Choosing the Right Prop

A propeller converts the turning power of a marine engine at the shaft (called torque) to usable power which moves the boat (called thrust). It works like a pump, sucking water from ahead, accelerating it and then discharging it astern. This produces thrust which pushes the boat forward.

• Different boats require varying amounts of thrust (and consequently varying amounts of slip) to propel them efficiently.
• Slow moving, heavy boats require high thrust and operate most efficiently at high slip.
• Fast moving, light boats require relatively low thrust and, therefore, operate most efficiently at low slip.
• Thrust is developed by accelerating a mass of water. It might seem that the same thrust could be attained by turning a small diameter propeller rapidly (giving a high acceleration to a small mass of water), or by turning a large diameter propeller slowly (giving a low acceleration to a large mass of water), but this is not necessarily so. Energy is lost in friction between the accelerated water and the surrounding water. Therefore, in selecting a propeller shaft speed, it is important to strike a favorable balance between shaft speed and boat speed. The resulting propeller size should have a pitch ratio (pitch divided by diameter) according to the following table for best efficiencies:
  
  

  Type of Boat	Pitch Ratio Range
  very heavy cruisers	.55 to 0.8
  average cruisers	.65 to 1.0
  medium and fast cruisers	.80 to 1.2
  high speed cruisers, & runabouts	.90 to 1.5

  
  
• Victoria Propeller uses a computerized propeller sizing program to help you choose the correct propeller for your boating needs. Based on your vessel data, we will provide a suggestion to help optimize your boat’s performance.