The first cruise we took on Makani was from Sausalito to Benicia. The Sausalito Yacht Club sponsored the cruise for members so they arranged for slips in Benicia and dinner at the Benicia Yacht Club. Normally, it’s a nice 3 to 4 hour downwind sail north through San Pablo Bay.
Of course, the wind disappeared and we ended up motorsailing the entire way. After going for about two hours at around 2500 rpm our tachometer dropped to zero. Not having any idea what to do, I shut down the diesel and ghosted along under sail. Eventually we approached the Carquinez Bridge which is around a dogleg to starboard. With the wind almost nonexistent and a lot of commercial shipping in the area, we decided to fire up the diesel. Started right up and the tach was reading perfectly.
We returned to Sausalito on Sunday, again having to motorsail all the back, because now the wind was right on our nose at 20-25 knots. Again after about two hours of motorsailing the tach dropped to zero, but this time we ignored it and continued home without incident.
Monday, I spoke to Hans List at List Marine and he explained the situation. On the Westerbeke, and other makes, the tachometer is run through the alternator. When running for a few hours the batteries were completely topped off, which caused the tachometer to read “zero.” We could have ignored it, or turned on some equipment that would have put a drain on the battery which would create charging and the tachometer reading in a normal fashion.
Interesting explanation, but not one I would have expected.