PROPELLER CALCULATION

When talking about propellers, they are described in terms of pitch and diameter. Diameter is obviously the distance from tip to tip. Pitch is the distance the prop would travel through water if the water weren’t a fluid.  Fluid dynamics, induced flow, Reynolds number will show  a propeller will never be 100% efficient. Optimizing propeller to get highest efficiency is requiring more than this article. The CSGnetworks propeller calculator is designed to give any missing value from the 5 variables needed for marine propeller calculations. They are engine RPM, prop gear ratio (normally for 1.5 to 3.0), prop pitch (the distance that a propeller would move forward in one revolution), prop slip (the efficiency of the prop, lower at high speed, but perhaps very inefficient at low speed) and the boat speed in mph. Enter any 4 variables and the 5th variable will be calculated. Defaults are set up so that boat speed is the missing variable.

 Engine brake power

P_B = M . 2 π n / 1000 K.W.

M = Engine torque (N.m)

n = rpm

Delivered power

P_D = P_B – P_L           – P_L = power loss due to mechanical arrangement and friction in bearings

Thrust power

P_T = S v_a /1000 K.W.    – S = space; v_a = advance speed

Effective power

P_E = R v /1000 K.W   – R = ships total resistance

Propeller efficiency

a. open water efficiency  η_o = P_T / P_D = Sv_a /( 2 . π. n ) . M_o     -  M_o = torque in open water

b. efficiency behind hull  η = P_T / P_D = Sv_a /( 2 . π. n ) . M         - M = torque behind hull

Transmission efficiency   η_T = P_D / P_S

Overall propulsive efficiency  η_oa = P_E / P_B

The wake

The wake is the phenomenon of dead water behind the ship . The wake speed is the difference

between the ship speed Vs and the speed of advance v_a

Wake estimation:

w = 0.5 C_b – 0.10   -  single screw

w = 0.55 C_b – 0.2   -  twin screw

Thrust estimation (single screw)

T = 0.5 C_p – 0.12  or

T = K w

K = 0.5 to 0.7 for stream lined rudder .

K = 0.7 0.9 for double plate rudder and square rudder post .

K = 0.9 1.05 for single plate rudder

The most efficient number of blades is one because the single blade has nothing to disturb the water flow ahead.    This is not realistic, a single blade would be impossible to balance. The two blade propeller is not practical because  would require a very large diameter of propeller  to get thrust. For this reason propellers have three or more blades.  They will also reduce vibration and increase acceleration for the same given pitch. The material the propeller is made also affects performance by allowing a better profile.

 

 

Propeller calculator

 

 

Powering and propulsion