An oversimplified explanation describes boat propeller theory as being similar to a rotating screw. But, it actually involves more than a turning screw. There are forces created by moving the water away from the blades of the propeller and these forces are what helps move the propeller forward, thus moving the boat.
Boat propellers include blades that turn on a shaft, which is powered by the motor. Propellers can have 3, 4, or 5 blades. Typically, the more blades on the propeller, the more propulsion can be achieved. While the motor provides the power, the propeller is where all the action takes place. Propeller blades displace water, to create the forces that move a boat forward.
The basic propeller shaft creates the torque, or energy required to turn the blades. But, the design of propeller blades is what creates the displacement of water. The displacement of water is what creates forces that move a boat forward. Propellers can turn either direction. They work by the same principles, regardless of the direction they turn.
The propeller works by turning torque into thrust. In other words, it converts power from the engine into an action. The action of turning the propellers creates force, by moving the flow of water downward and behind the blades. When this happens, the water that has just been pushed behind the blade creates a temporary hole. This hole fills with water, creating a low pressure system that lifts and pulls the blade forward. The momentum created by the low pressure is what moves the boat forward.
A boat propeller consists of blades. Each blade is designed with a sight curve and a particular shape, which helps move the water down and past it. The leading edge is the part of the propeller blade that hits the water first. The trailing edge is the part where the water leaves the blade. The curve of the blade usually begins at the trailing edge and extends all the way to the hub. the curve is also known as the cup. It's what gives the propeller blade its pitch, or angle.
When considering a boat propeller, it helps to first think about what the boat will primarily be used for. Boat propellers come in aluminum, steel, and sometimes brass. Aluminum propellers are the least expensive. They are usually meant for situations where the boat doesn't need to travel very quickly. Aluminum is easily dented, so this must be considered when a boat propeller is chosen.
A stainless steel propeller is sturdier and can withstand hitting rocks, sand, and debris. But, if the blade doesn't give, this may result in the propeller shaft being damaged. Repairing the shaft may be more time consuming and costly that replacing the propeller or fixing the blade.
The heavy duty brass propellers are usually reserved for heavy boating jobs. Most propellers designed for high performance boats will be made of stainless steel and may consist of 5 or 6 blades, for faster propulsion.
A propeller in motion creates a pitch. This is the forward movement of the propeller in one revolution. The propeller doesn't always move the exact distance it is designed to move, due to slippage. A good propeller will have as little as 10 to 30% slippage, but this depends on the design of the propeller and blades.
Boat propeller theory is not difficult to understand. But, it is not as simple as a turning screw. It's sometimes compared to the lift created by airplane wings. The difference is that the lift from a propeller is created by moving water.
If you have any questions on propulsion or boat propellers, reach out to the experts at Deep Blue Yacht Supply.