Because it sounds like a negative, “slip” might be the most misunderstood of all propeller terms. But slip is actually essential to propeller function.
Propeller slip is the difference between the actual distance the prop travels forward through the water versus the distance it should theoretically travel based on the prop blades’ angle of attack. The theoretical travel is determined by the prop pitch. A prop with a 21-inch pitch, for example, would travel 21 inches with one revolution through a soft solid. Think about a screw turning into a piece of wood. However, in the real world and in real water, the propeller does not act like a screw turning through wood. There will be a certain amount of slip, which is actually necessary.
This sounds inefficient. It seems that zero slip would certainly be ideal. However, slip is not a measure of propeller efficiency, and without some slip the prop could not move the boat forward at all.
To understand why, let’s back up and define the term “angle of attack.” One way to understand angle of attack is to think about an airplane wing. If that wing is a symmetrical airfoil moving through the air horizontally – a zero-degree angle of attack – air will pass symmetrically above and below the wing, resulting in equal pressure above and below the wing. No lift is generated to carry the weight of the plane. If we add some angle of attack by tilting the leading edge of the wing up, lift is generated. There will be negative pressure above the wing and positive pressure below the wing. You can experience the same effect by holding your hand out the window of a moving car. If you tilt your hand up, you feel lift.
The function of a boat propeller blade is like that of the airplane wing. If the propeller blade has no angle of attack, there is no slip, but also no positive or negative pressure around the blades. Propeller blades with some angle of attack create negative (lower or pulling) pressure on one side of the blade and positive (higher or pushing) pressure on the other side of the blade. The pressure difference creates lift at right angles to the blade surface, which both lifts the boat and thrusts it forward.
That angle of attack also results in some slip. Without some slip, there’s no thrust. Creating the desired range of slip is one of the key challenges facing a propeller engineer, and there are a number of ways this can be accomplished within all of the other design parameters (blade diameter, blade shape and area, blade rake and cup) the engineer needs to consider.
Calculating slip can be useful when diagnosing an issue with a setup. Calculated propeller slip within the range of 5% to 25% is typical and acceptable. If slip is greater than 25%, there is likely an issue with the propeller. It could be damaged or just wildly wrong for the boat-and-engine combination. Or, it could be ventilating because the outdrive or outboard motor is mounted too high on the transom. The online Mercury Racing Prop Slip Calculator makes it easy to calculate prop slip. Professional race teams and performance boat owners seeking to maximize speed can commission custom propellers designed for a very specific boat-and-engine combination. A custom propeller can operate much more efficiently and with better performance than any mass-produced propeller, but at significant financial cost.
Boat speed is really the best indicator of prop efficiency. The prop that makes your boat run the fastest is going to be most efficient at wide-open throttle, regardless of its slip. However, a prop that delivers the best top speed might not work well at cruising speed, when some props might keep the bow high and make for an awkward cruise.
Every motor/engine/prop combination will perform differently, and the ideal way to find the best combination for your boat is simply to try different propellers. You can also talk to your Mercury authorized dealer about your boat-and-engine combination, because your dealer has likely seen what works best on similar boats. They can help you find a good prop to begin testing.
Just remember that a little slip is essential for propelling your boat – and your fun on the water.