Electromagnetic Speed log - DIY - possible?

[transmitterdan]

Originally Posted by Nick_dreaming

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.

They do. They are called SOC microprocessors. Raspberry Pi is one example. Cost is <$40US. A few good opamps and I2C A/Ds to digitize the signals and you have the makings of any type of sensor desired. Any nonlinear effects can be programmed in software. The problem isn't a lack of cheap hardware. The problem is in knowing how to calibrate the sensor and the math needed to keep it accurate.

By the time one makes a water tight through hull sensor, figures out the needed math, builds the circuitry and programs the processor there will be many hours invested. And cost not a lot less than purchasing a sensor off the shelf. I would guess about 10% of those hours spent flipping burgers would buy an off the shelf speed sensor. Not as glamorous or "salty" but a much more efficient use of time.

[CyKlop]

I've been thinking about this subject a lot lately and I've been thinking that maybe a load sensor attached to a (very small!) drag inducer may work well for this application. It could, almost, auto-configure itself to compensate for marine grown and such. Heck, I wonder if it could be placed INSIDE the hull and simply measure hull flex as a measure of dynamic water pressure agains the hull. Hmmmmmmmmmmmmmm...

[Tillsbury]

Nke make an electromagnetic sensor (that fits an existing airmar socket, which is more than airmar do). But it’s reportedly very inaccurate. The more modern ultrasonic ones (nke also make one of these) are the better ones, but furiously expensive and probably beyond diy.

[GordMay]

Greetings and welcome aboard the CF, CyKlop.

[cuffy]

Airmar ultrsonic sensor
https://www.p2marine.com/airmar-ust-...ed-transducers

[CyKlop]

Originally Posted by Tillsbury

Nke make an electromagnetic sensor (that fits an existing airmar socket, which is more than airmar do). But it’s reportedly very inaccurate. The more modern ultrasonic ones (nke also make one of these) are the better ones, but furiously expensive and probably beyond diy.

Yeah, I looked at those, but I thought that maybe a pressure transducer might work better, sensing the pressure of the water as it flows past. It would, of course, add a tiny bit of drag, though. After all, a load cell costs less than $10.00, a bit more circuitry brings the price up to, say, less than $100.00, with no moving parts.

[transmitterdan]

A hull mounted strain gauge (aka load cell) would be inaccurate and hard to calibrate. Hull flexing is a function of much more than hull speed.

A “bendy” probe sticking into the water would provide a moment arm that could be calibrated I guess. But you still have something that could possibly catch kelp or lines.

Measuring Doppler shift of ultrasonic sound waves seems attractive and cheap.

[RaymondR]

Stick with the paddle wheel and blast it with ultrasound with one of those ultrasonic antifouling gadgets to stop the fouling.

[Lars_L]

I recommend a Microcontroller of type PIC16F18313. It has a 10 bit AD converter (= 5 mV resolution), circuit for RS232 communication (= NMEA 0183) and I²C which is then suitably connected to a Raspberry Pi Zero. Price less than $ 2.

But you can instead build a log of the type I describe
http://www.cruisersforum.com/forums/...ll-143401.html In post #16 I have a technical description.

Lars

[Lokebilton]

Good info here about this topic https://www.practical-sailor.com/mar...he-leeway-code

[partyvi]

People, did we get somewhere with this discussion? Did somebody even try to build some prototype?