Section 4 Hull Girder Loads

401. Longitudinal bending, shearing and axial loads

1. General

(1) For yacht of ordinary hull form with less than 12, with length less than 50 m, the mini- mum strength standard is normally satisfied for scantlings obtained from local strength requirements.

(2) For other types of yacht, with greater than 12, and for yacht with length greater than 50 m, the longitudinal strength is to be calculated as described below.

2. Bending moment from slamming pressure

(1) For yacht ≥3.0, a

slamming pressure is acting on an area equal to the reference area,

: as given in 202, Par 2

= 0.7 for crest landing

0.6 for hollow landing

(2) Crest landing moment

(A) The load combination illustrated in Fig 3.2.9 is to be required with actual weight dis- tribution along the hull beam. The longitudinal midship bending moment is to be as below. However, ( ) is not to be taken as less than 0.04 .

∆

: as given in 202, Par 2.

: one half of the distance (m ) from LCG of the fore half body to the LCG of aft

half body of the yacht. If not known, = 0.25 (0.2 for hollow landing).

: breadth of the slamming reference area, refer to Fig

Fig 3.2.9 Crest Landing

(B) The reduction of towards ends will be determined by weight distribution and the extent of .

(3) Hollow landing moment

Hollow landing is similar to crest landing except that the reference area is situated AP and FP, refer to Fig 3.2.10. However, ( ) is not to be taken as less than 0.04

. longitudinal midship bending moment is to be as following formula :

The

∆

kN m

: as given in 202, Par 2.

: mean distance (m ) from the center of /2 end areas to yacht LCG

Fig 3.2.10 Hollow Landing

Fig 3.2.11 Breadth of Midship Slamming Reference Area

3. Planning moment of hydrofoils

Hydrofoils longitudinal strength is to be calculated for the most severe conditions. As a rule, this will consider the yacht as sustained above the water surface by the foils, and stationary in the nav- igation condition, taking into account vertical acceleration as well as vertical components of the hy- drodynamic action of the water on the foils.

4. Hogging and sagging bending moments

Investigation of sagging and hogging bending moment (still water + wave), taking into account any immersed/emerged structures, may be required for all yacht.

(1) Monohull yacht

kN m

kN m

: as given in 203. 1.

: still water bending moment in the most unfavourable loading condition (kN m )

if hogging is not known =

if sagging is not known = 0 NOTES :

1) Documentation of the most unfavourable still water condition is normally to be submitted for information.

2) If still water bending moment is hogging moment, 50 % of this moment may be deducted where the design sagging moment ( ) is calculated.

(2) Twin hull yacht

kN m

kN m

: still water bending moment in the most unfavourable loading condition (kN m )

if hogging is not known = 0.5 ∆ (kN m ) if sagging is not known = 0

NOTES :

1) Documentation of the most unfavourable still water condition is normally to be submitted for information.

2) If still water bending moment is hogging moment, 50 % of this moment may be deducted where the design sagging moment ( ) is calculated.

: breadth of cross structure (m ).

and : factors for the effect of cross structure immersion in hogging and sagging wave

: height (m ) from base line to wet deck (top of the tunnel)

5. Shear forces from longitudinal bending

Shear forces of vertical hull girder is to be as following formula :

kN

: bending moment (kN m ) as given in preceding Par 2 (2) and (3).

6. Axial loads

Axial loads ( ∆ ), thrust and sea end pressure are to be considered in exposed areas.

≥

with linear interpolation for intermediate

402. Twin hull loads

1. General

(1) The transverse strength of the twinhull connecting structure is to be analysed for moments and forces as specified below.

(2) Design forces and moments are to be used unless other values are verified by model tests or

full scale measurements, or if similar structures provided are satisfactory in service.

(3) Superstructure is normally not to be included in the structure for transverse strength.

2. Transverse bending moment and shear force

(1) For the twinhull transverse bending moment of yacht with ≥ 3.0 and

m ,

refer to

kN m

: transverse distance (m ) between the centerlines of the two hulls.

: factor given in Table 3.2.5.

Fig 3.2.12 Transverse Vertical Bending Moment and Shear Force

(2) For yacht with ≥ m , the twinhull transverse bending moment is be the greater of fol-

lowing formula.

can here be taken as the waterline breadth at

and needs

to be taken greater than 3.

k Nm

k Nm

: transverse bending moment in still water (kN m ) horizontal split force on immersed hull, as following formula.

kN

: height ( ) from base line to wet deck (top of the tunnel).

(3) The vertical shear force in centerline between twinhull is to be as following formula :

kN

: factor given in Table 3.2.5

(4) For craft with length L ≥ 50 m the twin hull still water transverse bending moment shall be assumed to be:

kN·m

: Distance in m from centre line to local centre line of one hull (See

Fig. 3.2.13 Definition

of local geometry for one hull on twin hull craft

3. Pitch connecting moment

The twinhull pitch connecting moment is, refer to Fig 3.2.14 to be as following formula :

kN m

Table 3.2.5 Factors

and

Fig 3.2.14 Pitch Connecting Moment on

Twinhull Connection

4. Twinhull torsional moment

The twinhull torsional moment is to be as following formula

kN m

: as given in Par 2