RC Sail Boat speed measurement and transmission

Hi all,

I am looking for a telemetry solution for a Radio controlled Sailboat to measure and transmit on a PC or Smartphone, the speed of the boat, the angle and the speed of the wind, the onboard battery voltage. Is the Arduino + Xbee pro could be the right solution ? What kind of sensor could be suitable to measure boat speed with a 0,1 knot accurency ? Is anybody already done this kind of application and could help me ?

Look up pitot tube in Wikipedia.

Pascalou31: What kind of sensor could be suitable to measure boat speed with a 0,1 knot accurency ?

I reckon you need a GPS for any sort of accurate speed measurements and I don't know if it could be that accurate or if it would work at all over the short distances moved by a model sailboat.

I speak from the experience of money wasted on a paddle-type speed log for a real sailboat.

I guess a model sailboat has the advantage that you could easily clean a speed-detecting paddle. You would probably get better info about speed measurement on a model boat website.

Separately, if you want to transmit data directly to a smartphone or PC you will either need Bluetooth or WiFi. I don't know if Bluetooth would have the range you require.

AFAIK if you want to use XBee you will need an XBee receiver to relay the data to your PC - but I have no experience of XBees.

If you are content to have a shore-based receiver then I reckon using a pair of nRF24L01+ transceivers would be a lot cheaper than XBees. If you need the range the shore-based module could be the high-power type (with the external antenna). This Simple nRF24L01+ Tutorial may be of interest.

...R

@ GoForSmoke : OK thanks for this information, this is a beginning...

@Robin2 : GPS accuracy is not ok for model sailing boat to get a 0,1 knot accuracy then I've already visited a lot of model boat website and forum without finding the right informations.

Regarding the shore-based receiver I'm afraid the nRF24L01 doesn't have enought distance transmission power. We need at least 200 meters and as far as I can see nRF24L0 offer only 100 meters max.

Xbee pro has the range you need.

Full size sailboats can use ultrasonic transducers to measure speed. It might be viable for your model, depending on how big it is. Of course, being marine equipment, they are prodigiously expensive.

@wildbill : I know about ultrasonic transducers for full size boat but this is not suitable for small 1 meter model (race) sailing boats regarding size and weight...

Pascalou31: Regarding the shore-based receiver I'm afraid the nRF24L01 doesn't have enought distance transmission power. We need at least 200 meters and as far as I can see nRF24L0 offer only 100 meters max.

Droping from 2.4Ghz modules to 434Mhz ones will give you around 5 times further range, all other things being equal.

LoRa modules are not expensive, and whilst its often assumed they are good for low speed data only I have had 105km LOS from a LoRa module running @ 13.7kbps @7dBm.

Pascalou31: I've already visited a lot of model boat website and forum without finding the right informations.

Maybe because what you want to do just isn't practicable on a small scale?

I don't think you have told us the size of your boat, the type of place you will be sailing it.

I have had a pair of low-power nRF24 devices working at 110m range. AFAIK a combination of a low-power nRF24 on the boat and a high-power nRF24 onshore will give a range of a hundreds of metres. I believe those devices are used in some of the off-the-shelf 2.4GHz RC systems.

...R

The water pressure under a moving hull will be less relative to the hull than pressure under a stationary hull. That's what makes a pitot work. A sensitive pressure sensor might be easier to read than the water level in a tube.

If you point a tube out of the stern, it will pull even stronger as long as the hull moves forward.

I have one other idea, not sure how good. If I put 2 contacts in water and pulsed a very short weak current from one to the other there should be a little time for the pulse to cross between. If the water is moving so one contact is in front then the time for the pulse to cross should vary depending on which makes the pulse and which senses it.

This is like the sonic except with electrons and it will likely corrode the contacts over time but if the pulses are 1ms out of every 500ms (to measure both ways in 1 second) and INPUT_PULLUP weak the corrosion will be very slow.

GoForSmoke: If I put 2 contacts in water and pulsed a very short weak current from one to the other there should be a little time for the pulse to cross between.

I don't think an Arduino is fast enough to measure this delay by several orders of magnitude. The difference (if any) in moving vs. stationery water will need another few orders of magnitude faster measurements.

@Robin2 : The boat I'm talking about are IOM sailing boat (International One Meter). The speed to measure is between 0 and 4 or 5 knots (Miles per hour) and we sale mainly on small Lac ==> 30 to 50 000 square meter (not on sea)...

Differential GPS?

Pascalou31: we sale mainly on small Lac ==> 30 to 50 000 square meter (not on sea)...

I assume you're only interrested then in SOG (speed over ground) and not STW (speed through water). A GPS should be fine for SOG. Leo..

GoForSmoke: I have one other idea, not sure how good. If I put 2 contacts in water and pulsed a very short weak current from one to the other there should be a little time for the pulse to cross between. If the water is moving so one contact is in front then the time for the pulse to cross should vary depending on which makes the pulse and which senses it.

I don't think it will be such easy. "Normal" lumped circuit model assumes infinite speed of light. Your signal will be more like RF, not classic circuit. Also relativistic physics consider speed of light constant. What it means for this application?

Pascalou31: @Robin2 : The boat I'm talking about are IOM sailing boat (International One Meter).

I presume this is a racing boat so you won't want things poked into the water that would reduce the speed.

Do you want to measure the speed during a competition or only when practising?

...R

Your signal will be more like RF, not classic circuit. Also relativistic physics consider speed of light constant. What it means for this application?

Except that electrons (which are not photons) do not move through wire at C let alone through water, especially in pulses which must change the local magnetic field as they move. A steady current is different once the field is established. Inductance does not happen instantly, moving charge inducing field has a time term.

Still the speed may be beyond measure at 16MHz.

GoForSmoke: Except that electrons (which are not photons) do not move through wire at C let alone through water, especially in pulses which must change the local magnetic field as they move.

Electrons are slow but the field is fast. And the field is the important entity. It is like the Newton's cradle or water in a water tap. The water starts flowing "instantly" despite the molecules are slow and need seconds to get from the switch out of the tap.

Look up the Faraday/Lenz Law. Change to the field is resisted. Pulse must change the field.

There actually is a transmission delay when changing stationary water in a pipe to flowing, it is nothing like instant. It is not even instant in a steel rod, there is a finite time between push at one end and the other moving. The Veritassium (physics) channel on youtube has at least 2 videos on the subject. Atomic bonds are not ideal constructs nor are simple mental models true descriptions of reality.

Do you know what an inductor does to pulsed DC?

GoForSmoke: There actually is a transmission delay when changing stationary water in a pipe to flowing, it is nothing like instant.

Yes, I assume the speed of such propagation is the speed of sound in the water. Of course "capacitance and resistance" of the tubes plays similar role as in electronics. It is mostly unrelated to the actual speed of water molecules movement.

GoForSmoke: Atomic bonds are not ideal constructs nor are simple mental models true descriptions of reality.

Similarly simple mental modes are insufficient to describe behaviour of your measurement system. IMHO you need take relativistic physics (and maybe quantum) into account, as well as lot of parasitics features of the actual circuit. Maybe a RF theory may help but I doubt it.

My feeling is that such measurement setup would be affected by so many unknowns and parasitics, it is hopeless. Theoretic description is out of "normal hobbyist" reach, time differences out of Arduino's resolution etc.

I think Doppler effect may be used for such measuring - which can be more easily accomplished with (ultra)sound IMHO.

@Robin2 : You are right, I'm talking about racing boat and it should be better to find a solution which doesn't reduce boat speed in both racing and practicing mode.

We should also take in account that on those kind of boats the weight is very important factor. The total average weight of an IOM is around 4 Kg including a 2 to 2,5 Kg ballast, a LIPO 2S battery + 2 servo. So the total weight of the system should not exceed 100 to 150g or it will be prohibitive ...