Page 156 - Practical Design Ships and Floating Structures

acgo - acgm %go-acgm
acg = acgo - =acgm+ (7)

2. The hydrodynamic forces are proportional to the displacements, as the buoyancy ones, what
finishes:

with : y= sea weight density ; A , Vo , Vm = trimaran displacement and displacement volumes of the
outriggers and the main hull.
3. As far as the critic conditions individuation is concerned, two cases are considered for the shear
force and the bending moment (res. the torsion moment):
I) the heaving and roll (res. heaving, roll and pitch) hydrodynamic forces acting on the raising
outrigger, are opposite (as for the catamarans);
11) the roll (res. roll and pitch) hydrodynamic force acting on the impacting outriggers is equal to the
sum of the buoyancy and heaving forces on the same outrigger.
As far as the transverse bending moment is concerned, it has to be considered in conjunction with the
shear force: the effects of the roll inertia and hydrodynamic forces on the transverse bending moment
are null only at the middle section, which isn’t necessarily the most stressed section (as for the
catamarans).
4. In both previous cases, the hydrodynamic forces are assumed uniformly distributed on the fore part
(relatively to the centre of the trimaran) of the main hull and the impacting outrigger, when the
torsion moment has to be calculated.
5. For the longitudinal and transversal inertia moments (respectively I and It ), the following
expression are utilized:

A [o Lm’ +8q&,’+L,’)]
I, = 12gLm(Vm +2qj
Where L@ and Loo are the length of the fore and aft parts of the outrigger (relatively to the centre of
the trimaran). What implies the following expressions for the inertia loads intensity:
I. Transverse shear force and bending moment

12 l+a V
A cg
c.(x)=;[ a + O x 1 II case
cg b V +2V
m o
11. Transverse torsion moment
a) On the outrigger