Page 139 - Introduction to Naval Architecture

STRENGTH 125

approximately equal to the spacing of beams and frames in transversely
framed ships. In turn the longitudinals are supported by deep
transverse members at a spacing of about 3 to 4 metres. These and the
transverse bulkheads provide the necessary transverse strength,
The original Isherwood system was applied to tankers. The restric-
tion in cargo space due to the deep transverses made it less popular for
dry cargo ships. However, it is now most usual in these ships to find the
decks and bottom longitudinally stiffened and the side structure
3 6
transversely stiffened " .
If decks, stiffened by transverse beams, were supported only at the
sides of the ship, they would need to be very strong to carry the loads.
Their dimensions, or scantlings, would become large. Introducing some
support at intermediate positions reduces the span of the beams and
hence their strength requirement and leads to a more efficient
structure in terms of strength to weight ratio. This could be done by
pillars but these restrict access in the holds. Usually heavy longitudinal
members are used supported in turn by a few pillars and heavy
transverse members at the hatches. The hatch end beams are
themselves supported by longitudinal centreline bulkheads clear of the
hatch opening. In areas which are predominantly longitudinally
stiffened, deep transverse members are used for support,
Most structural elements contribute to the overall strength of the ship
girder and have some local strength function as well. For instance the
bottom and side shell must sustain water pressures normal to their
surfaces, acting as struts with end and lateral loading. Side structure
must withstand the loads due to coming alongside a jetty. Decks and
bulkheads must withstand the weight of equipments mounted on them.

FORCES ON A SHIP IN STILL WATER
The buoyancy forces acting on a ship must equal in total the sum of the
weight of the ship. However, over any given unit length of the hull the
forces will not balance out If the mass per unit length at some point is
m and the immersed cross-sectional area is a, then at that point:
buoyancy per unit length = pga and
the weight per unit length = mg
Hence the net force per unit length = pga - mg

If this net loading is integrated along the length there will be, for any
point, a force tending to shear the structure such that:

the integration being from one end to the point concerned.