A screw, in the context of a boat's propeller, refers to a specific type of mechanical device often made from strong and durable materials, such as steel or brass. This device is designed with a helical structure and is utilized for propulsion in the watercraft. The screw propeller converts rotational motion into thrust, pushing the boat forwards or backwards through water. Its rotational movement forces water backwards, and that reactively drives the boat with equal force in the opposite direction. The screw propeller is a fundamental component of marine transportation enabling efficient and controlled navigation.

1. The twin screws of the navy warship allowed it to power through the ocean at impressive speeds. 2. Seaweed had become tangled around the screw of the motorboat, forcing the captain to stop and remove it. 3. The underwater drone was equipped with a miniature screw for moving beneath the ocean's surface. 4. The motor yacht’s screw was intricately designed, carrying out its function efficiently and quietly. 5. A qualified marine mechanic was employed to regularly maintain and ensure the optimal performance of the cruise ship's screws.

The screw propeller design commonly used in modern boats was first patented in the 1830s by Swedish inventor John Ericsson and Englishman Francis Pettit Smith. The screw propeller replaced the paddle wheel design previously used on steamboats. It works by spinning and pushing water backwards, which propels the boat forward – a form of jet propulsion – demonstrating the third law of motion, which states that for every action, there is an equal and opposite reaction. Additionally, screw propellers are highly efficient because they convert more of the engine's power into thrust, making the boats faster and more fuel-efficient.