If it is a 1965 boat it is most likely an ammeter, and charging 10 amps after start up, and then dropping to near 0 sounds perfectly normal to me. It probably has a external voltage regulator regulator.
My brother went out with me today to help put the alternator back on, I will need to revisit to change the connections we had to rig the connections together. The heat shrink wrap I had was too small so I need to redo all the connections because the electrical tape is only temporary. He did notice a black mark on one of the connections which may have been loose.
One other thing we noticed is the red wire coming from the wire harness was larger than the one it was connecting to on the alternator. we had to reduce it in order for it to connect properly.
Anywho once we connected everything and mounted the alternator we started her up and the gauge was working fine. it would go up when the RPM's would and go down when at idle. the boat seemed to have more power and ran smoother than normal.
