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(This article was copied from the Glen-L Forum because it seemed like something you would benefit from. It was written by Glen-L Forum Member and Moderator “ttownshaw” who has built our Malahini outboard.

Those of you at the gathering knew that I was concerned about the prop I was running as it appeared I was only running about 3500K WOT. I have to admit there is still some fine tuning to be done to the motor before everything is dialed in but I was able to calibrate the tach (relatively close anyway) to ease my concerns somewhat. So, I’m going to detail the process for you. I am not, by any stretch of the imagination, an expert but I do feel comfortable discussing this process now that I’ve done it and proven it works for me. Also, there are multiple ways to calibrate a tach (like a non-contact rpm meter) but this is the process I used.

First, if you don’t know what you are doing then read, read, read…there are lots of sources of information out there; some are good and some are lousy. The more info you can get your hands on the more comfortable you will feel with trying stuff you haven’t done before. Then, ask people you know and trust for their opinion on what you are planning to do. Once I had the theory and process in my head I talked to Oyster and asked his opinion. He said it sounded good to him but thought I should confer with Caber-Feidh. Caber was gracious enough to confirm my theory and said it should work out as I had planned.

The process is really pretty straight-forward but I’m going to lay out some of the science for you so you too can have a basic understanding of the how/why’s of calibrating your tach and why this process works. There are also some oddities out there between different motors so I’m just going to focus on my motor (1983 Evinrude 70hp).

The Stator – most of you know what this is and what it does. But a brief overview…it is the charging system for your electrical system. The entire system is made up of coils of wire and magnets (just like an alternator). The number of poles on your stator is important in this process and calculation. Mine has 12 poles. All stators will have an even number of poles and you’ll understand why a little later. Two wires from the stator go to a distribution block (one yellow and one yellow/grey) on my motor and I suspect it’s the same for most motors. These two wires are the charging/electrical pulse system for your motor. It is an alternating current (A/C) coming from these wires. It changes to D/C a little further down the line but knowing it is A/C at this point is helpful. A/C simply means it is a cycle pulse of energy (+ and -) similar to a magnet. If you think of it like a sine wave you have a plus signal wire and a negative signal wire to give you one complete cycle (not of the motor but a measurable cycle like a sound wave). With me so far? Measuring the number of cycles of energy at a given rate/time is the frequency (just like a radio station).

So, if the cycle of energy is measurable in real time across these two wires it’s pretty easy to take this frequency measurement and convert it to a really helpful measurement (RPM). It is a very simple formula.

I’m going to cover the second part of the formula first as it is the easiest to understand. If you have a twelve pole stator (like me) then you will have 12 pulses for every complete revolution. This further breaks down to 6+ pulses and 6- pulses per complete revolution. So you will end up with 6 complete cycles (waves) per revolution of the stator. If you have a 10 pole stator then you will have 5 cycles per revolution. Pretty easy ehh? It’s really that easy! One word of caution…nearly all stators now days are built on this A/C principle of +/- pulses but some old rare stators use only + pulses; these are few and far between so more than likely you have what I do.

For the first part of the formula you have to convert the frequency measured across the poles to a specific constant (in this case we are using 60 seconds for RPM). Frequency is measured in seconds (how many cycles per second). So the first part of the formula is the measured frequency you get with a multimeter set to frequency across both wires. Again in my case it’s the yellow and yellow/grey wires. For this example let’s say I’m getting a frequency measurement of 219Hz. Now I have to convert this to a constant of minutes. So I multiply the frequency (219Hz/sec) times 60 seconds…this converts it to minutes.

Now that I have these two parts of the formula I can use them to come up with a helpful measurement (RPM). Drum roll please…here’s the forumula:

Hz x 60 seconds / 6 (wave cycles per revolution) = RPM
Simplified the formula for me is Hz x 10 = RPM. So if I have a measurement of 219Hz then my motor is running at 2190 RPM.

I know this is a long explanation but I thought some of you might appreciate the more detailed information.

With all of this said, I was able to calibrate my tach and confirm I’m actually running closer to a respectable 5300 WOT.

Additional Note:  I neglected to add information about my actual tach. Mine has a selector dial on the back to set the correct number of poles or cylinders (different manufactures have different descriptions). Reading the instructions for my tach I set it to 6. There is a tiny adjustment screw on the back allowing one to fine tune the indicated RPM’s. I suspect many tach’s will be fine out of the box but mine was off. It is also helpful to have a second person calling out (loudly since the motor will be running) the frequency measurements while the other person adjusts the tach. Obviously it is best to do this at rest and across several RPM ranges to get it dialed in closely. Try not to max out the motor’s RPM range when doing this as it’s not good to do this when there is no load on the motor; but RPM’s higher than idle are desirable. I think the maximum RPM I calibrated to was about 3000…even then I only held the motor at that range for about 20 seconds. I did all of my adjustment at the dock with a helper.