The long route to a steady diesel tach

[Barking Sands]

Just to share info.....

It wasnt exactly the route I intended to go but it is the route I took. I just wanted a steady and semi accurate tach signal. The old Det-Mar tach used the W signal off the alternator. It was very erratic and definitely not accurate. it had 4 adjustment positions but didnt really help or change anything.

I liked the idea of the Diesel Tiny Tach piezo signal generator. Its piezo device that clamps on any of the high pressure pipes. A small black box amplifies and conditions the signal to a useable output. The problem was my existing tach used a W signal. I am not sure how many pulses a W signal uses but apparently it is quite a few. Obviously the PZO amp which is essentially a 1 cylinder 4 stroke signal is not compatible for a tach that can only use W signal from alternator.

So I then bought the Dakota Digital tach adapter. It can take in nearly any signal; ignition pulse, hall effect, flywheel teeth, timing gear etc. It can then convert any 1-2-4-6-8-10-12 cylinder signal into any 1-2-4-6-8-10-12 tach. But come to find out it cant convert a 1 cylinder 4 stroke signal for use on a W tach. It could within a few hundred RPM but not quite. However at this point I was free to purchase nearly any normal tach such as a standard gas tach.

I chose the Jegs electronic digital programmable tach. $79 and programmable in several ways. Installed it today and was able to get the tach to work very steady and stable. The Dakota also has a fine correction built in. I have it currently corrected to 1.07 to bring up the RPM slightly. It needs a few more tweaks to hopefully keep it within around 10 RPM from the laser tach. Right now its as much as 35-40 rpm off.

The Dakota also allowed me to set the output to 6 cylinder and that way I can also replace the upper helm tach with a standard 6 cylinder gas tach which is pretty inexpensive.

Quite a round about way to get a stable and semi accurate tach signal. But it seems to work good so far and if I need to feed the signal to something else it should be compatible. All told it was around $410
Jegs tach- $79
PZO AMP with sensor $220
Dakota adapter $100

 

[C lectric]

Nice going.

Considering what you had before an inaccuracy of 30- 40 revs is still better that what you had.

Unless of course you are a Predicted Log Racer.

 

[Barking Sands]

C electric...I should be able to get it even closer with some more adjustment. But you are right. The original tach was not great. Half the time the needle was just wandering. As far as the route I took, which was less than ideal, for another $200-300 I probably could have just went straight for the Aetna system. However this set up allows me to purchase any tach. And there are many really nice ones out there that are very inexpensive.

I actually just logged a short video with a bit more detail.

 

[diver dave]

I’m not sure the alternator is “notoriously unreliable”. But the tach driven by one also reports broken belt or zero field current. I consider that a bit of a plus.

 

[Barking Sands]

I’m not sure the alternator is “notoriously unreliable”. But the tach driven by one also reports broken belt or zero field current. I consider that a bit of a plus.

Yes...also if your hour meter is in your original tach thats another reason to retain it. The other route I was considering was to retain the original W signal tach with hour meter and just get a small digital tach to supplement.

It may be slightly hyperbolic to say "notoriously unreliable" but they are the bottom rung of the ladder for tach accuracy/repeatability and reliability IMO.

 

[diver dave]

Been watching my Swiss made VDO tachs during this cruise. They are very stable.
But all is not well. Somehow, the boat ended up with transmissions that don’t match gear ratio. 2.19 and 2.49. And i suspect that the props are the same pitch. [emoji30]
Started to suspect trouble when the port engine drank less than stb motor. So, on this long trip i try to set rpm’s by power matching, not by rpm. Anyone got a couple prop shaft tachs to loan?

 

[MacPhid]

To measure the prop shaft rpm I use the same optical tachometer as Barking Sands uses in his video. I wrap dark masking tape or electrical tape around the prop shaft but leave a small gap between the ends. This mimics a reflective strip which the tach can read.

James

 

[diver dave]

Small problem. I got saildrives [emoji43]

 

[Piaro]

Been watching my Swiss made VDO tachs during this cruise. They are very stable.
But all is not well. Somehow, the boat ended up with transmissions that don’t match gear ratio. 2.19 and 2.49. And i suspect that the props are the same pitch. [emoji30]
Started to suspect trouble when the port engine drank less than stb motor. So, on this long trip i try to set rpm’s by power matching, not by rpm. Anyone got a couple prop shaft tachs to loan?

Just set up a couple of bike speedometers to act as a tach. Short version is to set the wheel circumference to 268mm and set display to MPH, that's it. So if the display says 21.3 MPH the rotational speed is 2130 RPM. Easy Peasey.


This guy did a really well done instructable write up on how to do it also:


https://www.instructables.com/Tachometer-made-from-a-bicycle-speedometer-cycloc/

 

[Marco Flamingo]

Yes...also if your hour meter is in your original tach thats another reason to retain it.

I'm seriously thinking of changing my tachometer because of the hour meter. It is a mechanical meter that reads tenths of an hour, but clicks every few seconds to keep track. After doing some engine room soundproofing, the clicking started to bother me. So I switched the lower tach with the hour meter to the flying bridge and moved that one down (no hour meter). Now have to go to the FB in order to get readings for the log. And when we use the FB we have the clicking up there. Small problem, but it's like a dripping faucet.

My engine is only at about 425 hours since a complete rebuild so my plan is to buy a tach with a digital hour meter and hook it to a 12 volt battery for 17.7 days. Engine hours would be accurate and no ticking. I found one with an old school needle for rpm and digital only for engine hours. But I think that because of the signal I'll have to replace upper and lower with the same brand.

 

[Barking Sands]

Marco...I have been doing exactly that on a digital hour meter for the last couple days. But reading your post right now reminded me I was supposed to shut it off last night! Woops. It will be ok..lol. the difference between 45 hrs and 60 isn't enough to be concerned about.

 

[mickand]

$18 from Amazon. I made up crude ali brackets to hold the sensor units, glued the supplied magnets to the balancers and included the readouts on my new helm. Going strong after 2 years.

 

[KnotYet]

$18 from Amazon. I made up crude ali brackets to hold the sensor units, glued the supplied magnets to the balancers and included the readouts on my new helm. Going strong after 2 years.

 

[Barking Sands]

Mickand...That was actually the first thing I bought. I still have it laying around somewhere. It seemed like it would be hard to read in the sun on the upper helm. But from the reviews they generally work good. Many machinists use them on their equipment with good results. I will be using the digital tach in the lower helm and a standard needle gauge tach in the upper helm.