| How to pick a boat prop | | | | can explode surface bubbles of air with a force that |
| Learning basic propeller lingo is a basic requirement for | | | | can chip the paint off the prop. This occurs when air is |
| choosing the right prop. Prop terminology is actually | | | | introduced through either cavitation or ventilation. The |
| quite simple, you'll hear a few terms like "Rake" and | | | | difference between cavitation and ventilation is often |
| "diameter area ratio." Propeller sizes are described | | | | misunderstood. Ventilation occurs when air bubbles |
| with two numbers. The first number is the diameter | | | | from the bottom of the boat transom or find their way |
| prop. If your prop has two or four blades, try | | | | into the Propeller and Vent it. It can also occur when a |
| measuring the distance from one blade tip to the | | | | prop is pulled in by surfacing the blade tips. You will find |
| opposite blade tip. If your prop has three or five blades, | | | | that cavitation occurs when the prop spins on its own |
| try measuring the distance from the center of the hub | | | | sort of like a car vat on a slippery road. The |
| to the tip of the blade, then Multiply that number by | | | | unmistakable symptom of cavitation is Excessive over |
| two. Pitch, the second number, is the distance in inches | | | | revving. You can correct cavitation by reducing the |
| that a prop moves forward in one full revolution. If you | | | | Throttle until the Propeller reconnect with the water. |
| have a prop with a diameter of 16 inches and a pitch | | | | Blades can be shaped in several different ways. The |
| of 24 inches, the prop configuration would be: "16x24." | | | | most common shape is the standard round ear or |
| The center part of the Propeller is called the hub. The | | | | elliptical blades. These props deliver an optimum |
| hub Centers the prop on the drive shaft. The hub is the | | | | balance of speed and thrust. |
| point on the Propeller where the engine exhaust flows | | | | Other propellers are fapered, which are designed to |
| through, as is the case with most of today's outboard. | | | | provide less drag and are usually more high-speed |
| There is also a barrel around the hub where the | | | | oriented. Props are also available with what is called a |
| blades are attached. | | | | cup, featured a lip on the trailing edge of the blade and |
| How a boat prop works | | | | improve the water thrust flu to reduce slipping - the |
| Prop blades push water in one direction and the boat | | | | amount of ineffectual Spinning usually measured by a |
| moves in the opposite direction, as the blades spin and | | | | percentage. For example, if a prop with a 25-inch pitch |
| push water back, they also create a vacuum on the | | | | is rotated four complete times it should (in theory) |
| forward side of each blade. This vacuum pulls on the | | | | move the boat forward 100 inches. In practice, it may |
| blade and help move the boat forward, this vacuum is | | | | only move 90 inches, Which would indicate the |
| so strong that an extreme low pressure area created | | | | prophas a 10 per cent slip factor. |