4.13.2 The size, shape and position of limber holes are not to affect the structural strength of the stiffening members in which they are fitted. Limber holes are, in general, to be positioned at the quarter span of the stiffener.

4.13.3 In way of limber holes, “Top Hat” stiffeners are to be boxed, so that, water does not pass through stiffener hollows to other parts of the vessel.

**4.14 Integral tanks (requirements for coatings) **
4.14.1 The surfaces of integral tanks are to be
provided with a barrier to reduce the ingress of
liquid. The details of the proposed system are to be
indicated on the submitted plans.

4.14.2 Fresh water tanks are to be coated with a non- toxic and non-tainting coat of resin that is recommended by the resin manufacturer for potable water tanks.

4.14.3 The design and arrangement of oil fuel tanks is to be such that there is no exposed horizontal section at the bottom that could be exposed to a fire.

Other fire protection arrangements for oil fuel tanks will be specially considered. For details of fire protection requirements see Chapter 10.

4.14.4 Where plywood bulkheads form part of a tank boundary, the surface is to be completely protected against the ingress of moisture with a minimum of 5 mm thickness of laminate to provide an effective fluid barrier.

4.14.5 Where outfit items are to be laminated to the tank surface, the heavy coating of resin is to be applied afterwards and the laminated brackets sealed to prevent the ingress of moisture.

4.14.6 The scantlings of integral oil fuel and water tanks are to be in accordance with Sections 7, 8 and 9.

4.14.7 Integral tanks are to be tested in accordance with Chapter 5, Section 5.5.

**4.15 Reserve buoyancy **

4.15.1 Details of materials to be used and the method of installation of reserve buoyancy are to be indicated on the submitted plans.

4.15.2 Where necessary, buoyancy materials are to be over-laminated in-situ to prevent the ingress of moisture.

**Section 5 **

**Table 5.1.4 (a) : Mechanical properties for **
**chopped strand mat (CSM) reinforcements **
**Mechanical property ** **[N/mm**^{2}**] **
Ultimate tensile strength 200 Gc + 25
Tensile modulus (15 Gc + 2) x 10^{3}
Ultimate compressive

strength

150 Gc + 72
Compressive modulus (40 Gc – 6) x 10^{3}
Ultimate shear strength 80 Gc + 38

Shear modulus (1.7 Gc + 2.24) x 10^{3}
Ultimate flexural

strength

502 Gc2

+ 106.8 Flexural modulus (33.4 Gc2

+ 2.2) x 10^{3}
Note: Gc is the glass content by weight of the
reinforcement within the laminate.

**Table 5.1.4 (b) : Mechanical properties for woven **
**roving (WR) reinforcements **

**Mechanical property ** **[N/mm**^{2}**] **
Ultimate tensile strength 400 Gc - 10
Tensile modulus (30 Gc - 0.5) x 10^{3}
Ultimate compressive

strength

150 Gc + 72
Compressive modulus (40 Gc – 6) x 10^{3}
Ultimate shear strength 80 Gc + 38

Shear modulus (1.7 Gc + 2.24) x 10^{3}
Ultimate flexural

strength

502 Gc2

+ 106.8 Flexural modulus (33.4 Gc2

+ 2.2) x 10^{3}
Note: Gc is the glass content by weight of the
reinforcement within the laminate.

5.1.5 The total thickness of a cured laminate is given by:

t = Σt_{i ,} (excluding the gel coat)
where,

ti = the thickness of ith layer

1.36 [mm]

Gc x 2.56 3072

wi

###

###

###

###

###

###

−=

wi = the weight of the reinforcement in the ith layer,
[g/m^{2}]

Gc = glass content by weight in the ith layer.

**5.2 Testing **

5.2.1 The test pieces of sandwich panels and laminates are to represent the actual construction in respect of the raw materials used ( reinforcements, resins, additives and fillers ), lay-up sequence, production procedures, workshop conditions, etc.

The shear strength and shear modulus properties of type approved core materials as tested and certified by the supplier may be accepted subject to satisfactory supporting documentation.

5.2.2 The testing is to be carried out as per the test methods specified in Chapter 3,section 4.

5.2.3 The extent of material testing will be considered in each individual case but shall normally include the following, as a minimum : In case of single skin laminates :

− the tensile strength, tensile modulus, bending strength and bending modulus and glass content In case of sandwich panels :

− the tensile strength and tensile modulus of the skin laminates

− the bending strength and modulus of the sandwich panel as a whole

− shear test for core materials and test for bond between the skin and core

In case of flange laminates of stiffeners and girders :

− the tensile strength and tensile modulus of the skin laminates

5.2.4 The testing is normally to be carried out at the temperatures mentioned in the relevant standards indicated in Chapter 3, Section 4. For structural members which may be subjected to elevated temperatures, e. g. in way of engine exhaust pipes, it may be necessary to carry out the tests at the relevant operating temperatures.

Indian Register of Shipping

**Section 6 ** **Hull Girder Strength **

**6.1 General **

6.1.1 Scantlings of hull members contributing to longitudinal and transverse hull girder strength are to comply with the requirements given in this section. In addition, members subjected to large compressive stress may also need to be checked for buckling strength.

6.1.2 In general, the longitudinal strength is to be checked for all crafts where L/D > 12 or L > 24 m.

For other vessels, longitudinal strength calculations may be required based on the deck flange area, form, construction arrangement and loading.

**6.2 Longitudinal bending strength **

6.2.1 The resultant longitudinal bending tensile or
compressive stress within any laminate is not to
exceed the allowable hull girder bending stress σ_{a} =
0.33 x σ_{u} {N/mm^{2}], where σ_{u }is the ultimate tensile
strength of the laminate.

6.2.2 The resultant tensile or compressive stress, σ, within any laminate is given by :

2] [N/mm 10

)x E (

yi Ei

*Ii*
*i*

*M*

=Σ σ

Where,

M = the Rule longitudinal bending moment [ kN-m], which is the greater of :

a) Total Bending Moment Mt = (Ms + Mw) given in Chapter 4, Section 4.1.2.

b) Bending Moment due to slamming Msl given in Chapter 4, Section 4.1.3

Ei = tensile modulus of the laminate under
consideration [N/mm^{2}]

yi = the vertical distance to the furthest point within the laminate under consideration from the neutral axis [m]

Ii = the moment of inertia of the laminate under
consideration, about the neutral axis [ cm^{2}- m^{2} ]

The distance of the neutral axis, Yna, from keel is given by :

) [m]

ai (Ei

i) iz ia E ( Yna

Σ

= Σ

Where,

ai, = the cross sectional area of individual laminate
[ cm^{2}]

zi = the distance to the centre of area of individual laminate from keel [m]

**6.3 Calculation of effective sectional properties **
**and longitudinal bending stress **

6.3.1 The effective sectional properties of a transverse section are to be calculated using the net area of all continuous longitudinal members after deduction of openings in accordance with Chapter 6, Section 2.3.

6.3.2 In case of sandwich panels, area of only the skin laminates are to be considered, ignoring the core material.

**6.4 Openings in longitudinal strength members **
6.4.1 The keel plate is normally not to have any
openings. In the bilge plate within 0.6L amidships
openings are to be avoided as far as practicable.

Openings in strength deck are to be kept well clear of craft’s side and hatch corners.

6.4.2 Circular openings with diameter equal to or more than 0.325 m are to have edge reinforcement.

Elliptical openings are to have their major axis in the fore and aft direction. Where the ratio of the major axis to the minor axis is less than 2, the openings are to have edge reinforcement.

6.4.3 Rectangular openings are to have their corners well rounded. Where corners are of circular shape, the radius is not to be less than 20 percent of the breadth of the opening and the edges are to be reinforced.

Indian Register of Shipping

**6.5 Shear Strength **

6.5.1 The shear stress, τ, at mid depth at any position
along the length is not to exceed the allowable hull
shear stress σa = 0.33 x τu {N/mm^{2}], where τu is the
ultimate shear strength of the laminate.

6.5.2 The shear stress τ, is to be taken as :

2] [N/mm As

=10Q τ

Where,

Q = the Rule shear force [kN], which is the greater of :

a) Total shear force QL = (Qs + Qw) given in Chapter 4, Section 4.1.2.

b) Shear force due to slamming Qsl given in Chapter 4, Section 4.1.3.

As = net sectional area of laminates of side shell
plating and longitudinal bulkheads, if any [cm^{2}]
**6.6 Transverse strength of twin hull craft **

6.6.1 The twin hull connecting structure is to have adequate strength considering the design loads and moments given in Chapter 4, Section 4.2.

6.6.2 The following stress levels are normally acceptable:

Tensile or compressive stress : 0.25 σ_{u }[N/mm^{2}]
Shear stress : 0.25 τ_{u} [N/mm^{2}].