Page 172 - Practical Design Ships and Floating Structures

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The tests were carried out on the main hull and on the aforementioned optimised trimaran
configuration for h/L=0.5-1.5.The seakeeping experiments carried out so far are to be considered as
preliminary, in fact only a trimaran configuration, the one optimised with respect to resistance has
been considered. To have a first rough indication about the comparative behaviour of a trimaran ship
in waves, with respect to a single hull, also the isolated main hull of the trimaran itself has been tested
for the same wave lengths. First results of this experimental investigation are reported in fig. 6 and 7.
In fig. 6 the heave response operators 4a and in fig. 7 the pitch response amplitude operators are
shown (in non-dimensional form with respect to the wave amplitude a and the wave steepness ku
respectively, as usual). From the reported results is hard to try any definite conclusion, but, in general
the lower values resulting from the trimaran curves can be noted.

4 HIGH SPEED CRAFT COMPARISON
The trimaran model resistance results have been utilised for a comparison with the equivalent
catamaran and monohull ships using the results obtained from previous model tests (Brizzolara et al.
1998; Cassella et al. 1998). Fig. 8 and fig. 9 show the residuary resistance to displacement ratios and
the total full scale resistances respectively. Model data have been analised by ITTC'57 friction line.
From the figures we can note the different trend for the curve relative to the monohull. It must be noted
that this vessel presents an hard-chine hull whereas the catamaran and the trimaran have round bilge
hull forms. It can be supposed that, at the higher Froude numbers the monohull is subjected to some
hydrodynamic lift. From fig 8 a relevantly lower residuary resistance is highlighted for the trimaran,
on the contrary the monohull definitely presents the higher resistance.
Due to the effects of different waterline lenghts and wetted surfaces, smaller differences among the
values can be observed in fig. 9. However, the same ranking among the various types of hulls
is generally evidenced, with the trimaran always presenting the lowest resistance up to Fn~=v/(gV''~)~.~
= 3 while, for Fnv>3 the monohull ship seems to become the best performance.

0.5 1.0 1.5 2.0 2.5 3.0 3.5 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Figure 8: Residuary Resistances Values of Figure 9: Total Resistances Values of
Trimaran, Monohull and Catamaran Trimaran Monohull and Catamaran