Table 1, Summary of principle particulars, escort tug Appendage option
Lwl, m m
T (max), m
Displacement, tonnes S.W.
Lateral area, m2
Hull only Hull and fin 38.19
14.2 3.8
38.19 14.2 6.86
1276 1276 125.4 157.1
Experiments to measure hull forces were carried out in the Ice Tank of the National Research Council’s Institute for Ocean Technology (Molyneux, 2003). The objective of these tests was to measure hydrodynamic forces and moments created by the hull and the appendages on a 1:18 scale model of the ship. The range of ship speeds was from 4 to 12 knots (with model speeds based on Froude length scaling). Yaw angle was varied between zero and 105 degrees, which covered the full range likely to be encountered during escort operation. The results of these experiments allowed basic force data for different hull configurations to be compared, in much the same way as a resistance experiment can give a measure of merit for different hulls at zero yaw angle. The test method was very similar to that proposed by earlier researchers (Hutchison et al., 1993). The fin was at the upstream end of the hull, for all cases when it was fitted. The hull remained in the same orientation when the fin was removed.
The models were fixed at the required yaw angle and measurements were made of surge force, sway force and yaw moment using a Planar Motion Mechanism (PMM). The load measurement system was connected to the tug on an axis along its centreline, at the height of the towing staple on the tug. The model was free to roll about the axis through the towing staple, and free to pitch and heave. Pitch angle,
roll angle, heave amplitude and
carriage speed were measured, in addition to the surge force Fx and sway force Fy. The model being tested on the PMM frame is shown in Figure 3.
A small negative value of yaw angle (usually five or ten degrees) was used to check the symmetry of the results, and if necessary make a small correction to yaw angle to allow for any small misalignment of the model on the PMM frame. Prior to each days testing, the PMM system was checked using a series of static pulls which included surge only, sway only and combined surge and sway loads. Also individual data points were tared using data values for transducers obtained with the model stationary before the experiment began.
Forces and moments were measured in the tug-based coordinate system and non-dimensionalized using the coefficients given below
Cl ALV Fx 0.5 2 Cq ALV Fy 0.5 2
Cq is the force coefficient normal to the tug centerline (sway) and Cl is the force coefficient along the tug’s centerline (surge). AL is the underwater lateral area of the hull and fin (if the fin was fitted), is the density of the water (kg/m3) and V is the speed of the ship (m/s).
When the measured force values were non-
dimensionalized, the results for all speeds reduced to small variations about a mean value of the coefficient (Molyneux, 2003). This implied that free surface wave effects are small for the range of speeds typically found in escort tug operation. This observation simplified the CFD predictions since only the hull below the design waterline needs to be considered, and the free surface effects can be ignored.
3.
CFD PREDICTIONS OF HYDRODYNAMIC FORCES
The surfaces used to construct the 1:18 scale physical model (Molyneux, 2003) were trimmed to the nominal waterline. The trimmed surfaces were imported as IGES files and cleaned up using the utilities available within GAMBIT (Fluent Inc., 2005a), the program used for creating the meshes. Dimensions for the surfaces were originally given in inches at model scale. The mesh was re-scaled in FLUENT (Fluent Inc., 2005b) to have units of metres, model scale and an origin at the leading edge of the waterline for the hull. All dimensions given in this report are metres, model scale.
A rectangular ‘tank’ was constructed around the hull. This had to be a compromise between being large enough that the boundaries had little effect on the results, and small enough that it converged to a solution in
a
reasonable time. The same domain size was used for tetrahedral
and hexahedral meshing strategies. Both
meshes were created using GAMBIT 2.1. The final mesh dimensions are given in Table 2. The same basic mesh geometry was used for the hull with and without the fin, and so views are shown for the case with the fin only.
Figure 3, Model tested on PMM (10 knots) ©2008: Royal Institution of Naval Architects B-43
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