Thanks for your note. My problem turned out not to be the alternator after taking mine to the shop to have it rebuilt (and uneeded parts on order). But there is nothing wrong with it. I have two copies of Nigel Calder's Boat Maintenance book but confess that I thought I knew most of the stuff in there from rebuilding boat systems for the last 45 years--I was wrong. I decided that maybe I should review Calder's trouble shooting guide and use it on my alternator just to make sure I did not overlook something. He starts out where you should: with the simplest, most basic reason an alternator might not work well: Are the connections from alternator to the battery clean and without resistance? I put a VOM on the + side and things were fine. I checked out the neg ground (engine block to battery) and guess what? Much resistance. I had rewired a lot of this boat over the last year, but never bothered with this most fundamental and important connection because it looked fine physically from above and was hard to access back on the bell housing. This boat had been sailed very little when I bought it a year ago and the bolt connecting the battery negative to the block had the Universal chipped brown paint on the top and appeared to be secure. HOWEVER, the bolt securing the battery cable was just stuck in the hole and, because it was a collared bolt that was too long, could not have EVER been tightened adequately to make a decent connection to the battery. I surmise the boat has been in this condition since it was built. Someone stuck a too long bolt slapped a nut on it and just left it hanging there--let the owner sort it out. This explains a lot of problems I have been having. It usually started well, but occasionally it would not-so I kept replacing connectors and lengths of wiring---and I had replaced the trailer connector and most of the starter and alternator wiring in an attempt to chase the problem. Sometimes the oil pressure light (I added a buzzer) would work and sometimes not. Now everything appears to work. Not sure what the moral of this story is. Has to do with being arrogant after all these years and forgetting to respect Occam's Razor in trouble shooting.
FWIW.