I found installing a new tach to be quite challenging with not enough info on line to make it simple for me so I want to share my weeks of experience to help the next guy wanting to do the same. My engine happens to be a Yanmar 2QM15 but I think this instruction will work for most diesel inboards. The RPM output can be found from the alternator if there is an electrical connector on it which sends a signal (my alternator needed an "R" which it didn't have so I needed a sensor which is no longer made by Yanmar. I think other alternators may be labeled "AC" for that connection. It's also possible to have an alternator shop install the tach output on the alternator but I found that to be expensive so I chose the sensor which was the original installation method. I found a diesel tach with an hour meter as well as the tach sensor on Amazon manufactured by a Chinese company called KUS and distributed by Springautoparts (see links below). If your boat never had a tach you'll need to pull the tach plug from the engine which will be above the flywheel where it senses the engine's rotation and order a sensor to match the plug. I went to ACE hardware and matched up my size and thread there. Springautoparts has a variety of sensors to match your plug size. Mine was an 18mm fine thread and it fit perfectly. I just screwed it in until it touched the fly then backed it off a quarter turn.
Maybe you can order these together and save shipping by contacting Springsautoparts directly. The shipping is pricey but fast....two days to us near Seattle.
https://www.amazon.com/gp/product/B01DKCDCZO/ref=od_aui_detailpages00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B01DP6FJ6K/ref=od_aui_detailpages00?ie=UTF8&psc=1
Maybe you can order these together and save shipping by contacting Springsautoparts directly. The shipping is pricey but fast....two days to us near Seattle.
https://www.amazon.com/gp/product/B01DKCDCZO/ref=od_aui_detailpages00?ie=UTF8&psc=1
https://www.amazon.com/gp/product/B01DP6FJ6K/ref=od_aui_detailpages00?ie=UTF8&psc=1