Propeller Basics

We’ve all experienced a lack of cooperation from the wind and current in a close-quarters maneuvering situation. So wouldn’t it be nice if your boat’s propulsion system would help out just a little? There are a few principles operating when our props are turning, but they may not always be obvious to us.

First of all, let’s start with the basics. There are a few names for a prop. It can also be called a propeller (obviously from which prop is derived), a screw or a wheel. Props are referred to by the direction in which they turn, by their diameter and by the pitch (angle) of their blades.

The turning direction is either “right-hand” or “left-hand.” In order to determine the direction that a propeller turns, you should observe the rotation from the stern. Moving forward, right-hand props turn in a clockwise direction and left-hand props turn in a counterclockwise direction.

The diameter of a prop is a measure of the size of the circle in which it can turn. It isn’t as simple as measuring across the blades, unless the number of blades is an even number. For example, a three-bladed prop with a diameter of 14″ will turn within a 14″ diameter circle. However, if you measure across two of the three blades, you will end up with something less than 14″.

You can measure the diameter across a 2-blade or a 4-blade prop because two blades are directly across from each other. In that case, the diameter of the circle would be the same as the measurement across the blades. Three and five blade props don’t have two blades that are directly opposite each other, so the dimension across them will be less than the prop diameter.

The pitch is another story. A prop’s pitch is measured by how far it would advance if screwed into a solid object for one revolution. A pitch of 12 would indicate that the prop would advance, or move forward, by 12″ for each revolution. Obviously, that wouldn’t happen in water. Pitch is simply a way to measure the angle of the blades on a prop.

What does this all mean? The issue of your boat’s failure to move in the right direction can be partially explained (not counting operator error). There is something called “prop walk” or “side thrust.” Because the prop’s blades are pitched, or angled, the thrust is not totally in the forward or reverse direction.

These forces cause a right-hand prop to “walk” to starboard and a left-hand prop to “walk” to port, moving the stern slightly with them. The majority of the thrust moves your boat forward, but some percentage pushes you to right or the left. In reverse, the “prop walk” is the opposite. A right-hand prop “backs” to port and a left-hand prop “backs” to starboard.

If you have a twin engine boat, your props are almost always “counter-rotating,” which means that one is right-hand and one is left-hand and they rotate in opposite directions. When both props are in forward or both in reverse, the “prop walk” is cancelled out.

So the next time the stern of your boat moves slightly sideways, you’ll know why. The trick is to anticipate it and use it to your advantage!

Until next time, we wish you clear skies, fair winds and calm seas!

 

World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit www.worldwidemarinetraining.com or call toll-free 866-249-2135.