Sorry you're so frustrated, but you need to step back and take a look at your entire DC system
, not just the alternator.
The type and sizing of an alternator depends not so much on the engine
, but rather on the type and size of your house battery bank
. What came originally on the Perkins 4-108 may be almost totally irrelevant.
I have a 4-108 on my boat, too. I have a house bank consisting of six Trojan T-105 flooded lead acid batteries, totaling 675AH capacity. Ideally, I'd want an alternator with about 25% of that capacity, or 170 amps. My setup, however, is smaller but has worked well for over 10 years.
It consists of a Balmar 100 amp alternator with a Balmar MC-612 external regulator. An external regulator is mandatory if you want to take advantage of the amperage capacity of your alternator.
The MC-612 is one of the best regulators on the market. And, since they've now come out with a new model which can handle two alternators, the original regulator is available at big discounts and is an excellent buy for those who don't plan to fit two alternators (check eBay).
One limitation of the 4-108 in fitting a larger alternator is the belt type and size
. The stock Perkins uses a 1/2" belt, and it's on the same power train as the water
pump. Most owners have found that 100-120 amp alternators are about the largest you can fit with this configuration. A larger alternator would mean you'd have to fit another pulley, mount, etc.
So....assuming your boat has a house battery bank typical of a 40-footer, you'd probably want to fit a 100 amp alternator with an external regulator in order to have the best charging
system you could without doing major surgery on the engine
One final note: the above pertains mostly to flooded and gelled battery types. AGMs are a whole different thing. Because AGMs can accept huge charging
currents, they can also tax your alternator to the point of burning it out....unless you take pains to ensure that current
is limited and temperatures are monitored. The MC-612 and other similar regulators can do this, by monitoring alternator temperature and by de-rating the alternator's output to any value you choose.