An echo sounder pings the seabed and times the return, and the depth it reads is compared with the chart because a wrong depth is a grounding. On one stretch it reads nine metres less than charted, and the shallow patch is the same length as the ship, and it keeps station with her at her own speed.
I am a deck officer, and the echo sounder is one of the instruments I sign for on my watch.
How it works.
A transducer in the hull sends a pulse straight down and listens for the return. The depth under the keel is on a display on the bridge and on a paper trace, and the trace is kept because a grounding inquiry reads it.
The route is over charted water of between twenty and forty metres, and the sounder agrees with the chart to a metre or two, corrected for tide, everywhere on the crossing except one stretch.
On that stretch the sounder reads nine metres shallower than the chart.
It comes in over about half a minute, holds for about eight minutes, and clears over half a minute. On the trace it is a step up and a step down, like a plateau, and it is the same every crossing, in both directions, on all three ships that work the route.
The plateau on the trace is about a hundred and forty metres long, at our speed. The ship is a hundred and thirty six metres.
I did not notice that for two years, until the trace from a slower crossing in fog, at half speed, showed the plateau twice as long on paper and the same length in the water.
It is not a bank. A bank is where it is. This is a length of shallow water the length of the ship that is under the ship for eight minutes at whatever speed the ship is doing.
What has been checked.
The sounder. Two transducers, forward and aft, on separate systems, both read it. A portable survey sounder over the side on a cable read it.
The bottom. The harbour authority's survey launch has run that stretch with a multibeam, which maps the seabed in a swathe, and the seabed is where the chart says, thirty one metres, flat sand.
The water column. A multibeam sees everything between the surface and the bottom, and it saw nothing between the surface and the bottom on the stretch when the launch ran it, because the launch is not the ferry.
I have had the launch run alongside us on the stretch, at our speed, with its sounder on. Its sounder read thirty one. Ours, forty metres away, read twenty two.
What that looks like on the trace.
A plateau nine metres above the bottom, with a top that is not flat. It has a shape, a long low profile with a rise about a third of the way along, and the shape is the same on every trace from every ship on the route, and I have laid the traces over each other and they match.
It is under us. It is the length of us. It goes where we go, at our speed, nine metres above the sand, and it has a shape, and the shape does not change.
What is done about it.
The sounder is not used for navigation on the stretch, by standing order, because a reading nine metres shallow on a stretch with twenty two metres under the keel is a nuisance rather than a danger, and the order says to use the charted depth.
The trace is kept. The superintendent has a folder of them.
What I do.
I keep the watch.
The one thing I have changed is that I do not go down to the transducer flat on the stretch.
The forward transducer is in a compartment low in the bow, and the sounder's return is audible there as a faint tick through the hull, once a second, and I used to go down on that stretch because the tick changes pitch when the depth changes and I wanted to hear it.
On the stretch it changes pitch, as the plateau comes in, and then there is a second tick, under the first, slower, from below the hull, at the interval of a sounder pinging up from nine metres down at something a hundred and thirty six metres long above it, and I stood in the compartment and listened to it measure us for eight minutes, and I do not go down there now.