My name is XXXXX XXXXX I will be helping you today. Most questions will involve troubleshooting, and usually take many posts back and forth. Post back with questions as needed.
Couple of questions back to you.
Have you been using the same battery this whole time, or different batteries? Are you trying to charge more than 1 battery at a time, or do you have just the 1?
Also, the cables that attach to the battery, what are you using for hardware to secure the battery cables? Are you using the wingnuts?
To be honest it can very well be those wingnuts that are the cause of your problem. (which is why I was specificly asking if you were using those or not).
Wingnuts do not provide a good connection to the battery, and they will come loose on the battery. What happens when the engine is up and running is that the alternator will see an intermittent connection to the battery.
Think of this like it was a little kids game, as your running down the water the battery tells the alternator "now you see me, now you don't, now you see me, now you don't" just like a kids game. What happens is this causes the voltage to spike inside of the regulator portion of the alternator because that voltage needs to go somewhere, and if it can't go anywhere because of an intermittent connection, the regulator portion goes up in smoke, and the alternator no longer works. It's a very common occurence and I run into to a few times a week.
The fix is to simply get yourself some stainless steel nylon locknuts from the hardware store along with some lockwashers. Sand down the battery cable crimp ends so that they are spit shine clean. Clean up the battery posts. And reinstall the battery cables with the locknuts. If you have a few wires going to the battery for other accessories, make sure the main battery cables are the wires on the bottom, directly on the battery posts, and not stacked on top of other wires. Also make sure your battery box is secured to the boat, and not sliding around. But it's those wingnuts, as simple as it sounds, that is the root core of your problem here. I have a toolbox drawer full of them.
Feel Free to post back with questions
So do you think this alternator is okay
Honestly I would not know that either way unless I tested it myself.
Do you have a basic ac/dc digital voltmeter? If so I can tell you how to test the alternator.
Simple take care of all the battery cable connections as I advised, put your volt meter on the battery, positive meter lead to Batt positive, negative meter lead to batt negative.
If the engine is not in the water, do not forget to hook up your flushing device.
Start and run the engine, bring the rpms up to about 1500 rpms. Set the meter to DC volts, and see what you are reading for the voltage at the battery. It should be between 14 and 14.8. Under or over indicates a bad alternator or a problem with the wiring inbetween the battery and alternator.
If that looks good, move on to test 2. Now turn your meter to AC Millivolts, and see what you read. Your looking for 250 AC Millivolts or less. If you get over 250 AC Milivolts the regulator/rectifier portion of the alternator is bad.
The reason why your checking AC milivolts at the battery is this. The alternator at its core makes AC electricity. It is the job of the voltage regulator/rectifier portion of the alternator to convert the AC the alternator makes to the DC that the battery stores. Now a little tiny bit of AC will always bleed through the regultor/rectifier, but you can't have to much. So we use 250 AC millivolts as the standard. Under that is good, over that means the regulator/rectifier is fried.
Post back with any questions.