Section 16 Masts

16.1 General

16.1.1 This Section may be used to determine the required scantlings for stayed or unstayed single masts of conventional design on NS1, NS2 and NS3 type ships. Additional requirements for masts are given in Vol 1, Pt 4, Ch 1, 5.3 Masts and externally mounted sensors or equipment.

16.2 Pole masts

16.2.1 The maximum allowable combined bending and direct stress in pole masts is not to exceed the value given in Table 3.16.1 Allowable stresses in masts. The maximum allowable shear stress is not to exceed 0,58 times the value given in Table 3.16.1 Allowable stresses in masts.

Table 3.16.1 Allowable stresses in masts

Item		Allowable stress, in N/mm²
(1)	Stayed mast:	0,50σ_y
(2)	Unstayed mast:	0,55σ_y
(3)	Cross trees, outriggers, etc:	0,55σ_y

16.2.2 The forces acting on the mast are to be calculated in accordance with Vol 1, Pt 5, Ch 3, 6.4 Loads for masts.

16.3 Unstayed masts

16.3.1 The total stress (σ_t) at any particular location is to be taken as:

σ_t

[(σ_b + σ_c ) 2 + 3q ² ]

N/mm²

where

σ_b	=	the bending stress at that location due to the bending moments acting on the mast
σ_c	=	the direct compressive stress at that location due to the vertical components of force. In general, the weight of the mast and cross trees may be ignored in this calculation
q	=	the shear stress due to torque in the mast. The effect of torque need only be considered where cross trees are fitted.

16.3.2 The total stress is to be determined at each change of plate thickness or other change of section along the mast. It is recommended that a plot or table of stress to a base of mast length be prepared.

16.4 Stayed masts

16.4.1 In the absence of stays the mast will deflect under the influence of the imposed forces. Where stays are fitted they will extend under tension, the amount of elongation being related to the deflection of the mast at the point of attachment of the stays. The distribution of forces in the mast and stays may therefore be obtained by consideration of:

The equilibrium between the deflection of the mast and the corresponding elongations of the stays.
The equilibrium between the imposed loads on the mast and the reactions in the mast and the stays.

16.4.2 These calculations are to be made using appropriately defined co-ordinate axes. Attention is drawn to the importance of assigning the correct sign to the angles and dimensions used. Any stay which would be required to work in compression is to be ignored.

16.4.3 Elongation of the stays is to be calculated on the basis of the area enclosed by a circle of diameter equal to the nominal diameter of the rope in association with an effective modulus of elasticity of 61300 N/mm² (6250 kgf/mm²). Consideration will, however, be given to the use of a higher modulus of elasticity where this is demonstrated by suitable tests to be applicable.

16.4.4 The total stress in the mast at any particular location is to be determined. Attention is drawn to the fact that increased stiffness of the mast leads to a rapid increase in stress in the mast with a corresponding reduction in the effectiveness of the stays. It is desirable, therefore, to design the mast for the required section modulus in association with the least practicable moment of inertia.

16.4.5 Wire rope stays are to be in one length and their construction is to comply with the requirements of Ch 8 Fittings, Loose Gear and Ropes of the Code for Lifting Appliances in a Marine Environment (LAME).

16.4.6 The scantlings of a stay are to be such as to provide the tensile force and elongation to meet these requirements. The breaking load of a stay is to be not less than 3,5 times the maximum calculated force on that stay.