Aquarius Sortie 16-10-09

Report R3

Tide meter remote reading system

Tide meter outline of equipment

The equipment is designed to remotely read tidal flows over a complete tide cycle. It consists of a sensor head fitted into a housing wired to a lap top

The sensor head is a paddle wheel and is designed to be fitted to the outside skin of the vessel

The unit is mounted in a simple pipe with wiring led from the top of the pipe to a lap top

Summary of state of play Feb 2010

Colin Moore’s report on first full trial - Aquarius Sortie 16-10-09

Gentlemen, Here follows a description of activities today together with (attached) result data and initial conclusions.

 

1.    AM we met Andrew at the boat and fitted the static tide gauge to the stern post ( that fitted for the previous sonar trials - Balls etc. ). We also fitted and tested a new smaller computer rack which Nick and I had made in the last few days.  All system tests were satisfactory, to the extent it was possible to determine alongside.

 

2.    We met again at the boat 1600 and made careful way under power out to mooring DH2 (immediately opposite the Webley). We teid Aquarius to this mooring by a single (therefore swinging) hawser, and cut power. High tide, of about 5 Metres, was predicted for 1845. All times in this report are local (BST) speeds are in knots and depths are in feet.

 

3.    The equipment was set to run, with a laptop recording smoothed speed, depth and water temperature data every 60 seconds. (The speed term is smoother internally within the DST800 device with a time constant of about 6 seconds. No additional smoothing was applied in my software.)

 

4.    Various attempts were made by Nick to obtain stable and seemingly consistent current readings using Tim Wright's tide gauge. We were unable to achieve such consistent readings from this instrument. We even resorted to reading the instructions! - but to no avail. Its impeller was spinning freely (even when in air with the wind) but we became convinced that it was under-reading current and observed that it read the value of 1.4 knots for a considerable proportion of the time. Reluctantly we had to conclude that Tim's instrument was defective, or that our method of using it was in error. The DST 800 sensor head was mounted starboard side aft at a depth of about nine inches below the surface. We attempted to use Tim's gauge at exactly that same point, and at other locations and depths. We could not achieve sensible looking data from it.

 

5.    The computer recorded the DST 800 data output every 60 seconds for the duration (almost 2.75 hours). Results are plotted at attached Graph 1. Results are tabulated in the attached text document (which you can open in WORD or WORDPAD). Notes within that .txt should adequately describe events to you.

 

6.    Graph 1 shows the water depth at the mooring and the measured current at a depth of about 9 inches as a function of time. The obvious quite interesting results include 1. Maximum current was observed at two hours before high water. 2. The actual high water seems to have been at 1915, which was 30 minutes later than the prediction. These graphs have been plotted manually, and not every recorded point is plotted. However, the curves appear quite smooth and consistent, giving us confidence in the recorded data and the equipment.

 

7.    Given that Tim's instrument did not seem to be working well, we resorted to "pooh sticks" dropped from the bow and timing their passage to the stern, noting that Aquarius is 29 feet long. Three such tests were carried out during the sortie, and on each occasion the indicated speed from the DST 800 was noted. Results are plotted at attached Graph 2. Given the crude nature of this method, these results are remarkably consistent. They indicate that the DST 800 as deployed at a depth of 9 inches outputs current data 2.25 times higher than the surface current as assessed from a "pooh stick".

 

8.    The reason(s) for this over-reading are as yet unknown. I have a theory - and it is only a theory. The DST 800 speed sensor is designed to be hull mounted in a boat, whereupon the tiny paddle wheel would extend only about 8mm into the water slipstream from the hull surface. Being that close to the hull, it would perhaps be in the boundary layer and therefore in water moving somewhat with the hull rather than "static". It may be therefore that DST have designed the thing with this in mind - and that its speed output figure is actually made higher than the rate of rotation of its paddle wheel would indicate to compensate for this consideration. In out application, the DST is not deployed from a hull but from the bottom of a 110 mm dia. vertical plastic pipe - the base of which would probably display very different boundary layer effects as compared to a proper hull.

 

9.    If the correction factor of 2.25 is applied, I think all our measurements and observations today start to make reasonable sense. However, we will need to carry out further calibration testing. One method would be to deploy the GPS tide buoys past the moored static tide gauge. 

 

10.    One final thing. On a few occasions the DST 800 paddle wheel appeared to stop rotating, although there was a reasonable current flow past it. Sometimes this "glitch" cleared itself, but on others we had to get an arm into the water and give it a little tweak. Maybe pieces of crud had fouled the wheel - but none was seen and no resistance was felt by any of us to un-sticking the wheel.

 

That is the summary of today's work. Tomorrow we will remove the equipment from Aquarius for now. (re-fitting need only take 15 minutes now).

 

Comments all appreciated.

 

 

Colin

Colin’ comments on Nicks comments

The "calibration" with the pooh sticks was as correct as circumstances allowed. Pooh stick times were recorded (in my contemporaneous notes) on three occasions and the displayed output of the static tidegauge was also observed and noted. Hence the three cal. points plotted at Graph 2 of my preliminary results. The fourth (phantom) point is zero/zero. It can not be denied that, based on this very limited data, there appeared to be a linear relationship between pooh stick surface speed and recorded gadget speed.

 

Although we did not carry out a pooh stick cal. at the time of maximum tidal flow, the maximum gadget reading was about 6 knots, which per "calibration" factor of 2.25 means an actual flow of 2.67 knots. Are we sure that the peak flow was actually substantially in excess of this? During all the (few) tide buoy tests, we never measured a surface flow much above 2 knots. This is a matter of subjective opinion, but I absolutely agree that we need to calibrate further.

 

At the end of the sortie, for around 22 minutes, the gadget registered a fairly consistent 1.7 ish knots. If we accept provisionally a cal. factor of 2.25, then this equates to a true current of 0.75 knots. Can we say that this was not what we were experiencing? That would correspond to a pooh stick time of 23 seconds for the 29 ft. of Aquarius's length.

 

That the reading "collapsed" to zero shortly after this did not surprise me enormously. The gadget maker does not specify a rigid minimum speed of operation. I think that the sales blurb says "below 2 knots". It is to be expected that the paddle wheel will stop abruptly at some low water speed when stiction resists flow induced motion of the wheel. I was favourably surprised at how low a flow rate it seemed to work down to.

 

The device details for which you asked Nick are : AIRMAR component DST 800 (plastic). You will find that the AIRMAR home base is in the USA and that their European distribution hub is in St. Marlo, France.

Colin’s comments on Chris’ comments

Thank you Chris. I think that I maybe did not explain things sufficiently.

 

1.    Pooh sticks vv. device did indeed appear to provide a linear relationship between pooh speed and device output. The tentative correction factor which I calculated (see Graph 2) is NOT 2.25 knots but a FACTOR of 2.25. Thus you have to divide the device indicated output by 2.25 to get the true water speed.

 

2.    The DST 800 is indeed small and designed to be mounted through a 2 inch dia. hole in the bottom of a boat's hull. The paddle wheel "buckets" then protrude into the water beneath the hull by no more than about 8mm. So when I was talking about a boundary layer effect, I was not referring to the river's boundary layer (as you say, water at riverbed is almost static, that at surface has maximum speed) but to the boundary layer which would apply to a boat's hull as it moves through water at any relative speed. Water in contact with the hull would tend to move with the hull, water some distance from the hull (say 1metre) could be regarded as static. Thus there will be some sort of boundary layer between the boat's hull surface and the main body of water through which that hull is moving. I guess that the 'thickness' of that layer would depend, inter alia, upon hull shape, exact location on the hull and hull speed through the water.

 

2A.    Nick did use Tim's device to assess whether the current varied to any extent between the surface and about 1 metre down. He reported no observable variation, so I think that out sensor was not mounted within a RIVER BOUNDARY LAYER. However, as commented, Tim's device did not seem to be working quite correctly.

 

3.    My theory was that when fitted as intended through a boat's hull bottom, the DST800 would probably be working within the boundary layer of that hull. If so, it would tend to under-read true water speed because the water within which the paddle wheel is sited would, to some extent, be travelling with the hull. So I thought that Airmar may have scaled the sensor output such that a compromise reading is obtained, based upon a 'typical' hull boundary layer effect. The DST 800 is designed to interface with various nav. displays made by other firms. Invariably all but the cheapest of these provide for the user to calibrate the speed reading up or down to suit his particular hull shape and location of his sensor.

 

4.    Our case is an extreme of location, since the unit is not fitted to a hull at all but to the flat bottom of a 110mm dia vertical tube, the bottom inch of which is only about 80 mm. dia. Thus there is virtually no hull boundary layer effect, and over-reading is, I think, to be expected.

 

5.   Applying the tentative correction FACTOR of 2.25 to logged data, we saw a maximum current at the Webley DH2 location of 2.33 knots. This seems to accord reasonably with Wallingford data and with our own limited use of the tide buoys. However, it was Nick's impression that the flow rate past us at peak was somewhat greater than this (eg. by looking at the wakes left by adjacent mooring buoys.)

 

6.    The predicted Aberystwyth high tide 16/10/09 was 4.6 M / 1918 BST. This would imply 1844 at Cardigan, I think. Hence my commenting upon the fact that our depth curve seemed to peak at 1915. Also up until the time we drew stumps, we could see that water was still passing us, slowly, inbound (by looking at bits of froth etc. carried with the tide). It is quite handy that the DST 800 measures Depth, Speed and Temperature (hence the DST) so I arranged for the system to log all three. You might care to look at the recorded data in the text file and make your own graphs.

 

7.    The reason for the apparent incorrect readings from Tim's instrument needs to be investigated. It is noted that his sensor has received an araldite repair previously. Maybe it simply needs a replacement sensor head. This should not be very expensive, if still available. I am sure that we need to carry out a further calibration sortie(s). Nick has an idea in mind to attach a flat plate to the bottom of the DST 800 sensor pole with a curved forward facing edge (bit like a simple surf board). To that end I have left the sensor pole with Nick while I am away for the next week / 10 days. Perhaps, Chris, you could call by and look at it. We put it in the lean-to garage / shed outside Nick's old home workshop.

 

8.    When we do decide upon a final design / calibration I can write that into the software (just as the better commercial boat speed displays allow). If the calibration factor turns out to be not just a simple linear FACTOR as presently envisaged, I can also deal with that.

 

Paddle wheel exposed to current for half its circumference

This would be the surface of the vessels hull