My tacho packed up last week and today on start the tacho read a slow reading, about 600rpm when the engine was doing about 1000. It then dropped to zero.

I checked the connections on the alternator and belt tension. Belt tension might be a bit loose but I am wary of over tightening. The connections look unchanged but the tacho connection is a bit suspect (no different than in the past but it could have a hidden problem). The condenser on the alternator looks pretty bad.

I then checked the voltage and it charges at 14.8v most of the time but at lower revs idle and slightly above it every 5-10 seconds the charge would jump to 15.4 for a second or so. When the voltage jumped there was a bit of noise from the engine as if the load increased. At higher revs it wasn't noticeable. I googled it and 14.8v is OK from what I read. 15.4 is high but not critically so.

The alternator has a 3 position switch fitted to either: fully use the regulator, partially bypass it or fully bypass it. The previous owner who had the boat built and fitted out was an electrical engineer, it was his idea and although a nuisance it is useful to put a quick charge in the battery. I went through the three settings and it worked as it always has with the voltage and load increasing as the regulator is bypassed and dropping when reset to normal.

The batteries are well charged and always are from a 40w solar panel.

I ran the fridge for 10 minutes to take some charge out of the batteries but it had no impact on the symptoms I experienced.

What would be the symptoms if the condenser failed?

To summarise:
the tacho doesn't work properly/at all,
the charge is 14.8v but jumps to 15.4 occasionally with a slight change in engine load, only noticeable at lower revs.

I hate electrics and was hoping one of you electricians could identify what the likely issues are with the tacho and with the alternator charging symptoms.