Lately I've been doing some very large boils -- 7 gallons in a 7.5 gallon kettle. To prevent boilovers, I've been using Fermcap-s. It's great, and I've had no boilover problems, but I have no idea how ...
I've heard that adding cleaned pennies to the wort can help prevent boil-overs. I guess they could provide nucleation points- I dunno. Any truth to this, or is it just an old wives' tail?