Measuring rpm (tachometers)

Trello · 24 April 2023, 18:11

What opinions do people have on measuring rpm on an outboard?

As far as I know, there are three different types of tachometers:

VIBRATIONAL
Examples are the Vibra-Tach and the Briggs and Stratton Treysit.
These comprise a thin reed which is placed against the engine (seemingly the cowling is fine for this), and the length of the reed is adjusted until it's vibrations are at their maximum - at this point, a measurement can be read off, giving you (plus or minus some calculations), the rpm.
Looking at their use on videos on the net, disadvantages seem to be that it takes some time for each measurement to be taken, and that measurements are discrete - that is to say, you can't have continuous measurements of changing rpm.
They do not seem to be very practical for use on a moving boat.

OPTICAL
Examples are the Clarke CT1 laser tacho, and the Multicomp Pro MP780522.
This type involves putting a reflective marker on a rotating part of the engine (reflective stickers are provided) and pointing a handheld device at that part of the engine as it rotates. The device emits a laser which is reflected back to it every time the reflective sticker comes round to face it - the device then merely counts the number of reflections. In this way, this type really does directly count revolutions (per minute).
On an outboard, I'm not sure that there is a suitable point on which to connect the reflective sticker. Like the vibrational type, this type seems more suited to static, terrestrial engines.

INDUCTIVE
Examples are Nine-Rong Jayron JR-HM026 Digital Hour Meter Tachometer and various "Tiny Tach" models.
This type involves having a coil of wire or a probe close to the high tension lad going to the spark plug of the engine - a voltage is induced into the wire/probe and this is measured and counted by an attached device. From the number of sparks, the revolutions per minute can be calculated.
These seem to be the most appropriate for measuring rpm on a moving boat - you get continuous measurements, and, if you use the type with the coil, you don't have to be pointing anything at, or touching something with, the engine. There is an added advantage of many of the models being able to give a running total of the hours of use of the engine too.
However, these too, have their problems. Reading some of the threads on this forum suggest that many of them have problems with accuracy (dubious readings), reliability (stop working within months if purchase) and waterproofing.
[And in terms of accuracy, I read somewhere (possibly on this forum), that the ones that have TWO leads (an earthing lead as well as the standard lead to coil around the HT cable) are more likely to give a plausible reading. Does anyone have any further knowledge about this?].

What tachometers are people using, and with what degrees of success? Are there any tachometers that don't fall into any of these three categories, and that might be useful for an outboard?

Fenlander · 24 April 2023, 18:50

I have never had a problem with operation or accuracy on the two wire inductive type. But full time water splash can mist them. Hence me mounting them under the hood once initial rev, running in and prop trials are complete. Then it becomes a tuning aid and hour meter.

Steve509926 · 24 April 2023, 21:18

Quote:

Originally Posted by Fenlander

I have never had a problem with operation or accuracy on the two wire inductive type. But full time water splash can mist them. Hence me mounting them under the hood once initial rev, running in and prop trials are complete. Then it becomes a tuning aid and hour meter.

+ 1

Peter_C · 25 April 2023, 03:36

I had one similar to the last photo. It was mounted to the tiller handle with double stick tape. Worked great and it was nice to have an hour meter too. Mine never misted up *shrugs* Probably should purchase another one for my kicker as they are nice to have.

Current boat has a regular tach gauge that gets mounted in the console to the left of the steering wheel.

Pikey Dave · 25 April 2023, 09:34

Quote:

Originally Posted by Fenlander

I have never had a problem with operation or accuracy on the two wire inductive type. But full time water splash can mist them. Hence me mounting them under the hood once initial rev, running in and prop trials are complete. Then it becomes a tuning aid and hour meter.

Likewise [emoji106]

Tommy c · 28 April 2023, 06:35

I have one like in the last photo. I've noticed that the hour usage is always on display so don't know how long the battery will last or, if the battery does run out, whether the hour log is still stored.

Steve509926 · 28 April 2023, 07:20

Hi Tommy, when I changed the battery on the last one I had (before it misted up) the hour log was stored.

Fenlander · 28 April 2023, 08:31

>>>I have never had a problem with operation or...

Just thinking back of the several I've bought one was faulty/old stock from new with a duff battery. Post 48 on this thread...

https://www.rib.net/forum/f36/best-t...ype-71268.html

Trello · 28 April 2023, 19:33

Thanks, Fenlander.

That's a really interesting thread. I'm pretty sure I read it some time last year, but I didn't find it when I did a search last week (search term "tachometer" - I should have used "tach" !). If I'd found it I would have added to it, rather than starting this thread - but no harm done.

So the consensus would be, would it, to use either a proprietary "Tiny Tach", or a mid-priced copy, but to avoid the really, really cheapo ones?

It was also interesting to hear (in post 20 of that thread), that you can use a timing light to confirm that your tach is reading correctly - which makes sense really, because the timing light, as far as I understand it, is also detecting electrical pulses in the HT lead.

Trello.

Steve509926 · 28 April 2023, 19:55

Hi Trello, I found absolutely no difference between the £40 tiny tach that I first bought and the £10 tiny tach I've now got. It wouldn't surprise me if they are made in the same factory in China but just sold by different middle men.
The first one misted up after approx 10 hours, the 2nd one was exactly the same. The one I have now is smeared in petroleum jelly to prevent water ingress and after doing my prop rpm tests, it's now wrapped in a plastic food bag and taped to the inside of the cowling.

Trello · 28 April 2023, 20:13

Thanks for that, Steve.

When I bought my boat in the autumn, it had a cheap-looking tachometer model attached to the hood of the outboard - but the screen was blank. I assumed that the battery was dead, and it was.... but a fresh battery didn't get it to work either. I assume that water had got into it at some stage, and fried the electronics.

The reason for starting this thread was to help me decide what to replace it with. And I'm thinking that getting a Tiny Tach copy is the way to go, but to get one that doesn't have a replaceable battery and also to do some zealous post-production waterproofing, as you describe.

Trello.

dhaglund · 12 June 2023, 21:25

I have measured rpm in three different ways.

First using a Tiny-tach clone. But if the outboard has a tach output signal, then that can be measured easily with a multimeter that can measure frequency (Hz). The better ones can. I used that method on a Tohatsu MFS25B. All I had to do was to take the frequency x 10 to get the rpm.

The last motor that I measured was a Suzuki DF20A. For that I used the Suzuki SDS software and cable to get the data from the ECU. That worked well but was a bit cumbersome since I had to bring my laptop and run without the cowling.

The inductive type would work for all engines but is also the least reliable. So I guess it all depends on what motor you want to measure.