Propellers; How They Work And Things To Know
If there is one thing boaters have in common, it is the propeller. Unless you only do your boating under sail, you are the happy beneficiary of a turning propeller below your boat.
Propellers, also called “screws” and “wheels”, are essentially pumps. They draw water in via low pressure caused by the turning blades and then discharge it creating thrust. The discharged water, easily seen behind an outboard motor, comes out as a high pressure stream, pushing against the water behind the boat. The pure physics of it is more complex than that, but that is a reasonable description of what is happening.
The discharge stream in a typical inboard powered boat is directed across the rudder, which is normally in-line with the propeller’s thrust. With multi-engined boats you have multiple rudders. Turning the rudder causes that high discharge water flow to push against the exposed side of the rudder and assist the boat in turning. This is why powerboat rudders can be smaller than sailboat rudders. Outboards and I/O’s of course turn the drive unit and propeller together to change direction.
Propellers come in different sizes, shapes and materials depending on the duty they are asked to perform. A typical propeller is described by its diameter and pitch—or distance the propeller “screws” through the water. Rake and cup are also parameters that affect a props performance. It’s important to size your propeller correctly to the hull and motor combination to get the most efficient use of your engine, fuel and hull. When you change engine horsepower you’ll need to re-assess your propeller. Click here for more information about propeller sizing and performance.
Propellers, even though they are immersed in water, can suffer various kinds of wear and damage. Cavitation, or the creation of a vacuum next to the propeller surface, will cause bubbles to expand and then collapse, in effect eroding the propeller surface. Ventilation (propeller exposed to air either from the hull, exhaust or breaking the surface) also causes bubbling and erosion. Cavitation and ventilation will be apparent as the propeller ‘slips’ while trying to perform. And of course, there is all the junk in the water to bang into, bending or breaking blades and causing damage to shafts and lower units.
There are some pretty interesting propeller set ups available to boaters; Arenson drives, outboard motors on brackets, pod drives, the new Volvo IPS system that uses ‘pulling’ instead of ‘pushing’ propellers, evil looking stainless high performance propellers, and more. There are even composite or plastic propellers. Most of these are attempts to create more efficiency from the propeller and give you better performance.
Boat and engine manufacturers will recommend the right propeller for your boat, and there are many propeller shops and professionals to help you choose a different propeller, or alter or repair one as needed.