I turn without legs and move the might of machines Toothed and tight I pass force between unseen means I spin with another yet keep things in line What small hard helper makes engines align?

A guide through the blue. I'm the first one you see. You'll hear me or see me while bound to your seat.

I'm the jewel of the sea Made by clams from grains of sand What am I?

In a certain video game franchise, the undead are blocked by this shell, it may be tall, armored, or explosive, but its defense is tough to crack

I'm known for my planks, though I never set sail. I can join you in marriage, but I'm not a minister. My work is often measured, but I don't keep time. What am I?