James wrote: "At full output they (his alternators) range between 275F and 315F. Ridiculously hot." Yes, indeed. Ridiculously hot! The wires of the stator and field coils in an alternator are coated with enameling as insulation. Plastic insulation would melt and fail at way lower temperatures. The primary internal alternator component with a limiting temperature specification is that enamel insulation coating. Its quite ordinary to hear people say they "burned up their alternator." Generally, what that means is the enameling on the enamel-coated wires failed, and the coil shorted out. Running these machines at 275F to 315F may not cause them to fail today, but if you take that machine apart, you will see clear evidence of damage to the enameling, and you are looking at fore-shortening the service life of the alternator. The Balmar ARS-5 cuts the outback 50% when the alternator frame temperature set-point is reached (103C). The Balmar MC-614 cuts output back in 10% increments until the temp set-point is reached, so maybe not 50% unless ambient air temperatures are very high. Using the frame temp sensor is the only sensible way to run externally regulated alternators. Small frame alternators are hideously inefficient; less than 50%. Much of the energy they consume is output in the form of waste heat. Richard's solution of forced ambient-temperature cooling air is a very good approach to get better alternator performance. What that does is remove the waste heat faster, which means the output of the machine can be higher while maintaining that high temp set-point spec. I'm not sure why any of this matters to trawler owners. This discussion is a sailboater's issue, where auxiliary engines are run for a very short percentage of the cruising day. Trawlers run their engines continuously through the cruising day, sometimes multiple days at a time. What difference does 1/2 hour make to reaching battery full-charge state? That is, if it takes 5 hours instead of 4-1/2 hours to fully charge the battery bank, so what? At the end of the day, the batteries are still fully charged, which is - after all - the goal. I wonder, are we polishing the turd here? Jim Peg and Jim Healy, living aboard Sanctuary http://gilwellbear.wordpress.com Monk 36 Hull #132 MMSI #367042570 AGLCA #3767 MTOA #3436