[DanBC]

> E.g. there don't seem to be a lot of crackpots that go into plumbing or whatever.

There are plenty of reality TV shows about crackpot plumbers - either idiot DIYers who don't know what they're doing and who ruin a home or cowboy rogue builders who con their customers and provide substandard work.

[JadeNB]

> either idiot DIYers who don't know what they're doing and who ruin a home or cowboy rogue builders who con their customers and provide substandard work.

The idiot DIYer may be a crackpot, but the rogue builder isn't. A crackpot is someone who has no expertise, but (crucially) believes that he or she does (or at least that he or she has made a discovery in a domain believed to belong to experts); a con man is not a crackpot.