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Propeller Knowledge:
Blade Area & Pitch : Corrosion : Engine Loading : ISO 484 Standard : Dynamic Balancing : Propeller Terminology : Propeller Pitch
The Delicate Balance Between Blade Area & Pitch

In order to achieve maximum performance it is necessary to balance the blade area with the effective pitch of the propellers.

The propeller must produce sufficient Thrust to move the vessel. Thrust is like torque in a car engine. Torque is necessary to get the wheels turning and to keep them turning once the car is moving. Blade area on a propeller produces the thrust that gets the boat moving and keeps it going once underway.

Insufficient blade area will result in excessive slip, lost efficiency, poor performance, and increased fuel consumption.

Blade area is directly related to diameter and number of blades and is often referred to as D.A.R.
As diameter increases so does blade area. Because of hull limitations sometimes the correct diameter propeller is impossible to install.

In these cases additional blades will accomplish the same purpose.

Thrust is not speed. Pitch is speed. Pitch is like the gear box on your car. There must be enough pitch to achieve the speed and performance a vessel is capable of. Both pitch and blade area combine to load the engines. A balance must be achieved so that enough thrust is generated to move the boat while enough pitch is present to attain the design speed.
Blade Area and Pitch
Blade Area and Pitch

For any given engine, vessel, gearbox combination there is an optimum pitch, diameter, and blade area propeller. If the correct propeller is not available or impossible to install it then becomes necessary to compensate in pitch for any change in diameter or blade area. As this occurs the propeller becomes a compromise and efficiency begins to degrade.

For instance say the optimum propeller for a the F/V SPEED KING is a 32 RH 35 x 3” 4-blade 0.80 D.A.R. propeller [That is 32” diameter x 35” pitch x 3” bore]. Now, if the largest diameter propeller that can be installed is a 30” diameter then pitch must be added to compensate for the loss in diameter or a propeller.

Pitch will never compensate entirely for loss of diameter. At some point too much diameter or blade area will be lost decreasing thrust which in turn will increase slip to the point that propeller efficiency plummets. On the other hand too much blade area will generate induced drag and reduced pitch which slows the boat down.




Boating Magazine
“We picked up 1 mph at top the top end and nearly 3 mph at the boat’s cruising speed of 1900 rpm. In addition, we were able to run similar speeds at lower rpm, which helped fuel consumption.”
Power & Motor Yacht
“Computerized scanning can make your props more efficient and your boat perform better.”
Shallow Draft
“Ignoring the propeller on today’s sophisticated vessels is ignoring the most important piece of equipment on your vessel.”
Marine News
“Combined with vessel performance and engine loading information the Prop Scan EPS software can calculate changes that will decrease propeller slip and match engine loading requirements thus increasing vessel performance.””

 
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