What is a right-hand screw propeller or left-hand screw propeller?

The terms right-hand screw and left-hand screw are defined as such: When viewing from aft, if the propeller turns to the right at the top of the sweep then it is deemed a right-hand screw. Named “right-hand” because the natural rotation of your right hand is pinky down and thumb up (clockwise). Conversely, the left-hand screw is counterclockwise when viewed from aft.

See the animation below which makes it clear. Tap on the Forward and Reverse buttons.

Most sailboats are right-hand screw.

So why then does the aft of the boat move to the port when reverse is applied? It is propwalk, and it is not what you expect. Some people think that the turn of the prop is pulling the boat sideways. But that is not the case because the pull in one direction balances the pull in the opposite direction. It turns out that the action is what airplanes experience called the p-factor as well as another phenomenon that was fully defined by one of the world’s most experienced prop engineers, Dave Geer of Geer Marine, Inc. — Dave has written a full paper on what makes your boat go sideways…. For more in-depth reading on this, here is Dave Geer’s propwalk white paper for further interesting reading.

Take the NauticEd Maneuvering Under Power online course to learn more Learn how to easily handle propwalk, back your boat into a slip on a windy day, and generally look like a professional in the marina.