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Old 15-01-2018, 00:45   #1
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Electromagnetic Speed log - DIY - possible?

Good morning all,

I have been thinking about how to replace my old (but still very usable) airmar (Raymarine ST50) Paddle wheel speed log with one that does not have the normal drawbacks.
And I don't want to spend the money that NKE (and alike) are asking.
So for the alternatives to the paddle wheel and I ruled out the pitometer as it again would suffer from marine growth and I can't quiet understand how the doppler sensors work and they also look like it would require some very technical skills to build.

So My question is, is it possible to build a electromagnet speed log system?

From a old US patent (https://www.google.com/patents/US3110876), I can see that I need to build a coil (low voltage hopefully 5vDC/1amp) and place a voltage sensor within the magnetic field, exposed to the water and measure the increase in voltage (again very small ?) as the water flow increases. A very simple description of the process.
I would also like it to fit into the same thru-hull as the airmar, hopefully using a airmar blank as the housing.

From you guys with a lot more electronic knowledge than me what kind of coil specifications would It require (size of wire, numbers of turns) then following that how close to the surface in contact with the water does it need to be for the magnetic field to be affective? I guess the placement of the voltage sensor within the field has to be chosen.
Once those 2 components are positioned , what magnitude of voltage are we likely to be measuring? Do I need to be looking for a special sensor to amplify the output?
I was thinking/hoping that a Raspberry Pi (or similar) would be able to read the sensor , do the calibration and maths required and then output the NMEA sentence.

Is this just too far beyond the skills of a DIYer?

Any thoughts and ideas on how to do it?

Thanks in advance for all comments.
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Old 15-01-2018, 04:20   #2
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Re: Electromagnetic Speed log - DIY - possible?

Greetings and welcome aboard the CF, Nick.
This sounds, to me, to be beyond the skills of the "uninitiated" DIYer.
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Old 15-01-2018, 13:33   #3
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Re: Electromagnetic Speed log - DIY - possible?

Thanks GordMay for the quick response.

To me, like a lot of good ideas the concept for this is fairly simple. I was after some feedback to the level of precision required and if anyone had had any experiences with these type of devices.
It's more the hardware setup that I am trying to get a idea of, as I'm sure there are bits of software which I can hack to calibrate and produce the correct output for something like "Signal K" to feed back into my NMEA network.

Thanks
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Old 15-01-2018, 14:09   #4
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Re: Electromagnetic Speed log - DIY - possible?

Just assume you WILL have to do experiments, and that it’s probably weirdly non linear due to boundary layer and bottom growth, so the function to find boatspeed from whatever you measure will require computation, then you may have some success.

However: why not just use GPS speed, and forget the thru hull transducer completely? That’s what I do.
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Old 15-01-2018, 14:37   #5
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Re: Electromagnetic Speed log - DIY - possible?

Quote:
However: why not just use GPS speed, and forget the thru hull transducer completely? That’s what I do.
Because speed through the water is an important metric, separate from speed over the ground. Maybe not important to you, but important to some of us.

Jim
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Old 15-01-2018, 15:47   #6
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Re: Electromagnetic Speed log - DIY - possible?

u4ea32,

I guess maybe where you sail, it is usually short sails. Anyhow, Jim told you it is important to us, and my imagination popped up with you saying "Why?" So, very simply, here is why, for me, since Jim may answer in greater depth. For me, the difference between SOG and Speed Through The Water (STTW) lets me know how to revise my anticipated arrival time, which is useful for the entrances with bars where we sail. Sometimes, it might mean choosing a different layover. It also provides a clue as to biological fouling, if your STTW is always slower than your SOG. It could just be the paddlewheel, or it could be the boat. (an overnight soak in white vinegar gets rid of tubeworm). So, indirectly, it can be a reminder for routine maintenance.

Those are my reasons; others will have more.

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Old 15-01-2018, 16:45   #7
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Re: Electromagnetic Speed log - DIY - possible?

When sailing where there is a significant current, STW gives us valuable data beyond what we get from GPS. It tells us if our sails are trimmed for optimum speed. When used along with GPS data it can be used to tell us about the current and helps us disambiguate heading, course, and leeway as well as Apparent Wind vs True Wind vs Ground Wind. If you sail where the currents are negligible, or you don't worry about these finer points, then GPS and heading are sufficient, STW isn't all that necessary.
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Old 15-01-2018, 18:38   #8
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Re: Electromagnetic Speed log - DIY - possible?

Hi Nick,

This is certainly possibly. However, there may be some limiting factors which make it a tough project for DIY. For starters, the magnetic field must be perpendicular to the flow of water for the Faraday Effect to take place. This means that most likely you would need to place the electromagnet (or even permanent magnet) in a housing that sticks out of the hull a bit farther than a normal hall effect wheel would go. On the bright side, there would be no moving parts. I think the trickiest bit would involve measuring the VERY small voltage differences that would then be translated to water speed. To be somewhat accurate, this involves some pretty sophisticated circuitry. (hence the challenge for a DIY project) In comparison, I think a traditional hall effect sensor with a paddle wheel would be about a 2 or 3 (out of 10) on the DIY difficulty scale. Your electromagnetic speed indicator might be more like an 8 or a 9.

The problem with measuring tiny voltage differences is that temperature plays a big part- most likely bigger than the speed voltage value itself. You would probably need a temperature compensation circuit in addition to a super high impedance differential amplifier and a sophisticated filter to smooth things out, especially at low speeds. As an embedded systems designer, I would probably do the fancy math (filtering, temp comp) in software on a microcontroller (like Arduino, or Raspberry PI) with relatively simple (and few) components such as a high quality op-amp and a thermistor. One other simplifying idea would be to try a strong permanent magnet (like neodymium) for the B field. (That might cause a problem with your navigational compass though.)

There exists a hand-held version of this device, which you could buy for about $3500. However, I think even this fancy handheld version has issues with calibration. (different water salinity/conductivity vastly changes the zero setting)

Having said all that, if you need a long winter's project... do it! You might toss it all for a simple hall effect wheel in the end, but it could be a learning experience.

Cy
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Old 16-01-2018, 02:28   #9
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Re: Electromagnetic Speed log - DIY - possible?

If you just want a project go for it but don't expect it to be easy. If it was easy, there would be commercially available ones and they would be standard kit.

Paddle wheels are a joke in terms of reliability and outside of racers, most cruisers just ignore them as even a few days without cleaning and clog up enough with growth to throw off the results by significant margins. If you really want to dial in sail trim for a race, STW is useful. If you are just cruising from point A to B, the GPS output (SOG) is typically good enough and really what you need to predict arrival times and such.
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Old 16-01-2018, 22:36   #10
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Re: Electromagnetic Speed log - DIY - possible?

Of course I realize there is a difference between SOG and STW. But I wonder if the difference is worth knowing, because SOG is both accurate and precise, while STW is precise but not accurate, and your ability to determine the variable inaccuracy of your STW transducer is highly dependent upon SOG.

So one (SOG) is certainly useful information, and the other (STW) has unknowable error at any given time.

Why worry about something that is wrong, but you just can't tell how wrong it is, and when its in error?

That is why I don't bother with STW.

For the initial poster: The calibration of the electromagnetic speed log will require runs in zero current and deep water and will use SOG for that calibration. The calibration will be dependent upon many environmental factors, including (as noted above) temperature of water, temperature of all the various circuitry inside the boat, salinity of the water. At least. It is probably also dependent on the same things that make mechanical STW sensors so inaccurate: growth, condition of paint, sea conditions, which tack you are on, how much you are heeling, asymmetry of the foils and hull, and boat speed especially high speeds and very low speeds.

With so many challenges to STW, and no need for it for navigation (as mentioned, SOG is more useful for routing than STW as SOG will actually tell you when you will get somewhere), I just don't see the use for it any more. I have not used STW instead of SOG for two decades at least, sail or power, racing or cruising, high speed multi or slow speed mono, in the Pacific, Atlantic, Solent (!), and Caribbean. I don't use it because its just not as trustworthy: you can trust it to be wrong, but you can't expect to know how wrong it is.

Consider this: No Olympic sailor has ever used a STW instrument while racing in the Olympics. If they don't use it, why do you think you need it?

Consider this: Very close Gran Prix classes like TP52 use STW instruments in addition to GPS. They spend inordinate amounts of time trying to calibrate these systems. But in the end, they use feel and wisdom instead of trusting or even using the third digit of precision on any measurement, and they try but never eliminate the difference between port and starboard readings.

Like an anemometer: it will tell you a number, but you need to use totally different data to decide if you need to reef: heel, helm, leeway. None of which is provided by the anemometer.

An STW transducer gives you a number, but what it is measuring is not boat speed, or water past the hull itself. It does not tell you the angle of the flow, it does not give you any flow field information (flow is faster further from the hull surface), it does not give you sufficient information to solve for flow changes due to lift effects from a nearby keel (higher lift means larger speed differences of the water flow). At best, it only tells you the vector in the "direction" of the transducer within the turbulent boundary layer.

YMMV.
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Old 17-01-2018, 03:47   #11
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Re: Electromagnetic Speed log - DIY - possible?

What I wish is for a STW system that could be "calibrated" automatically. It should be a simple programming trick to put the system in calibrate mode then sailing/motoring at various speeds when the current is known to be zero. Using SOG data a calibration table could be made and this would improve STW accuracy for those times when it matters.
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Old 18-01-2018, 00:04   #12
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Re: Electromagnetic Speed log - DIY - possible?

Good morning all,

Many thanks for pointing the correction factors needed to over come, as mentioned surely this and possible others could be handled by some clever circuitry and an additional probe for salinity, we could get the water temp from either another probe or from existing NMEA data.
Some of the physical limitations are the same for a paddle wheel to a lesser or greater degree, aren't they (laminate flow, difference readings due to heel/tack/hull shape)?

So yes, it is beyond my means and abilities.

Still for interest sake, what about the hardware setup?

I'm sure I read somewhere that a inducted magnetic field is stronger than one produced by raw earth magnets?
Would the "whole" magnetic field be required to be outside the hull or would just part of it? (Let's say the bottom half of the field)

Thanks again.
Nick
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Old 18-01-2018, 02:22   #13
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Re: Electromagnetic Speed log - DIY - possible?

Nick,

I feel sure that if this idea could be implemented with "simple" circuitry it surely would have been done. So do some more research on whether it was ever commercialized. If so, then there may be information out there on the circuits used. Also, most "good" ideas generate better ideas. So check the patent history to see if this patent was referenced as prior art for a subsequent patent. The USPTO.gov web site has search tools that can do that. So does the EPO.

If its necessary then a salinity probe isn't trivial either.
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Old 18-01-2018, 19:03   #14
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Re: Electromagnetic Speed log - DIY - possible?

Quote:
Originally Posted by Nick_dreaming View Post
I'm sure I read somewhere that a inducted magnetic field is stronger than one produced by raw earth magnets?
Would the "whole" magnetic field be required to be outside the hull or would just part of it? (Let's say the bottom half of the field)

Thanks again.
Nick
Hi Nick,
As I mentioned earlier, you could purchase such a device today and stick it on the bottom of your boat. It would just be really expensive, like $3K and up.
Here is just a sensor for about $3K::
https://www.hach.com/fh950-portable-...CABEgJAAPD_BwE
Here is an entire handheld probe for about $5K.:
https://www.hach.com/fh950-portable-...SABEgLWBPD_BwE

The high prices most likely reflect the tricky circuitry needed to measure tiny voltage differences. Your project, in the end, would be trying to re-invent the wheel but just cheaper. This might take a a lot of effort, and then to sell such a device, you would need to license the patent. Still, let us all know if you do it!

To answer your magnet question, the magnetic flux lines simply need to be perpendicular to the flow. Most flow sensors that use this method immerse the magnet in the flow to get every last bit of the flux lines to cross the stream. (Ghostbusters?) The reason is that all effort is given to get the maximum voltage change, because its so small in the first place.

You could try a permanent neodymium magnet, but an electromagnet could be controlled and calibrated I suppose. The only advice here would be to get the most magnetic flux as you possibly can, and make it parallel to the flow.

I suppose you could try arranging the magnet INSIDE the hull, with N an S poles of the magnet to port and starboard. This might arrange a lot of the flux perpendicular to the flow. Then, the only thing you would need outside the hull would be two small conductors to act as voltage differential sensors. (one inside the flux lines and one outside the flux lines) I suppose one other problem might be keeping the conductors clean. If you got bottom growth, you would probably have trouble as the gunk might behave as an insulator.

Finally, there is an INCREDIBLE and FREE visual magnet modelling application called FEMM. It does the same 2D finite element math and graphing that motor designers use. Did I say it was free? It is a bit tricky to use a first, but also incredibly powerful. The author, Dr. David Meeker is a brilliant engineer who shares all his hard work with the world. (talk about good karma!)

If I was going to design the magnetics in your speed sensor project, I would definitely use FEMM. (I've used it on other projects) You could even model the voltage drop versus speed with FEMM. Here is a link for this free application, as well as an example of a model of a simple motor below...
Finite Element Method Magnetics: HomePage
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Old 22-01-2018, 00:35   #15
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Re: Electromagnetic Speed log - DIY - possible?

Hi all,

I thought in this day and age somebody would be making cheap clever bits of circuitry to do something close to what You guys have pointed out to handle some of the correction factor required.

What do you think of this circuit board;
https://www.amazon.com/EZO-Electrica...29P8WWPHRJ4QKY

I also found on Amazon, salinity probes, not hugely expensive.

Here are the commercial versions of the speed log which appear to be marketed towards our type of boats (I.e. Not super tankers);

Speed Logs

http://www.nke-marine-electronics.co...-speed-sensor/

No idea of the cost, but at least they are out there.

Can the alignment of the magnetic field be North South vertically ?

Thanks
Nick
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