Erraid is where the Stevenson's obtained, queried and shaped the rock for Dubh Artach. Later the island became the lighthouse station and keepers with their family lived there. On top of the island is the communication tower which was used to check the light was lit at Dubh Artach. If it was not lit, the keeper would try and contact Skerryvore or Tiree so that someone could communicate to passing ships. It was the Torran Rocks which were more feared. The houses are now owned or managed by The Findhorn Foundation. I walked out to Erraid via the east shore opposite Knock Vologan after walking round from the Fidden campsite. The sound dries out at low water quite soon after high water as the depth is not that significant.
In May I anchored in Tinkers Hole and departed through the north passage. UKHO Raster charts, Raymarine Lighthouse Charts and Navionics can all be used, but show a reef with 0.2m (UKHO) or 2.2m (Navionics) extending out northwards from one of the rocks. Antares shows it as an isolated rock with deep enough water around it for my 2m draft boat. I had done this passage way back in the late 80's and it was good to be reassured by Antares what was actually there.
Some pics. We do sail in a beautiful part of the world.

Erraid Crossing FromKnock Vologan About 1 Hour Before Low Water

The Dubh Artach Quarry on Erraid

The Communication / Watch Tower To Check That Dubh Artach Was Lit

UKHO Raster Chart of North Passage Options From Tinkers Hole

Antares Charts North Passage Options From Tinkers Hole and The Reef Is Now An Isolated Rock