CETN I-63
March 1999
Bulk Carrier, θ = 45
LPP = 824 ft, B = 105.8 ft, T = 42 ft, h= 45 ft
2.5
600
Heave, RAO
Heave, Phase
Roll, RAO
Roll, Phase
Pitch, RAO
2.0
480
Pitch, Phase
Total, RAO
USACE, RAO
1.5
360
1.0
240
0.5
120
0.0
0
8
10
12
14
16
18
20
22
24
Wave Period - seconds
Figure 3. Excursion RAO curves and USACE guidance for bulk carrier
As an example of the difficulties present in estimating shallow-water squat, Figure 4 shows squat
curves for a bulk carrier (Harkins and Dorrell, in preparation). Squat values from 10 empirical
equations are extracted from PIANC (1997), USACE (1995), and Ankudinov et al. (1996) for a
shallow fairway h/T = 1.2, where h = water depth, and T = vessel draft. Similar empirical
relationships exist for trenched fairways and canals; these types of geometry restrictions usually
result in greater values of squat when compared with an unrestricted channel having the same
depth. In general, shallow-water squat is usually greater than deep-water squat and is
approximately proportional to the square of the vessel speed. From Figure 4, for a speed of
10 knots, the squat estimates vary from 1.7 to 3.7 ft, a difference of 2 ft; for a speed of 15 knots,
squat varies from 3.7 to 9.8 ft, a difference of 6.1 ft. This ambiguity in squat would limit vessel
speeds by 4-5 knots based on the extreme estimators. Overly conservative squat estimators
would result in either unnecessary dredging costs or reduced throughput capacity. It is necessary
to revisit these predictors and arrive at a consensus for a unified squat estimator for both design
and operational purposes.
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