How accurate is your Tacho??
Jan 17, 2015 12:20:53 GMT
Post by Trevor on Jan 17, 2015 12:20:53 GMT
Hello
On my boat (2008, SO42DS) the original Hitachi alternator was replaced by a 120 amp Balmar with external regulator by the previous owner. I had a few problems with this and to cut a long story short, I now have a new alternator and a new external regulator.
During this period of discovery I noticed that my Tacho was fed from a stator output from the alternator. I thought the tacho would be driven by a source directly to the crank shaft but that is not the case. My first thought related to the possibility that the pulley on the Balmar could have been a different size to the original on the Hitachi which would lead to false readings of the Tacho.
I decided to dig deeper. I knew the alternator had to run much faster than the crankshaft at idle if the batteries were to get any charge because the alternator only starts to produce anything over about 1500 revs.
In order to understand just what the speed of these devices was I bought off Ebay a digital tacho. It cost me $20.
I had to put a little reflective sticker on each pulley so the digital tacho could "see" a repetitive flash of light.
The speed shown on the crankshaft was 818 revs per minute.
And the alternator speed was 1488 RPM.
The tacho was reading 1000 RPM so it was reading 25% high!!
When I went to find some calibration method I found nothing. 
But I did find this calibration sticker on the key switch indicating it was correctly set for my engine.
So, when you replace the alternator the tacho can be out. Mine is out by 25% and all of this time I have thought my revs at WOT was about 3,000rpm with a dirty bottom. I am actually 25% less than that which means I am well overproped. I did check at different revs and the speed difference between the crankshaft and the tacho is out by that linear factor throughout the rev range. I am now concerned about prop size and pitch and understand I have no way to calibrate my Tacho accurately.
I thought this was worth mentioning on the forum because my research in this area did not reveal much solid information at all and it just seems like such a possibly common issue with skippers who have swapped out the alternator.
Regards,
Trev
On my boat (2008, SO42DS) the original Hitachi alternator was replaced by a 120 amp Balmar with external regulator by the previous owner. I had a few problems with this and to cut a long story short, I now have a new alternator and a new external regulator.
During this period of discovery I noticed that my Tacho was fed from a stator output from the alternator. I thought the tacho would be driven by a source directly to the crank shaft but that is not the case. My first thought related to the possibility that the pulley on the Balmar could have been a different size to the original on the Hitachi which would lead to false readings of the Tacho.
I decided to dig deeper. I knew the alternator had to run much faster than the crankshaft at idle if the batteries were to get any charge because the alternator only starts to produce anything over about 1500 revs.
In order to understand just what the speed of these devices was I bought off Ebay a digital tacho. It cost me $20.
I had to put a little reflective sticker on each pulley so the digital tacho could "see" a repetitive flash of light.
The speed shown on the crankshaft was 818 revs per minute.
And the alternator speed was 1488 RPM.
The tacho was reading 1000 RPM so it was reading 25% high!!
When I went to find some calibration method I found nothing.
But I did find this calibration sticker on the key switch indicating it was correctly set for my engine.
So, when you replace the alternator the tacho can be out. Mine is out by 25% and all of this time I have thought my revs at WOT was about 3,000rpm with a dirty bottom. I am actually 25% less than that which means I am well overproped. I did check at different revs and the speed difference between the crankshaft and the tacho is out by that linear factor throughout the rev range. I am now concerned about prop size and pitch and understand I have no way to calibrate my Tacho accurately.
I thought this was worth mentioning on the forum because my research in this area did not reveal much solid information at all and it just seems like such a possibly common issue with skippers who have swapped out the alternator.
Regards,
Trev
