how do I log the sea's depth every 2hrs (say) over 2 or 4 weeks?

[Shearwater]

My boat is moored in a Med Sea marina in about 2 metres of water. There's a very clearly marked waterline on the pontoon supports but I am often surprised by the variation in depth of the water, just after Christmas there was an extra 30 cms or so of water that made climbing over the pulpit very difficult. This was not (I suggest) a tidal surge as it remained that depth the next day and then subsided back to 'normal'. On some other occasions the level is below the 'norm' but only a few cms.

I forgot to look at the surface pressure charts at the time and there may well have been an anticyclone somewhere Italy way and a depression with us. Similarly the adjacent and huge River Ebro may have disgorged rain water from the hinterland - I understand seawater has its own viscosity complemented by the terrain of the sea bed etc but I really don't know what I'm talking about here but I realise the sea surface is not necessarily 'flat'.

What I would like to do is log the depth (ie 2 metres + or - 40 cms) every so often on a machine that will remember the data over a period of weeks to account for the times I can't get to the boat or just forget in the excitement of going sailing. I can afix something to the pontoon near the boat's bow and have access to 240 and 12v. What do I need? I don't want to buy something fancy and if I can make a unit myself from off the shelf bits then I'll give that a go. What do I need? Advice gratefully received, thanks in advance. Oh, it might have to be duck proof!

 

[FullCircle]

Try sending a mail to Tim Thornton at TeamSurv. He will tell you what kit they supply to ordinary boats for surveying local area depths.

http://www.teamsurv.eu/Home/tabid/56/language/en-GB/Default.aspx

 

[rogerthebodger]

If you can connect NMEA 0183 instruments to a PC or laptop using NavMonPC you can log various parameters including depth at various rates and then export the data into a spread sheet for analysis.

http://www.navmonpc.com/

 

[CreakyDecks]

If you fixed a length of 1/4" ID pvc tubing down the leg of the pontoon you could put a pressure sensor (about £20) on the top of the tube and measure the change in pressure as the water rose and fell. The difficult bit would be logging the results on something you'd be happy to leave unattended for weeks on end!

 

[alahol2]

Are there any real time tide stations in the area with internet archives?

 

[AngusMcDoon]

If you can connect NMEA 0183 instruments to a PC or laptop using NavMonPC you can log various parameters including depth at various rates and then export the data into a spread sheet for analysis.

And if you have Seatalk instruments you could use a Seatalk to USB converter to do much the same.

 

[Swanrad2]

Old school - is all you are really interested in is the range then?

Two lengths of ratchet style bar (non-engineering term) - one with teeth allowing upward movement, one with teeth allowing only downward movement. A plastic cork float on each attached to a metal ring allowing vertical movement with tidal rise and fall but only in one direction - i.e. one moves upwards with the highest 'tide' when the water fall away it locks there and the other downwards only.

No idea if it would work, but I'd have more fun trying than I would wiring a a seatalk network to a laptop and suspect save a few quid as well?

 

[lw395]

How about a purely mechanical system, use a chart recorder like a barograph, and move the pen via a linkage from the seabed?

 

[AngusMcDoon]

...and suspect save a few quid as well?

Bet I could do it cheaper via Seatalk than some Heath Robinson contraption made with gaffer tape, chewing gum, bits of bicycle chain and a second hand cat flea collar.

 

[hartcjhart]

is this any good for you

http://easytide.ukho.gov.uk/easytide/EasyTide/ShowPrediction.aspx?PortID=1705&PredictionLength=7

 

[AntarcticPilot]

is this any good for you

http://easytide.ukho.gov.uk/easytide/EasyTide/ShowPrediction.aspx?PortID=1705&PredictionLength=7

No; he's looking to measure the non-tidal variations in depths that are not predicted by this kind of prediction software.

Of course, a tide-gauge is the professional way to do it. A depth-sounder isn't reliable because of wave action and variations in water temperature (which affects the speed of sound in water); tide-gauges either use a pressure sensor located on the sea-bed or a depth sensor in a well connected to the sea.

 

[AngusMcDoon]

depth-sounder isn't reliable because of wave action and variations in water temperature (which affects the speed of sound in water); tide-gauges either use a pressure sensor located on the sea-bed or a depth sensor in a well connected to the sea.

The tide height changes slowly enough to take multiple readings to average out transients like waves. The speed of sound in water could be compensated for from the water temperature reading from a log transducer if anyone was really interested, but I expect the water temperature change from one week to the next is pretty minimal, especially below a boat - 3% over a year absolute temperature in the UK? I always seem to run aground at the same indicated depth, summer or winter. I doubt the OP is looking for sub 1% accuracy. Depth sounder and logger seems to me to be the easiest way to go for accuracy of a few %.

 

[AntarcticPilot]

The tide height changes slowly enough to take multiple readings to average out transients like waves. The speed of sound in water could be compensated for from the water temperature reading from a log transducer if anyone was really interested, but I expect the water temperature change from one week to the next is pretty minimal, especially below a boat - 3% over a year absolute temperature in the UK? I always seem to run aground at the same indicated depth, summer or winter. I doubt the OP is looking for sub 1% accuracy. Depth sounder and logger seems to me to be the easiest way to go for accuracy of a few %.

I agree that a depth-sounder is probably good enough for what the OP wants, provided the sampling interval is short enough to ensure that wave frequencies aren't aliased. But for accurate measurement for determining tidal constants and determination of sea-level for geodetic purposes, what I said is correct. You are correct that sea-water temperature can be compensated by measurment, but incorrect in assuming that the rate of change will be slow in shallow water, where both temperature and salinity can vary rapidly with fresh-water run-off. Shallow water is usually appreciably warmer at the end of a sunny day, as well.

 

[Swanrad2]

Bet I could do it cheaper via Seatalk than some Heath Robinson contraption made with gaffer tape, chewing gum, bits of bicycle chain and a second hand cat flea collar.

I prefer zip ties to gaffer tape - so much more versatile.

 

[AngusMcDoon]

...where both temperature and salinity can vary rapidly with fresh-water run-off. Shallow water is usually appreciably warmer at the end of a sunny day, as well.

The pressure sensor you mentioned earlier will be affected by those variables as well, unless compensated for.

 

[AntarcticPilot]

The pressure sensor you mentioned earlier will be affected by those variables as well, unless compensated for.

The density variation isn't as great as the velocity variation; about 1% maximum. Salinity and temperature give several percent variation in velocity - about 5%. However, I believe it is compensated for in tide-gauges anyway.

 

[nigelmercier]

If you only wanted to measure the maximum, you could paint a stick with water soluble paint.

 

[Andrew G]

I always seem to run aground at the same indicated depth, summer or winter.

Great comment! I've noticed that in the S hemisphere too. Strange that.

More seriously, for zero cost - if you are happy to leave a PC running - if your gear outputs NMEA you can just use Hyperterminal and save the data to disk. Or if its SeaTalk use one of Angus' SeaTalk devices to log the data to disk. As suggested above you can then use Excel etc to analyse the data (if it is running for a while it could be a lot of data - would need to think about the capacity of the hard disk!).

Andrew

 

[Steve Clayton]

Your pontoon is fixed/static so make up a marker covering the range and fit to pontoon. Fit a webcam to toe rail and point at marker. Webcam to laptop and software to take photo every 2 hours. Just got to figure out night-time shots (may not be so mportant)

 

[CreakyDecks]

You could always buy some steps.