tide guage

		This was originally going to be a conventional tide gauge, with the buoy floating inside a tube to remove the effects of the waves. I was worrying about barnacles inside the tube when I thought of not having a tube and letting the buoy bounce up and down with the waves. This has the advantage that the buoy is visible and the mechanism will be constantly moving so it will be more interesting to watch. It also produces a more interesting recording on the chart. If you want accurate data on tides, you wouldn’t want a gauge like this. You would probably just have an ultrasonic sensor, with some software to convert the data into a graph. There would be nothing to see, just an LCD screen showing the result. But for people on holiday, its much more satisfying to have a mechanical contraption that produces a 'picture' of the tides.

Making this gauge seemed to involve a lot of sums. Sums to do with buoyancy, friction, sea levels and other stuff. No sophisticated maths, but I kept double checking everything as I’m usually mathematically much too careless. The whole design was also a more theoretical exercise than I’m comfortable with. There was no way to try a prototype out on the pier. I literally had no idea what would happen when we finally raised the finished pole. My initial reaction was intense relief – the marker did go up and down with the waves and turn the shaft to the recorder. Only gradually did I become aware of the problems.		The pen cut through the paper on the first day, leaving no recording. Fortunately that's one of the easier problems to sort out.

The main problem is that the buoy’s guide wires (4mm diameter) are too thin. Over the length of over 6m from the top of the pier to the bottom bracket they are just far too stretchy. I can see them twisting up or even breaking in a strong gale.		With a temporary ball point pen refill its started recording. The far swing to the right is a really low tide just after the new moon. The thickness of the line shows the height of the waves. I'm looking forward to a rough sea to see what it looks like.
Even worse, I soon found that the steel wires were acting as saws, cutting through the bronze bushes. This is photo shows just 10 days wear! The rope has cut nearly 3mm into a 25mm long bush.		Ten days later, we've just replaced the 4mm guide cables with stiff 8mm cable and replaced all the bushes with roller assemblies. I'm still nervous about winter gales, but it will now be fun to see how it does in fairly rough seas. .

Two weeks later I found the answer was that it didn't do well at all. The 8mm cable broke in the first rough sea and when I rescued the buoy, I found the rollers had already almost worn through. The rope was my fault. Rigging cable can be bent once or twice, but I had inadvertently made it constantly flex, so it fatigued.

When I stripped the rollers down, I found the stainless shafts were also wearing. I fear the silty North Sea water (which is famously gray, never clear) is acting like grinding paste. If this is true, I may have to give up and replace the buoy with an ultrasonic sensor.

I've decided to have one more attempt. I've replaced the rollers, with underwater grade Igus polymer bushes, and I'm now trying polyester rope instead of steel rope. Unless we get divers who are expensive, we can't reach the underwater frame to fix the ropes on, so we are going to try tieing heavy weights on the bottom.

Two weeks later, the polyester rope has broken. From the point at which it broke, it had obviously been rubbing against the underwater frame.
I'm feeling a bit optimistic though. There were no signs of the rope wearing where it had been sliding up and down the buoy. Also, watching it bob up and down in a rough sea, the stretchiness of the polyester rope looked much more suitable for coping with big waves than the rigid rigging cable.

With the Fisher Space Pen the markings are looking much better. There's a great contrast between calm and rough weather. The markings also show another problem I haven't yet solved - the markings drift slowly over towards the right hand side of the paper, particularly in rough weather, and mostly near low tide.

I then had to wait for I diver to remove the underwater frame before trying anything new. George was reluctant to do it in the winter, so it wasn't until the following June before he came. By that time I was able to look at the whole project more objectively. I decided the guided buoy was just not practical in the long term. I would be nervous about it every time there was a storm, and I couldn't think of a good solution to the problem of the rope occasionally slipping a bit on top pulley, causing the recording to veer to the right (see photo on left).

Tide gauge MK2 looks much less dramatic. All that remains of the original is the chart recorder box. An underwater pressure sensor sends an analogue output to a servo motor that moves the pen. I had never tried using analogue sensors of any sort, or servo motors, so it took me a while to puzzle out how to use them, and I killed the first servo motor (Ł200) by connecting the sensor voltage in reverse polarity. It was finally installed in mid October 2008. One improvement from the original version is that its now easier to see when each tide occurred, because the servo draws a pale line all the way to the right once a day at midnight.

The long struggle to get it to work now makes me really proud of it. I particularly like the way a rough sea look properly angry.

Current tide and wave data (not from my recorder, but from a nearby offshore buoy) can be found online at cefas wavenet:
(Position the mouse cursor on map off the East Anglian coast and the current data from the 'southwold approach waverider' buoy should show on the left of the screen.)

I was rash to be so confident. About a year later (July 2009) it stopped working. At first the sensor was tangled it was impossible to pull out of the water. Three months later it suddenly came up easily - it had been freed when an 8mm diameter shackle snapped into 3 pieces! The power of the sea is awesome, no wonder 6 ft high tsunami waves do so much damage. The sensor was still OK. It had stopped working because it had become completely enveloped in a mixture of shells, tiny animals and muddy sand - sadly I didn't have my camera when it came out of the water. The problem was that the hole in the sensor (top right in the photo) that lets water onto one side of the diaphram had got completely blocked.