How Boat Props Work

Correct Propeller selection and tuning is key for optimized boat performance. Here are some prop basics every smart boater should know!

More power and speed.
Better acceleration and fuel economy.
Reduced vibration and engine strain.


BOAT PROPELLERS DEFINED

A boat propeller is a fan mechanism that transmits power by converting rotational motion into thrust. Usually consisting of 3 or more blades, a boat prop spins around a central shaft to create dynamics similar to a rotating screw or airfoil. When blades spin, a pressure difference between the forward and rear surfaces is produced, accelerated the water behind the blade to create force.


PROPELLER'S ANATOMY

Hub

Solid cylinder located at the center of the propeller to which each prop blade is attached. Hub shapes include cylindrical, conical, radius & barreled, with the center of the Hub Bored to accommodate the engine propeller shaft.

Blade

The ear-like, curved pieces that extend outward from the prop Hub. Most recreational craft propellers are available in 3, 4, and 5 blade configurations.

4-Blade and 5-Blade Props (or multi-blade props) offer advantages for high horsepower outboards. mounted high on the transom, the prop runs closer to the surface, providing additional bite and stability at higher speed. They can also improve out-of-the-hole acceleration, while maintaining plane with fewer engine rpm's. Because there is more drag , multi-blade props reduce top-end speed. A high-end three-blade prop usually runs a few miles per hour fasted per hour faster that the same pitched multiple–blade design.

Blade Number

The number of blades attached to the propeller hub.

Blade Back

Suction side or Forward side of the blade (the surface facing the bow).

Blade Face

Pressure Side, Pitch Side, or Aft side of the blade (surface facing the stern).

Blade Root

Also called the Fillet area. The area where each blade attaches to the hub— the region of transition from the blade surfaces and edges to the hub periphery.

Blade Tip

Maximum radius of the blade from the center of the hub to the outermost blade point. The point of separation between the leading and trailing edges.

Leading Edge

Edge of the propeller blade adjacent to the forward end of the hub. The leading edge leads into the flow of water when providing forward thrust.

Trailing Edge

Edge of the propeller adjacent to the aft end of the hub. When viewing the propeller from stern, this edge is closet. The trailing edge retreats from the flow of water when providing forward thrust.

Cup

Small radius of curvature located on the trailing edge of the blade. This curved lip on the propeller provides an improved "bite" on the water— reducing ventilation and slipping and providing better "out-of-the-hole" acceleration. A cupped prop can also work well when trimmed up so that the prop is near the water, resulting in higher top end speed.

Radius

The distances from the center of the hub to the outer-most blade tip.

Diameter

Diameter It is the first number listed when describing a propeller. It is the distance across the circle that a propeller makes when rotating. Prop Diameter can me calculated my measuring the Radius (distance from the center of the hub to the outermost tip of a blade) x 2.

Pitch

Pitch is the second number listed in the propeller description.
Pitch is defined as the theoretical forward movement of a propeller during one revolution if there was no "slippage" between the propeller blade and the water. Note: Many mistakenly believe that switching to a larger pitch prop will increase speed. But too too large a pitch can result in excessive engine strain, reduced RPM,. slower speeds, fouled spark plugs, and premature engine wear.

Prop Pitch Tip:

A simple rule-of-thumb to follow when experimenting with prop pitch: At wide-open RPM, increasing the prop pitch reduces RPM by roughly 200 rpm's per one inch of pitch. So switching from a 21- to a 23-pitch prop will drop maximum RPM level approximately 400 rpm's. Going down in pitch size will have the opposite effect and increase maximum achievable RPM.

Skew

The transverse sweeping curve of a prop blade. Skew forms an asymmetrical shape that can be viewed by looking at the prop blades directly from the fore or aft.

Aft Skew or Positive Skew

Blade sweep in the direction opposite as prop rotation when moving the craft forward

Forward Skew or Negative skew.

Blade sweep in the same direction as rotation when moving the craft forward

Rake

The degree that the blades slant forward or backwards in relation to the hub. Rake affects the flow of water through the propeller.

Aft or Positive Rake

Common for planning and combination hulls, Aft Rake provides increased top-end speed while assisting in trimming the bow upward for less wetted surface and drag.

Forward or Negative Rake

Common in commercial craft with displacement hulls, forward rake trades through-water speed for steady power that can aid in holding the bow of the boat down and level.


PROPELLER PERFORMANCE

Slip

Slip is the difference between actual and theoretical travel of the propeller blades through water. A property matched propeller should move forward through the water at 80% to 90% of the theoretical pitch.

Track

Track represents the spread distance between individual blade rake distributions.
Always a positive value, Track is the absolute difference of one blades actual individual blade rake distributions to the corresponding blade rake distributions on a neighboring blade.

Rotation

The Direction a prop rotates when viewed from the stern facing forward.
Right-hand propellers rotate clockwise to provide forward thrust.
Left-hand propellers rotate counter-clockwise to provide forward thrust.
Note: Left-Hand Propellers are primarily used on twin engine boats to cancel the steering torque that would be caused if both propellers spun in the same direction.

Ventilation

When surface air or exhaust gasses are drawn into the propeller blades, resulting in loss of speed and rapid RPM increase. Ventilation causes include excessively tight cornering, over-trimming of the engine, and mounting an outboard motor too high on the transom.

Cavitation

Often confused with ventilation, Cavitation is a phenomena where water vaporizes or "boils" due to extreme reduction of pressure on the back of the prop blade. Partial cavitation is normal on most speed-oriented props, but excessive cavitation can result damage to the propeller’s blade surface. Causes include incorrect power/propeller match, incorrect pitch, damage to blade edges, and propeller imbalance.

Thru-hub Exhaust Propellers

Propellers designed with an open barrel hub that serves as an outlet for engine exhaust to escape without making blade contact, improving acceleration.

Non Thru-Hub Exhaust Propellers

Used for inboards with shaft-driven propellers and through hull exhaust, and on some outboards that don’t route the exhaust through the lower unit torpedo.

Over/Thru-hub Exhaust Propellers

Props that allow a controlled combination of thru-hub and over-hub exhaust flow at lower RPMs, boosting prop performance during initial acceleration and allowing for a better prop bite on some engine/boat combinations.


PROPELLER MATERIALS

Aluminum Props

Aluminum props are popular and affordable option for small to medium-sized boats. Soft and foregiving, aluminum absorbs impact energy to help protect drive component, making them a smart choice for boating areas where prop damage is common due to grounding or debris. Under load, aluminum props actually bend when spinning, reducing the overall pitch size by up to one inch, and limiting maximum available pitch size to 23".

Stainless Steel Props

A more expensive option that delivers increased rigidity and performance, bow lift, and out-of-the-hole power—especially on heavier boats. Because Stainless Steel props don’t flex under power, they are also available in larger pitch sizes.

Bronze Props

Used on early boats, Bronze propellers are still used for many modern inboard applications, including ski boats that prefer them because of strength, durability, and minimal flex. They are available in a wide variety of size and pitch combinations, and provide excellent corrosion, salt water, and fatigue resistance, and, making them popular for larger inboard, open water vessels as well. Disadvantages include initial expense, damage realized if collision with an obstacle does occur, and difficulty of repair.

Nibral Props

Nibral is an alloy that combines bronze, aluminum and nickel for superior strength, durability, and corrosion resistance. Available in many sizes, pitches, and blade configurations, Nibral boat props combine the effectiveness of bronze and aluminum with the strength of nickel. A common choice for stern drive boats wanting to upgrade from aluminum.

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