There are a number of strong tidal races in the area, "'Tum ba nach "'is one that extends north from Sheep Island to around halfway across the Sound of Sanda.
22.
In the 18th century Thomas Pennant recorded that sea-going vessels of up to were being hauled over the isthmus in order to avoid the dangers of storms and tidal races in the seas surrounding the Mull of Kintyre.
23.
When the ship's movement is determined only by currents or the wind and particularly by storms, a common result is that the ship is unable to avoid natural hazards like rocks, shallow water or tidal races.
24.
Tidal flow is disrupted by the feature; at 10 metres ( about 5 fathoms ) deep and long, it causes a tidal race to the south of Portland Bill, the so-called " Portland Race ".
25.
Since the wreckage of their vessel washed ashore from the south, the saga specifically states that the ship was thought to have been lost south of Shetland in " Dynr�st ", a treacherous tidal race otherwise known in more modern times as Sumburgh Roost and Da Roost, lying between Sumburgh Head and Fair Isle ( ) . in the swell, and kept vibrating backwards and forwards in these seething tides for weeks at a time.
