Digital tachometers

[Kuncicky]

The tachs on my old Gulfstar don't work very well. The port doesn't work at all. The starboard will work if I goose the engine up and down a time or two. I think they are original on this old (1973) boat, and probably the time has come to replace them.


What I am wondering is whether or not it is feasible to replace them with some newer technology digital tachs. The engines are old 4-265 Perkins. Anyone have any experience with this?


Thanks for any suggestions.


Bill

 

[rgano]

I would guess you have alternator-feed tachometers. My Ford-Lehmans had tachometer senders operated by the cam shaft, I think, and when I looked into digi-tachs, it looked like the conversion was going to be a bit costly for the perceived benefit since mr ofo y analog tachs were working, but your case sounds like the time would be right for a conversion.

My current engine has a magnetic tach sensor back aft at the bull gear making my conversion from analog to digital quite simple, just set the new tach for the proper number of teeth on the gear.

Lacking the magnetic sensor, you might be looking at an optical sensor mounted so as to look at a painted spot on the flywheel. Wait a minute and somebody with Perkins knowledge with be along.....

 

[Max1]

There are many tachs out there that run on alternator pulses. Digital or analogue, I dont think it matters.

 

[syjos]

I'd check with Aetna Engeering, maker of digital tachometers.

They manufacturer digital tachs that gets pulses from a sender or alternator.

It all depends on whether the Perkins has a tach drive.

There are also digital tachs that receive pulses from a transducer that counts revs from a reflective mark on the crank pulley. Less expensive than the Aetna.

 

[catalinajack]

There are many tachs out there that run on alternator pulses. Digital or analogue, I dont think it matters.

It absolutely does not matter until you suffer alternator drop-out and are left with no signal. That's the whole point, is it not?

 

[Ski in NC]

Need to know if tach signal comes off alternator or from a tach signal generator or flywheel tooth counter. All are possible.

The one tach that does not work could be a symptom of a dead alternator, which is a different problem.

Turn of batt charger and let volts on batt decay a bit. Monitor voltage. Start engine with dead tach and see if volts rise from 12.6 up to 13.5 to 14.5. That tells you alt is working.

Common for alt driven tachs to not read on first start, as some alts need a "goose" of rpm to wake up exciter. Then it should work on down to idle.

Also common if gennie and charger are left on, batts get topped up and alts "go to sleep". Then tach signal goes away and tach goes to zero, engine still running.

 

[ancora]

I'd check with Aetna Engeering, maker of digital tachometers.

They manufacturer digital tachs that gets pulses from a sender or alternator.

It all depends on whether the Perkins has a tach drive.

There are also digital tachs that receive pulses from a transducer that counts revs from a reflective mark on the crank pulley. Less expensive than the Aetna.

We have had Aetna digital tachs on our boat for 22 years and they work flawlessly. Digital tachs are a "must have" for Predicted Log contests.

 

[Nomad Willy]

There is a downside to digital tachs.

When your’e busy keeping track of all there is to see that could be danger to your boat you don’t need to know EXACTLY what rpm your’e running. Just a very small glance at an analogue tach will tell you what you need to know. If you’re testing to see if your small modification to your prop made a difference then the digital thach is king.

But most of the time on a pleasure boat all you need to know is “about” what rpm are you running. The difference between that and exactly what you’re running is not information you need to know.

I became aware of this while flying my UL aircraft. The difference is in how quickly you need the information. Digital tachs are great but it takes time to mentally process the numbers and decide if the readout is good/bad .. acceptable or not. With the analogue instrument you can read and “digest” what you see very quickly if you’re ok or at an omg moment.

On the UL I had a digital tach and a relatively crude airspeed indicator that was the standard of the sport.
At a very quick glance I could tell if I was near stall speed w/o processing numbers. And w the digital CHT I can watch it and easily observe whether my CHT was climing or falling. Very important w high performance engines. AND how fast the change was happening. Get the CHT too high (about 400 degrees) and your engine can stop faster than you can imagine. Then your a glider.

So IMO the type of instrument you have should depend mostly on how you use it and how fast you need the information. Not on whether it’s high tech or modern.

 

[syjos]

I have both.

The analog tach has the hour meter on it so when I added the Aetna digital, left the analog one in place.

A quick glance at the large, bright, red, LED tach numbers, while scanning the gauges, provides the info fast enough for me.

I rarely look at the analog tach.

 

[rgano]

I needed an accurate tach to properly run this single engine boat just over the hump RPM of 2800 RPM and at a bit less than 3100, and the analog tach with its inherent inaccuracies just was not hacking the program. I don't spend time obsessing over it; I just get it near where it needs to be but over 2800 for sure. Maybe I am used to digits, having digital speedometers in my cars since 1986.

I wanted to have a pair of them in my twin engine trawler to better synch the engines, but I never got around to it, using my ears instead.

I added a digital temp gauge to the generator (which had no gauge at all) because digits are so much precise a way to watch trends as I add and take off electrical loads.

 

[fgarriso]

Stay with analogue tacks they are much easier to read.

 

[jleonard]

Stay with analogue tacks they are much easier to read.

Exactly. When I cruise my Lehman doesn't care if it's running 1743 rpm or 1757 rpm.
The needle reads 1750 and that's accurate enough.

 

[catalinajack]

I needed an accurate tach to properly run this single engine boat just over the hump RPM of 2800 RPM and at a bit less than 3100, and the analog tach with its inherent inaccuracies just was not hacking the program. I don't spend time obsessing over it; I just get it near where it needs to be but over 2800 for sure. Maybe I am used to digits, having digital speedometers in my cars since 1986.

I wanted to have a pair of them in my twin engine trawler to better synch the engines, but I never got around to it, using my ears instead.

I added a digital temp gauge to the generator (which had no gauge at all) because digits are so much precise a way to watch trends as I add and take off electrical loads.

Rich, analog tachs are availble with a high degree of accuracy if used with a signal generator rather than an alternator and perhaps even with an alternator. I have a VDO on my lower helm and an ISSPRO on my upper helm, both programmable. Once dialed in, an easy process, they stay there. I stopped tweaking when I got closer than 25 RPM as shown on a phototach. Yes, the old Motorolas and Stewart Warners were inherently inaccurate but not the modern programmable units.

As others have said, I do agree that a simple glance at an analog tach tells the story instantly while a digital tach, although precise, takes a moment to process. Accuracy is important and/or useful. Precision is not except in extremely unusual use cases such as a predicted log event.

 

[FF]

"There are many tachs out there that run on alternator pulses. Digital or analogue, I dont think it matters."

True , but none are very accurate .

The alt. RPM to engine RPM will vary with the temperature of the alt, the construction and load of the alt as well as the condition of the belt.

The working diameter of the pulley changes with the pulley temperature, and belt slippage .

For most folks there "close enough", but syncing twin engines is better done by ear than alt. driven tachs.

 

[tiltrider1]

To the OP. First determine whether you have mechanical driven tachs or electrical tachs. If electrical tachs, determine if they run off the alternator or I there is a signal generator on the engine. Once this is determined you will know what type of replacements are available to you.

As far as digital vs analog. I have both on my boat and on my dingy. I really have no preference. However, on the dingy were the throttle position changes frequently I find analog easier to follow than the digital tach.

 

[ancora]

I have both.

The analog tach has the hour meter on it so when I added the Aetna digital, left the analog one in place.

A quick glance at the large, bright, red, LED tach numbers, while scanning the gauges, provides the info fast enough for me.

I rarely look at the analog tach.

I installed the Aetna digital tachs on my flybridge and found a 50 RPM difference on one of the analog tachs at the lower station. 50 RPM is way to inaccurate fro my needs.

 

[jleonard]

I installed the Aetna digital tachs on my flybridge and found a 50 RPM difference on one of the analog tachs at the lower station. 50 RPM is way to inaccurate fro my needs.

Well that's the difference, I always made sure my tachs were calibrated and within about 25 rpm. For me that is accurate enough

 

[rgano]

I had dual station Stewart Warner tachs for my trawler's twins which could not be made to remain "calibrated" by which I mean to always show the same RPM as indicated at a specific RPM measured using my phototach. These analog beasts were fed not from alternators but rather from senders on the engine run off the cam. Move ahead a few years, and I trashed all four of the ancient, drifty things in favor of a whole new set of SW analog tachs along with two new senders - I know, slow learner. These remained semi-accurate with little drift as determined by the phototach. They never read the exact real RPM, and my solution was to get them close to my favored 1600 RPM cruising speed and attach a small label at each tach with the analog tach reading necessary to achieve some normal running RPMs. For instance, for phototach RPM readings of 1200, 1500, 1600, and 1750 in my port Lehman 120, the lower helm might have a label with 1250, 1525, 1575, 1610, and 1700 meaning that if I wanted 1500 RPM on the engine, I needed to run the analog tach up to 1575 on the dial. There were four different sets of labels, and the system worked well for me.

The really sorry Faria analog tach I inherited with my single engine Mainship was impossible from more than one perspective. I was never ok with being "close" on the trawler (not in my nature), and I darned sure was not going to be putting up with falling off plane in this boat because the tach was off a hundred or so RPM, and I did not want to over rev this high-RPM motor if I did want to run it a bit hard. The digitach, accurate to within ONE RPM is a joy to run this sort of boat with.

 

[catalinajack]

I installed the Aetna digital tachs on my flybridge and found a 50 RPM difference on one of the analog tachs at the lower station. 50 RPM is way to inaccurate fro my needs.

There are analog tachs and there are analog tachs. As I said earlier, programmable analog tachs can be dialed in so close to precision that the difference is wholly insignificant.

 

[mickand]

Amazon sells a complete digital kit for $19. I've had mine for about 11,000 miles now and they work well. Made up a crude ali bracket to mount the sensor.
Tape is over the magnet while the adhesive set up.

 

[DW1979OA]

Tuning by ear on the fly bridge didn’t work for me. I also found that the $19 Amazon digital tacs were the answer. Also allowed me to calibrate the installed analog tacs that had wondered the calibration over the years.
Also used Actisense to put the rpm on the mfd. The digital tacs were most helpful getting it calibrated accurately. The display choices could be a number or an analog dial or both.
Recently converted to Glendinning controls and the sync function is a nice to have feature.