10. State requirements about power output of marine propulsion plant according to propeller and ship’s hull?

Suppose NK (hp) is the necessary power to maintain ship's speed at V (m/s), with total resistance R (m/s), then; 

Nk =RV/75 (hp).

According to result of 1.4, thrust power of propeller is that:

Np = Nk / ηh (hp).

The power on propeller hub:

Ncv = Np/ηp = RV/75ηhηp  (hp).

Power of the main engine:

Ne=Ncv/ηtr = RV/75ηhηpηtr

Power of the main engine depends on type, functions and displacement of the ship. If the ships have the same displacement, but functions and types are different then their powers are also different. Therefore, definition "relative power" is used:

α = Ne/D     (hp/T - displacement).

With D: Displacement of ship    (T).

Or:     α= Ncv/ηtrD  (hp/T - displacement).  

The power on the propeller hub NCV is determined according to 1.4. However, to simplify the power on the propeller hub can be calculated by Naval formula:

Ncv=(V^3D^2/3)/Cw  (hp).

Where:

- V: Ship's speed   (knot/h).

- D: Displacement  (T).

- Cw: Naval coefficient.

Finally:  α = (1/Cw. ηtr) . (V^3 /3√D)  (hp/T - displacement).

This result shows that at the same displacement, the higher the speed, the larger power is.