# Measuring speed through water

1. Nov 28, 2012

### focus123

Hi, I'm an amateur rower exploring the ways of measuring the speed of the boat through the water. I am attempting to design and make my own speedometer system using probably Arduino, I know that GPS and impeller are the two main ways of measuring velocity, however I am curious to explore other alternative methods.
Any suggestions would be greatly appreciated,
Thanks!

2. Nov 28, 2012

I have seen air flow rate measured with 2 thermistors, one measures the static temperature and the other uses a higher current input to actually heat the thermistor - the air flow cools then thermistor, and the resulting temperature, read as a resistance ( or voltage across the thermistor) is a function of Heat input, ambient temperature and flow rate.
Would take some calibration but I can see no reason this would not work for water - other then water cools much better than air, so it may be difficult to heat the thermistor enough to get a differential reading.
Just a thought

3. Nov 28, 2012

### focus123

Thanks for the quick reply, sounds like it could work, however, how large would the current need to be to heat significantly?
I have been looking into magmeters, used to measure volumetric flow rate in pipes and was wondering if anyone knew a method of using these to measure velocity of an object through water as opposed to the velocity of the actual flow?
Thanks again

4. Nov 29, 2012

### Staff: Mentor

Hi focus123. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

You might be able to put a pressure transducer on the bow of the boat, the faster you row the greater the force pushing on the bow. Maybe a U-tube manometer would give a visual display.

The venturi principle might offer opportunities in laminar flow. As water flows faster around the bow, the pressure in a sensor tube on the side of the bow should drop.

Perhaps the average height of the bow wave could be calibrated to be a measure of speed? Would be a good exercise in filtering and signal processing.

If you took two small metal or wood plates and formed them into an angle pointing in the direction of the boat's travel, the faster that arrow is pushed into the flow of water the closer the plates would be forced together. Arrange for a spring to oppose this. Calibrate the angle of the arrow for speed. Or use a force transducer to measure the force pushing the plates. (A single plate on the side of the bow could be forced harder against the hull the faster the boat travels.)

Drag something aft on a cord. The faster you row, the greater the tension in the cord.

Ultrasound and doppler shift might be feasible.

A tiny propeller facing forward would spin faster with faster flow over it.

There are a few ideas.

Last edited by a moderator: May 6, 2017
5. Nov 29, 2012

### jim hardy

old fashioned boat speedometers are a simple pressure gage with a square-root scale.

Read up on a "pitot tube" sensor
and Bernoulli's equation

basically the pitot tube will deliver to the pressure sensor a pressure equal to
(velocity)^2/2g , in units of height of fluid (in your case water). The nonlinear scale extracts √ and converts units.

6mph being 8.8 ft/sec, i get about 1.25 ft of water. One can get pressure sensors with that range.
Perhaps a tube on leading edge of rudder facing forward.......
Study upon pitot sensors though, with such a small range the depth below water of your pitot will have to be accounted for.

That's just one approach...... and it's sorta old fashioned.

Polaroid used to sell the ultrasonic rangefinder from their cameras for experimenter purposes. If you have a good electronic hobbyist buddy you coould pick up particulates in the water and calculate speed

The magnetic flowmeters that i knew all had large heavy iron cores so as to make substantial magnetic flux at 60 hz.. doesn't sound deirable for a competition watercraft. However a google on "magnetic velocity sensor seawater" returned lots of articles... here's a pretty good one that demonstrates the theory. Just might be plausible with today's precision IC amplifiers. Higher frequency relieves the need for high flux.

http://new.aslo.org/lo/toc/vol_20/issue_2/0174.pdf [Broken]

old jim

old jim

Last edited by a moderator: May 6, 2017