Page 537 - Practical Design Ships and Floating Structures

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depths tested. The novel result leads to a new hypothesis for the total resistance of ships in shallow
water:
For a ship moving at a subcritical speed and in not extremely shallow water, the total resistance could
be considered as a unitfirnction of the effective velocity and independent of the water depth .
For the two subject ships tested, this hypothesis holds for a speed Fnh 50.7 and a water depth h/T
2 1.5. If this hypothesis could be systematically validated for other ships, it would substantially
impact the resistance prediction of ships in shallow water.

70
60
- 50 -
z
40
I +.
d 30 aF
20
10
0
00 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 2.5
VE.IW v,, WSI
Figure. 3: Total model resistance as a function Figure 4: Total model resistance as a function
of the effective speed for the subject inland of the effective speed for the subject container
ship ship

3.3 Unit Form-Factor Based on the Effective Speed
The uniform total model resistance should also lead to a unit form-factor, if the model speed VM for
the identification of the form factor is replaced by the effective speed VEM . The well-known Hughes-
Prohaska formula reads now
(3)
LFOME
here the model resistance is normalized by p 12. S, . V& instead of by p/2. S, . Vi . The Froude
number FnE and Reynolds number R, for the ITTC friction line refer also to the effective speed.
The resulting form factor should be thus called as an effective form-factor.
2.5

2.0
1
YL
E
0

1.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 00 0.1 0.2 0.3 0.4 0.5 0.6
Fn'/CF,
FnE'/CFaM
(a) Conventional evaluation based on the (b) New evaluation based on the effective
towing speed speed
Figure 5: Hughes-Prohaska form-factor at different water depths for the inland ship