As usual, excellent write up MS. I've got a couple questions, don' t laugh if they seem naive please.
The new alt can put out 140 amps a hour, the bank is 500ah. Assuming you were involved in sizing the bank the owner is educated enough to have sized it so he doesn't cycle it below 50% (250ah).
In the first hour the alt dumps in 140ah, bank is at 390ah / 78% - we'll call that 80% because it's close enough.
At that point - 1 hour and a couple seconds - it's no longer in bulk charge mode (I'm ignoring the efficiency ratings and drops amps) and has gone to absorption mode.
This all depends upon how the owner wants to run the boat. Many owners want to be able to cycle between 50% & 80-85% with less than or equal to 1 hour of engine per day. It also depends upon what you have for batteries.
If that 500Ah bank were Lifeline batteries they want to see a MINIMUM charge capacity of 0.2 "C" or 100A hot output in bulk mode. This requires at least a 120A alternator to meet the "minimum" for Lifeline.
Odyssey TPPL AGM's are even tougher to satisfy the "minimum" requirements. Odyssey wants to see a min charge current of 0.4 of "C" so a 500Ah bank of Odyssey batteries technically needs a 225A alternator to meet the 200A min charge current. The boat above has a 400Ah bank and technically should have the 180A alt to hit the 160A / 40% of capacity recomendation but the owner decided 140 was close enough and preferred the low RPM performance of the 140A vs. the 180A. I did confer with Odyssey on this too and they said while 40% is preferred and advised that the 30% we were at was far better than most installations.
Isn't the 140 amp output kind of overkill?
Again for Lifeline, Odyssey or other AGM batteries it can actually be quite underkill not overkill.
It saves some diesel running time certainly, but is it worth the presumed 2k of cost?
It may not be worth it to you but may be to others. For this owner it cut him from about 3+ hours to hit 85% to about an hour. His stock alt really suffered when heated up and maxed at about 28-30A when hot. That is two+ hours, per day, of less wear and tear on the engine, alternator, noise pollution for others if in an anchorage AND more time under sail only vs. motoring or motor sailing.
If he's got 100ah of solar capability this makes sense - run the motor a hour in the morning while it's still in bulk acceptance, then absorption rate with the solar to get almost full by sun down.
Even without solar it makes sense, if it makes sense to your wallet, you don't want to listen to an engine all day long and prefer to sail more than motor. Solar always makes sense as an additional charge spurce provided you can fit the panel real estate.. The only time it may not make sense is if your alternator is exceeding bank acceptance in bulk and the capabilities are going untapped. Even then a larger alt will run cooler and work less hard so alts that exceed acceptance of the bank can still be a good value over the long haul because they simply last longer.
I ask because I'm going to have similar needs on my boat (500 ah bank, 200ah 'daily diet).
A 140A alternator on a flooded bank still makes a lot of sense as the 500Ah bank will take about 125A of charge current for a period of time in bulk. Putting it back as quick as possible means shorter run times. Also many HO alts will output LOTS of current at low RPM meaning less engine RPM at anchor if you need to top up on the hook.
Also - what's the take off the engine/shaft horse power, I'm assuming there is no free lunch.
About 1HP per 25A +/-. It should not affect a properly sized engine, in most circumstances. If that is an issue a field cut off switch can be added so if you need more HP you simply cut the regulator. I have a number of boats with cut-offs and they owners claim the never use them or find a need to use it.