The phenomenon of the Moder Dy - In Conversation with Shetland inhabitants:
Charles Simpson (Cunningsborough): It’s a reverberation; a back echo or reflection of a wave hitting an immoveable object (the land).
Davey Smith (retired fisherman, Scalloway): Well to navigate you need something fixed that can be seen, and I don't see how you can navigate from something that you can't see.
George Duncan (fisherman, Burra): It’s something that has now been lost – old fishermen could read the surface of the water – the pattern – the way the current was running. It was an experience handed down in the same way they could read the weather 24 hours ahead. Used ‘meids’ as well – fixed positions on the land by which to navigate. I remember my faither taking me out in a boat when I was a 'perrie' boy and pointing into the sea and saying to me 'there’s the moder dy'. I couldn't see anything. It was also used by Polonesian fishermen. It was a back wave, and I was shown a diagram showing the direction of the waves. (George drew this using a bit of chalk that we found on the road).
Hazel Hughson (Shetland Arts Officer): ...currents far underneath - from deep underwater; Fishermen could feel it in their feet; it's not on the surface.
Alister Goodlad (Tronda): It was a way of steering. Can be seen from a plane – wave always comes in the same direction; SW it will always come from WSWesterly??? (not sure I wrote this down correctly). If the wind stays in same direction you can gauge where you are. Think that it is mainly a phenomena of the west side of Shetland. There is the Wyville Thomson Ridge between Shetland and Faroe; the ridge affects the flow of water due to the changes in depth and temperature. It’s possible that as a result, a ripple effect may be created in the water. (The ridge separates the Faroe-Shetland Channel to the north from the Rockall Trough to the south. Its significance lies in the fact that it forms part of the barrier between the colder bottom waters of the Arctic and the warmer waters of the North Atlantic`). Does this phenomena only occur on this side of Shetland?
Dodo Watts (retired fisherman, Scalloway): Well, it’s far-fetched!. We used radar and charts - taking longitude/latitude readings.
Ian Napier (Senior Policy Advisor, NAFC Marine Centre: Re. the Moder Dy): My suspicion is that it is more likely liked to be the deep ocean swell coming in from the open Atlantic. The currents past Shetland are pretty much one way - I am not aware of any significant counter-flows. The long Atlantic swell wave-length waves will probably start to 'feel' the bottom as they cross the continental shelf, causing them to turn towards the shore. The result is that the direction of the Moder Dy would probably be relatively constant, regardless of the local wind / sea conditions. This would presumably give those with the necessary skills / experience a fixed (more or less) reference to judge directions. Sailing 'with' the Moder Dy should take you back to land. There are interesting parallels with the ability of Polynesian sailors to use swell patterns to navigate between oceanic islands in the Pacific.
Addendum: It is likely that the Moder Dy would work best on the west side of Shetland if - as I suspect - it depends on long-wavelength waves coming in over long distances from the open Atlantic Ocean and encountering the (relatively) shallow waters of the continental shelf. We could be talking here about waves with wavelengths of several hundred metres that may have travelled for 1000s of miles. Such waves could have periods of 15 seconds, or more. Waves generally start to 'feel' the sea-bed when the water depth is less than half their wavelength.
To the south and east of Shetland (in the North Sea) there is not the space or water depth for such long waves to form. That said, waves always tend to turn towards the shore (or at least shallower water) so I would not entirely rule out the possibility of some sort of usable phenomenon occurring anywhere around Shetland. But in the relatively enclosed and shallow North Sea the wave regime is always likely to be more 'messy' (confused) and thus more difficult to interpret. I suspect the relatively 'clean', long, open-ocean swells from the Atlantic would be a better guide (and easier to separate from the more local waves, which be running in a different direction).
Other thoughts from boat-builders, an artist, fishermen in Lerwick / Scalloway:
- Ground sea swell – a back swell – used in fog to navigate. Now forgotten.
- A swell running into shore.
- I’ve never seen it – but the old fishermen could see an indication towards shore.
- It’s a whole experience of the sea – auditory… seen… and felt.
The Moder Dy (the mother wave)
(Extract from poem by Jessie ME Saxby written around1880/90’s)
..…. Gray, gray was the lift. And the mist lay low
Like a shroud ower the weary sea;
It heaved its breeest wi’ a lang-drawn breath,
Like a body that’s going ta dee
We could see no meedes, nor glimpse o’ the laund –
Nae guide was the guidelss wave;
“We’ll never see hame,” said Ned o’ the Knowes –
I wheshted the fule wi’ me nave.
Then Mauncie he stimed weel into the lift,
And then upon the sea;
“Noo, boys look oot for the Moder Dy
And mark hoo she rins,” said he.
“The eight peerie dys that geng afore
Rin this way an’ that, ye keen;
But she flowes straight wi’ a lang, free sweep
That ‘ill lighten wir herts an ‘een.
“We need na compass, or glim frae the lift,
There’s light in her fleecy kame;
She maks for the laund, that’s as true as deth,
And she’ll guide wiz safely hame”……..
Image and description found on www.foulaheritage.org.uk: The Moder Dye, used by Shetlanders before the days of compasses to find the land in times of fog. The radar photograph above shows how this was done. The photo was taken the day after the night hurricane Flossie passed through Shetland in September 1978. The heavy westerly sea swells were about 250 yards apart and show up well on this photograph. The swells reflect around the north and south ends of Foula (left centre) to produce an overlapping pattern which continuously broadens out until it reaches the Mainland of Shetland (top right). Any line drawn through the points of overlap, where the two swells peak, leads from the Mainland to Foula.