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Boat Propellor Anatomy - The Basics

parts of a boat propeller - the basics


Glossary:

  • Outer Hub = Exterior hub
  • Inner Hub = Hub inside (sacrificial centre)
  • Splines / Ribs =  Connections between inner & outer hub
  • Diffuser Ring = Flare ring on the back 
  • Diameter = Distance across the circle swept by prop blade tips
  • Pitch = Distance prop 'theoretically' travels in one revolution 
  • Rake = Angle blades are set relative to outer hub
  • Hole Shot = From dead stop (in the hole) to up on the plane

Blades: 

  • Blade Tip = furthest bit from the hub
  • Blade Back = Blade side facing forward
  • Blade Face = Blade side facing backwards
  • Blade Root = Attachment point to hub
  • Leading Edge = Forward part of the blade
  • Cupping = Curl in the blade towards the trailing edge


Diameter:

Image ©: evinrudenation.com
Image ©: evinrudenation.com

Diameter is the circle swept by the tips of the prop blades. 

Increased diameter means increased thrust but also increased drag.


Pitch:

Image ©crowleymarine.com
Image ©crowleymarine.com

Pitch is the theoretical movement forward of a propellor during one revolution if it was going through a hard surface like a drill goes through wood. However as it is travelling through water there will be slip therefore the distance is less than the pitch. This slip varies from boat to boat.. 


Cupping:

Image ©purepropeller.com
Image ©purepropeller.com

This is the curved lip which allows the prop to have a better bite on the water. It reduces slipping, ventillation and allows for a better hole shot. 


Rake:

Image ©solaspropellers.com
Image ©solaspropellers.com

Rake is the angle that the prop blades slant in relation to the outer hub. Rake affects the flow of water through the prop and affects boat performance. 

Assuming 'aft' rake (as most outboard props) this reduces the wetted area (friction) and ensures a higher top end speed. Props with Less rake are generally fitted to displacement craft. 


Image © Cabela's LLC
Image © Cabela's LLC

OUTER HUB: Centre barrel of the prop that the blades are attached to. 

INNER HUB: Many props have a sacrificial centre between the outer hub and the splines so if you were to hit something this would break therefore protecting your engine. 

RIBS/SPLINES: Joining points between the Inner and Outer Hubs. 

On through exhaust props, some of the exhaust travels through the gap between the inner and outer hub. 

BLADE FACE: Confusingly though this is called the blade face it infact faces aft (backwards) i.e. away from your boat. It is the 'pressure' side of your blade

BLADE BACK: and again, a little confusingly, this is the face that faces forward i.e. towards the stern of your boat. It is the first part of the prop to meet the water in each turn and is the 'suction' or negative pressure side of the blade. 

Diffuser Rim:

Image ©offshoreonly.com
Image ©offshoreonly.com

The diffuser rim is the Flared section on the back end of some propellors, this flare helps stop exhaust from travelling back onto the blades which would reduce thrust. 


test yourself:

Download
Parts of a Prop Test.pdf
Adobe Acrobat Document 367.6 KB

check yourself:

Download
Parts of a Prop Test Answers.pdf
Adobe Acrobat Document 371.4 KB


additional Resources:

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