Page 163 - Practical Design Ships and Floating Structures

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The effect of the running trim on the resistance was also investigated and the trim was measured for all
the tests. Figures 5 and 6 show the difference AT between the running trim and the trim at rest. There is
an evident correlation between the running trim and the trimaran resistance; this last decreases as
AT (positive when bow up) increases.

1.50 1.50

1.25 1.25

1 .oo 1 .oo

0.75 0.75

0.50 0.50
0.5 0.6 0.7 0.8 0.9 1 1.1 0.5 0.6 0.7 0.8 0.9 1.0 1.1
Figure 5: Running trims for different clearances Figure 6: Running trims for different staggers

For the tested clearance 0.10 (Fig. 3) we have the minimum resistance for the stagger -0.125 in the
Froude number range 0.70-1.00 probably due to a larger lift effect than in the other trimaran
configurations. However, the lay out of the stagger -0.125 can be considered of hard realisation for a
trimaran in the full scale, Therefore the following resistance model tests were carried out considering
only two more realistic staggers 0 and -0.0625. Due to interference phenomenon the hydrodynamic
resistance decreases when increasing clearance (Fig. 4).

In the trimaran with Wigley outriggers only one configuration has been considered (Clearance 0.10,
stagger 0) and it has been tested in the Froude number range 0.60-0.80; its resistance is about 2.5 %
lower in comparison with trimaran with the 64 derived outriggers (Fig. 7).
5.5

5.0
4.5

4.0

3.5
3.0
0.5 0.6 0.7 0.8 0.9 1.0 1.1
Figure 7: Comparison between the Wigley and Series 64 outriggers

Due to a large spray observed at Fn > 0,80, no other model tests were carried out on Wigley side hull
trimaran. This phenomenon, the operating Froude number range of the trimaran ship (0.80-1.00), and
design considerations suggested that the Wigley side hull is not realistic, so no further experiment on
this hullform was carried out.