Every answer here comes from boat owners who had a similar problem and wrote it down.
Each one links back to the thread it came from — read it there, and thank the person who solved it.
With thanks to the members of
CanalWorld,
Sailboat Owners and
YBW, who wrote all of it.
Gathered by the book. We read and obeyed every forum’s robots.txt, honoured each site’s crawl-delay, fetched one page at a time, and identified ourselves honestly with a contact address. Where a site had put up a gate, we took it as a no. Zero violations.
Just like any other forum-based help, this is a guide to a starting point — for fixing the problem yourself, or for the conversation with your mechanic. We can’t accept responsibility for this content, obviously.
Built in Europe by people who'd rather be messing about in boats.