Iveco Marine Diesel Engines, Installation Handbook

Page: 19 / 132

MARCH 2004 INTRODUCTION
1.19
MARINE ENGINES INSTALLATION
Gliding hulls
These hulls, due to the shape of their bottom and the power installed, can reach a gliding trim by
exploiting the hydro-dynamic phenomena, starting from an initial displacing condition.
The gliding hulls move the water only when stationary or at low speeds; as soon as the boat speed
increases, the floating angle changes and the water pressure lifts the boat stem.The pressure increases
with the speed square and at the same time the gliding surface is reduced; the pressure centre moves
from the stem to the boat centre of gravity which, at full speed and if correctly balanced, reaches the
horizontal trim.
Yachts, patrol boats and sea rescue boats belong to this category as they are required high speeds.
Displacement
It is the actual weight of the water moved by the boat fully laden and corresponds to the total weight
or mass of the boat fully laden.
The displacement is a weight and should not be confused with other terms, such as tonnage, which
refer to the volume measurements.
When unknown, the displacement of a boat can be calculated by making reference to the boat “block
coefficient”.
This coefficient, usually referred to with C
b
, represents the relationship between the actual hull volume
and that of the parallelepiped, circumscribing the hull and limited by the floating length L, by the float-
ing width B and by the waterline D.
Therefore, as:
it is derived that the displacement is (for the sea water):
Displacement W = 1,025 · L · B · D · C
b
• L, B, D, in m.;
• W in metric tons;
• Sea water density 1,025.
or:
with
• L, B, D in feet;
• W in tons.
Figure 11
hull.volume
Displacement.W