough increases in engine RPM may add ZILCH to the equation of you are up against battery bank acceptance.