It measures the minimum depth in which a boat can operate without going aground.
Single engines are more economical than twin engines, but twins provide more security.
A kill switch is a safety switch attached to a lanyard that shuts off the motor if the driver leaves his or her seat. Don't make the amateur's mistake of trimming up at low speeds and then trying to put the boat on plane. The motor will roar and the water will fly, but you'll go nowhere fast. A boat likes to keep spinning the way it's backing. If you're backing to starboard, spin the wheel to port before putting it into forward.