Hull–propeller interaction factors $w$, $t$ and the hull efficiency $\eta_H = (1-t)/(1-w)$ that completes the propulsive-coefficient chain.
Formula
$$ w = 0.5 C_B - 0.05 \text{ (single screw)}, \quad w = 0.25 C_B - 0.05 \text{ (twin screw)} $$
$$ t = 0.60 \cdot w + 0.01, \quad \eta_H = \frac{1 - t}{1 - w} $$
$$ V_A = V (1 - w), \quad T = \frac{R_T}{1 - t} $$
Symbol legend
| Symbol | Meaning | Unit | Source |
|---|---|---|---|
| $w$ | Taylor wake fraction | - | Harvald regression |
| $t$ | Thrust deduction fraction | - | Harvald regression |
| $\eta_H$ | Hull efficiency | - | result |
| $C_B$ | Block coefficient | - | hydrostatics |
| $V$ | Ship speed | m / s | from knots |
| $V_A$ | Advance velocity at propeller disk | m / s | result |
| $R_T$ | Bare-hull resistance | kN | resistance calc |
| $T$ | Thrust required | kN | result |
$\eta_H$ above 1.0 is normal for single-screw full hulls - the accelerated wake gives the propeller water at lower velocity, making more thrust per kW than would be produced in open water at the same ship speed.
Sources
- Harvald - Resistance and Propulsion of Ships (1983).
- ITTC - Propulsion Committee Standard Procedure.