c Shear Strength For craft without continuous longitudinal bulkheads, the nominal total shear stress fs in the side shell plating may be obtained from the following equation.
fs = (Fsw + FsFw)m/2tsI
fs = nominal total shear stress in kN/cm2 (tf/cm2, Ltf/in.2)
I = moment of inertia of the hull girder section in cm4 (in.4) at the section under consideration m = first moment in cm3 (in.3), about the neutral
axis, of the area of the effective longitudinal material between the horizontal level at which the shear stress is being determined and the vertical extremity of effective longitudinal material, taken at the section under consideration.
ts = thickness of the side shell plating in cm (in.) at the position under consideration.
Fsw = hull-girder shearing force in still-water in kN (tf, Ltf)
Fw = Fwp or Fwn as specified by 3/6.1.4b, depending upon loading.
Fs is as defined in 3/6.1.1b
d girder Shear Strength - FRP Craft Hull-girder shear strength will be specially considered on fiber reinforced plastic craft over 61m (200ft.) in length.
e Craft of unusual proportion Craft having unusual proportions will be specially considered.
3/6.1.6 Hull Girder Torsional Loads
Torsional calculations may be required for craft with large deck openings. Racking load calculations may be required for craft with tall superstructures.
3/6.3 Primary Hull Strength - Twin-Hulled Craft
3/6.3.1 Longitudinal Hull Girder Strength
The longitudinal strength requirements for twin-hulled craft are as given in 3/6.1. With the following modifications
i B is to be taken as the sum of the waterline breadths of each hull.
ii For craft less than 61m (200ft) longitudinal shear strength need not be considered unless they have unusual or highly concentrated loads. For craft over 61m (200ft) the shear strength will be specially considered.
iii Items as listed in 3/6.1.3 may be included in the longitudinal strength calculation for the total cross section of the hulls, with the addition of the cross deck bridging structure.
3/6.3.3 Transverse and Torsional Bending Moments and Shear Force
The transverse primary bending moments and shear force for the hull bridging structure (i.e. cross structure) are obtained from the following equations:
Mtb = K1∆Bcln kN-m (kgf-m, ft-lbs) Mtt = K2∆Ln kN-m (kgf-m, ft-lbs)
Qt = K1∆n kN (kgf, lbs)
Mtb = design transverse bending moment acting upon the cross structure connecting the hulls.
Mtt = design torsional moment acting upon the cross structure connecting the hulls.
Qt = design vertical shear force acting upon the cross structure connecting the hulls.
K1 = 2.5 (0.255, 0.255) K2 = 1.25 (0.1275, 0.1275)
∆ = craft displacement in tonnes (kg, lbs).
Bcl = distance in meters or feet between the hull centerlines.
L = length of craft in meters or feet as defined in 3/1.1.
n = vertical acceleration at the craft’s center of gravity, see 3/8.1.1
3/6.3.4 Catamaran and SES Transverse and Torsional Section Modulus and Shear Area
The required transverse section modulus and shear area for catamarans and surface effect craft are obtained from the following equations:
SMtb = Mtb/σa cm2m (in2ft) SMtt = Mtt/σab cm2m (in2ft) Ast= Qt/τa cm2 (in2)
SMtb= required section modulus of the cross structure
SMtt= required torsional section modulus of the cross structure
Ast = required shear area of the cross structure Mtb = design transverse bending moment acting
upon the cross structure connecting the hulls.
Mtt = design torsional moment acting upon the cross structure connecting the hulls.
Qt = design vertical shear force acting upon the cross structure connecting the hulls.
σa = 0.66σy, tensile or compressive stress.
τa = 0.38σy, shear stress.
σab = 0.75σy, combined or von Mises stress.
σy = minimum yield stress of the material, for aluminum the yield stress is to be for the unwelded condition.
3/6.3.5 Items included in Transverse Moment of Inertia and Section Modulus Calculation The following items may be included in the calculation of the transverse section modulus and moment of inertia provided that are continuous or effectively developed over the entire breadth of the cross structure or wet deck, and have adequate buckling strength:
Deck plating, main deck and bottom shell of wet deck Transverse stiffeners on wet deck
Transverse bulkheads or web frames which span the wet deck
Transverse girders or box beams
Continuous transom plating and horizontal stiffeners attached
In general, the effective sectional area of the deck for use in calculating the section modulus is to exclude hatchways and other large openings in the deck.
Superstructures and house tops are generally not to be included in the calculation of sectional properties of the cross structure. Craft having unusual configuration or large bracketed overhead “bent” type structures will be specially considered
See appendix 3/B for guidance on calculating the offered torsion strength of the craft.
3/6.3.6 Craft With More Than Two Hulls
Transverse and Torsional Strength of craft with more than two hulls will be specially considered.
3/6.4 Strength Considerations for Hydrofoil Borne Craft
3/6.4.1 Longitudinal Strength
The craft’s weight curve showing full load, light ship, and partial load if more severe is to be submitted. The support reactions for each of the hydrofoils are to be shown. The resulting shear and bending moment diagrams, as derived from these curves, are to be submitted for approval.
Hull deflection under the condition of maximum bending moment is not to exceed 1/200 of the distance between the forward and aft foil attachment points.
3/6.4.2 Calculation of Loads from Hydrofoil Appendages
The maximum forces transmitted by any hydrofoil to the craft structure is given by the following equations:
FL = CUCLV2AP
FD = CUV2(CDFAFF + CDSAFS) + (Wetted surface drag) FL = maximum lift force on craft exerted by
hydrofoil in kgf (lbs). This force is assumed to act perpendicular to the plane of the foil.
FD = maximum drag force on craft exerted by hydrofoil plus strut in kgf (lbs). This force is assumed to act directly aft from the center of the foil.
CU = 13.847 (2.835)
CL = peak coefficient of lift for the foil selected.
CDF= peak coefficient of drag for the foil selected.
CDS= peak coefficient of drag for the strut section selected.
V = maximum craft speed in knots.
AP = plan view area of foil in m2 or ft2 AFF = frontal area of foil in m2 or ft2 AFS = frontal area of strut in m2 or ft2
Total drag of foil and the strut (or similar appendage) is the drag term as shown above, plus the frictional drag of skin friction coefficient, as a function of wetted surface and Reynolds number.
The bending moment of the foil and appendage that acts on its attachment to the hull is to be calculated and compared to the strength of the connection. A safety factor of 2.0 against the maximum combined lift and drag loads is the minimum acceptance criteria. Calculations for bending moment, stiffness, and shear are to be carried out and submitted by the designer.
Additionally, calculations supporting the “Fail-Safe” performance of each foil attachment structure are to be submitted.
Watertight integrity of the shell is to be maintained in the event of a collision of hydrofoil appendages with a solid object underwater. A design safety factor of 3.0 against the yield strength or 2.0 against the ultimate strength of the foil strut bearing are to be used to assess the strength of the foil for the collision condition.
3/6.5 Effective Decks
To be considered effective for use in calculating the hull girder section modulus, the thickness of the deck plating is to comply with the requirements of Section 3/9. The deck areas are to be maintained throughout the midship 0.4L and may be gradually reduced to one half their midship value at 0.15L from the ends. Only that portion of deck which is continuous through the transverse structure may be considered effective.
3/6.9 Operating Manual
Craft are to be furnished with an operating manual providing guidance on;
a loading conditions on which the design of the craft has been based, including cargo loading on decks, loading ramps, and double bottoms.
b permissible limits of still-water bending moments and shear forces, for craft 61m (200 ft.) in length or greater.
c maximum operational speeds for the various sea-states (significant wave heights) in which the craft is intended to operate.
d other operational limits as applicable such as distance from place of refuge.