I think I know the answer. I anchor a lot. The anchor chain is rusted around the 30m mark. The rust extends about 5m either side. The manual windlass probably throws off specks of rust and the wind spreads it around.
Solution - re-galvanising, although one company was reluctant to 'contaminate'...
First noticed this in Ireland about a month ago. Now encountered this in Devon. A light grey very fine dust has appeared on the deck and coachroof. This contains some iron or iron compound that reacts with salt water to produce pin-pricks of rust on these areas. The spots can easily be removed...
No plans but I have the book: Boat Building by Percy W. Blandford (Foyles Books). This has the plans laid out on a grid that can be scaled up. Not as accurate as the proper plans.
Please send a PM if you would like this book.
I have an unused GMI10. It is an early version and needs a separate power connection, even if connected to N2K. I was going to put it in the for sale section at £150.
Comes complete with all cables, manuals etc.