I can describe a solenoid, draw one, but not tell you what function it performs in a circuit.
A solenoid is basically a relay--I learned that when I had the starter solenoid replaced on my parents' car. It takes a heavy-duty switch to handle all the current needed to power the starter motor and turn the engine over. (Old cars didn't have stater solenoids--just a big switch on the floor that you'd activate by stepping on it.) The ignition switch that you activate by turning the key of your car couldn't handle the current needed, so it just activated the solenoid, which can handle the necessary current.
And forecastle is pronounced foc'sle, I believe.
Yes. I think it's pronounced that way because it's a very old nautical term that's been shortened over time. Kinda like gunwale, which is pronounced ɡənəl.