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Propeller Basics

Outboard-Sterndrive propeller basics.

KTrapp
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Propeller Basics

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  1. Propeller Basics

  2. Understand how to decipher propeller specifications. • Understand common propeller terminology. • Understand common non-propeller terminology. Propeller Basics Objectives

  3. Propeller Basics Identification What is a 13-1/4” x 17” RH 3-Blade AL propeller?

  4. Propeller Basics 13-1/4” x 17” RH 3-Blade AL propeller? Diameter Two times the distance from center of the hub to blade tip. (Always measured in inches.) Pitch Theoretical forward movement of a propeller during one revolution, assuming there is no slip. • Always the first number in propeller description. • Always the second number in propeller description.

  5. Propeller Basics 13-1/4” x 17” RH 3-Blade AL propeller? Left Hand (Counter) Rotation The propeller spins in a counter-clockwise rotation. Right Hand (Standard) Rotation The propeller spins in a clockwise rotation. Left Hand Right Hand • Why are there LH rotation propellers? • Two propellers spinning in the same direction will create steering torque. • Two opposite-direction propellers eliminate steering torque on twin engines. • Determining Rotation • View the propeller as it would be installed, with a blade straight up. • If the RH side of the blade is closest to the hull, it’s RH (standard) rotation.

  6. Propeller Basics 13-1/4” x 17” RH 3-Blade AL propeller? • 3-Blade • Provide good “hole shot” and top-end speed. • Better top-end speed than 4-blade propellers. • 4-Blade • Less steering torque than 3-blades. • Less vibration at high speeds than 3-blades. • Better fuel economy. • Common on: • Recreational boats with 3, 4, & 6 cylinder outboards and I/O engines • Common on: • Recreational bass boats, and boats with high HP engines.

  7. Propeller Basics 13-1/4” x 17” RH 3-Blade AL propeller? • Aluminum (AL) • Less expensive than stainless steel. • Blades sacrifice themselves to help prevent lower unit gear damage. • Easier to repair than stainless steel. • Good “general-purpose” propeller. • Stainless Steel (SS) • More durable than aluminum. • Withstand more damage caused by small rocks, sand, loose objects, etc. • “Minimal give” to the blades. • Best used in deep, familiar, or salt water. • Important to carry spare aluminum propeller • Better top-end speed than aluminum. Avg. Life 2-3 Years Avg. Life 6+ Years

  8. Propeller Basics Common Propeller Terms Cup Small curvature located on the trailing edge of each blade. Slip Difference between actual and theoretical travel of propeller through water. • Provides better “bite” on water – reducing slip and cavitation. • A properly matched propeller will move between 80-90% of theoretical distance.

  9. Propeller Basics Common Propeller Terms Leading Edge Edge of the propeller blade that is adjacent to the forward end of the hub. Trailing Edge Edge of the propeller blade that is adjacent to the aft end of the hub.

  10. Propeller Basics Common Propeller Terms Rake Angle of a propeller blade face relative to the hub. Blade With No Rake • 0° to aft rake more common in OBSD propellers. This contributes to bow lift. Diffuser RingFlared ring used on thru-hub exhaust propellers which aids in reducing exhaust back pressure and in preventing exhaust gas from feeding back into blades (which produces cavitation). Outline of Blade With Rake Forward (Bow)

  11. Propeller Basics Common Propeller Terms • Ventilation • The situation where surface air or exhaust gas is drawn into the propeller blades. • Causes loss of speed. • Causes RPM to climb rapidly. • Can result from: • Excessively tight cornering • Motor mounted too high on • transom • Ventilation Holes • Exhaust is drawn out the vent hole upon acceleration. • Less force is required to push through resulting aerated water – propeller rotates faster at slower speeds resulting in more HP.

  12. Propeller Basics Common Propeller Terms • Cavitation • The phenomena of water vaporizing, or boiling, on a propeller due to extreme reduction of pressure on the back of the propeller blade. • Can result from: • Incorrect prop for engine • Incorrect pitch • Physical damage to blades

  13. Propeller Basics Exhaust Types • Thru-Hub Exhaust Propellers • Consist of round barrel to which blades are attached. • Exhaust passes through barrel and out the back. • Exhaust does not contact blades. • Results in good acceleration and hole shot. • Over-Hub Exhaust Propellers • Propellers have blades attached directly to small tube that fits over the propeller shaft. • Used for attaining maximum top speeds.

  14. Propeller Basics Exhaust Types • Over/Thru-Hub Exhaust Propellers • Combination of Thru-Hub & Over-Hub exhaust. • Allows some exhaust to escape at lower RPM. • Allows propeller to be slightly easier to turn during initial acceleration. • Non Thru-Hub Exhaust Propellers • Non Thru-Hub exhaust props used on: • Inboards using shaft driven • propellers • Sterndrives using through hull • exhaust • Some outboards not routing • Exhaust through lower unit

  15. Non-Specific Propeller Related Terms Hydrofoil Shaped foils that lift the hull clear of the water to increase boat speed. Cavitation Plate A permanent horizontal plate on the outboard that sits at water level when the boat is on plane, which prevents propeller cavitation. • Make trim adjustments more quickly. • Creates more lift to get the boat on plane quicker. • Helps achieve effortless steering.

  16. Non-Specific Propeller Related Terms Jack Plate Designed to raise and lower outboard motors easily. Trim Tabs Also known as “sacrificial anodes”, they sacrifice themselves by corroding to protect other metals. • Improve gas mileage. • Increase speed. • Allow for shallow water access. • High performance trim tabs are required for select applications.

  17. Non-Specific Propeller Related Terms Changing Trim Tabs

  18. Non-Specific Propeller Related Terms Trimming The Engine Trimming “In” Excessively Affects Steering Takes Longer to Plane Less Fuel Efficient Trimming “Out” Excessively Bow is Raised, Unable to Cut Through Chop Speed Decreases “Neutral Trim” Boat Rides Level with Most of the Hull on the Surface Speed, Hull Design, Weight & Balance, Water Conditions All Impact Optimal Trim Setting

  19. Non-Specific Propeller Related Terms • Porpoising • Term used to describe boat bouncing caused by hull or trim, not water conditions. • Can be partially controlled by propeller type.

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