The effects upon speed and power performance by a propeller in poor surface condition are difficult to assess in practice.
Some years ago British Ship Research Association (BSRA) developed an integrated system of analytical procedures for modelling propeller characteristics in real flow conditions behind ships' hulls. The following case studies are based on BSRA's analytical program. Calculations have been made on a fuel price of USD $185 per tonne.
- A 10 year old 320.000 DWT crude oil carrier trading between Arabian Gulf and Europe.
- Speed 12-13 knots (cargo/ballast).
- The propeller is four bladed (fixed pitch).
- Engine: Steam turbine main propulsion plant.
- New building: 45.000 DWT bulk carrier, trading between the Mexican Gulf and Japan.
- Speed 13 knots.
- With a four bladed fixed pitch propeller.
- Engine: Diesel main propulsion plant.
- A 12 year old, 1600 TEU capacity container feeder, operating in the North Atlantic.
- Operating at an average speed of 19 knots (three knots below design speed).
- Fitted with a five bladed pitch propeller.
- Engine: Steam turbine or slow speed diesel main propulsion plant.
In the table below, the results from the BSRA analysis method have been used for calculating the cost in annual terms of excessive roughness relative to Rubert Grade A (highest smoothness practically achievable, for definitions see Roughness Definitions).
| Rubert Grade | Cost of propeller roughness relative to Rubert Grade A ($ per annum) | |||||
|
Constant
speed operation
|
Constant
power operation
|
|||||
|
Ship
A
|
Ship
B
|
Ship
C
|
Ship
A
|
Ship
B
|
Ship
C
|
|
| B* |
15
000
|
3000
|
5000
|
10
000
|
3000
|
10
000
|
| C |
54
000
|
14
000
|
24
000
|
36
000
|
13
000
|
48
000
|
| D |
106
000
|
32
000
|
55
000
|
71
000
|
29
000
|
109
000
|
| E |
155
000
|
52
000
|
88
000
|
103
000
|
48
000
|
174
000
|
| F* |
205
000
|
70
000
|
122
000
|
135
000
|
62
000
|
240
000
|
| * Approximate | ||||||
