Prop Primer: A Short Course in Propeller Terminology

If you’re new to boating, or you’re considering buying a boat that’s unfamiliar to you, you’ll need to devote some attention to the matter of propellers – those types of Archimedes Screw that push most boats around on the water. I recently discussed the basics of propeller choice, so in this column I thought it would be helpful to offer you some plain-spoken explanations of some of more common terms used when talking about propellers.

Driven by an inboard diesel, this is a left-handed 16x16 propeller. The numbers mean that both its diameter and pitch are 16 inches.

It might seem a bit esoteric, but many times a used boat doesn’t come with the right prop and you’re left to sort it all out. This primer should help you decide whether your new boat is propped correctly, and how you might make adjustments to get just the right blend of prop and engine performance.

Aeration: Also known as ventilation, aeration is the introduction of air to the propeller blades. In high-performance applications, a little aeration is OK, even beneficial. But on recreational boats, it’s not necessary and ends up feeling like a slipping transmission. Propeller blades won’t bite—because the water isn’t staying attached to the surface of the propeller blade—and they end up spinning freely. It happens most often during hard turns or when you trim the drive up too far.

Cavitation: Often mistaken for aeration or ventilation, cavitation is a disturbance of water flow to the face of the propeller blade or blades. When it happens, you’ll feel excess vibration. Physically, cavitation occurs when low-pressure bubbles form on the blade surface, which can generate enough heat to burn the face of the propeller blade. If you have ever seen a painted aluminum prop with all those little dings and pits on the face of the blade, you have seen the results of cavitation. Chief causes of the phenomenon are bent or nicked blades, which initiate the disturbance of water flow across the blade surface.

Cup: If you look closely at the surface of the blade of a propeller, you’ll notice it’s a little convex. That’s cup. It is measured with—wait for it—a cupping gauge from the flat face of the blade to the inner edge of its lip. The more cup, the better the “bite,” but as with so many things, there are limits to how much cup you can have.

Diameter: If you have a four-blade prop, the diameter is measured from tip to tip of the opposing blades. If it’s a three-blade, it’s the theoretical circle the prop creates when it rotates, or double the measurement from the center of the hub to the blade tip. A larger-diameter propeller can generate more stern lift when the boat is on plane.

Hub: The hub is the center, cylindrical portion of the prop that slips on the propeller shaft. During the manufacturing process, the blades are cast as part of the hub.

Mercury’s Bravo Three system uses twin contra-rotating props, with a larger-diameter, left-hand, four-blade prop in front, and a smaller-diameter, right-hand, three-blade prop behind. Now there’s some prop talk for you.

Left-hand/Right-hand: These terms refer to prop rotation. Generally speaking, marine engines rotate clockwise, which is why most are fitted with a right-hand prop. Stern drives such as MerCruiser’s Bravo Three or Volvo-Penta’s DuoProp drives have a right- and left-hand propeller spinning in opposite directions on concentric output shafts. If you have twin engines, propellers generally rotate inward (right-hand prop on the port side and vice versa) for balance and easier handling.

Over-propped/Underpropped: If your engine seems to have trouble pushing the boat on plane or won’t reach redline, it’s probably over-propped, which means you have too much diameter or pitch or both. Conversely, if your engine hits its rev-limiter and frequently ventilates the prop, you’re likely underpropped — not enough diameter, pitch or both.

Pitch: Heady stuff, pitch is the theoretical distance in inches a propeller would travel forward after one revolution, if it were moving through a soft solid. Imagine it moving through gelatin. Generally speaking, the greater the pitch, the more power you need to spin it.

Rake: Rake comes up a lot, and it is the angle of the propeller blades where they meet the hub. The greater the rake, the less need for drive trim and the less chance for ventilation. A higher rake angle also will reduce stern lift.

Slip: Remember pitch, and how it was the theoretical distance a propeller would move forward as it rotates through a soft solid? Slip is the percentage difference between pitch and the actual distance it travels in water, which as you know, is not a solid. If you have 11-percent slip, which is actually a pretty efficient prop, on, say, an 18-inch-pitch propeller, the actual distance traveled is about 16 inches.

Slip calculator: You can eliminate the guesswork when you calculate prop slip with Mercury’s handy prop-slip calculator. Just plug in your propeller pitch, the gear ratio of your drive, engine rpm and speed, and it will provide the slip figure.

The Mercury Marine website has other valuable information in their own primer, Explore Propellers.

Brett Becker

Even more about propellers: