Cruising Guides

Principles of Yacht Design Xeh

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Mast through deck

+ Thinner mast which gives better mainsail efficiency.

+ Smaller outer dimensions/wall thickness give a lighter mast.

+ Smaller foresail sheet angles are possible.

- More difficult to trim, especially lengthwise. _ High horizontal forces in deck level.

- Risk of heat and water leakage.

Forces on the shrouds The forces conic from the wind pressure on the sails and dynamic additions from wind and sea. Two load cases are considered in Fig 11.3: in case 1 the rig is loaded by only a foresail, and in case 2 the rig is loaded by a deep reefed mainsail. In case I the transverse force T, is simply the righting moment divided by the distance from the waterline to the uppermost shroud, as illustrated in Fig 11.3(A). it does not matter what kind of foresail is carried, since the dimensioning force comes from the righting moment.

In case 2, with the reefed mainsail, the transverse force T^ is calculated by dividing the righting moment by the distance from the waterline to the geometric centre of the mainsail, approximately ]A of the luff up from the boom. This force is then distributed between the head of the sail, Thead, and the boom, Tboom, according to Fig 11.3(B). When Thead lies between two shrouds, the force shall be distributed between the two shrouds proportionally to the distances from the shrouds" attachment points to the location of the force, Fig 11.3(C), and the resulting forces are Thu, acting on the upper shrouds, and TM, on the lower shrouds. The boom force is working on the deck and on

the lower shrouds, where we are interested to know the load on the shrouds. This load, Tbu, is a fraction of the boom force proportioned as the ratio of the boom height above deck to the distance of the shroud to the deck, Fig 11.3(D).

We now have all the components forming the transverse loads on the rig. Regardless of rig type, the dimensioning force is T{ in load case 1. In case 2 the dimensioning force is different combinations of T,UP Thl and Tbu, depending on rig type according to Fig 11.4. Rig type F 0 only has F| as a dimensioning force, type M-l and F-l have F1 and F:, and type M-2 and F-2 include force F3 as well. For the dimensioning of the shrouds we use the maximum forces F1? F2 or F3 from load case 1 or case 2. Note that there is no F3 force on a double-spreader rig in case 2, if the reefed mainsail does not reach the upper spreaders (see notes 1 and 2 in Fig 11.4).

When calculating the shroud forces in the following figures, 11.5 to 11.8, it is essential to calculate the two above mentioned load cases

Case 1: The rig is loaded by only a foresail.

Case 2: The rig is loaded by a deep reefed mainsail.

The load is to be applied at the geometrical centre af the reefed sail.

Fig "I 1.3 Transverse loads

F : Fractional Rig

D

tmensionîng Fo

rces F1

f2 f3

...... ... ...

Type of Rig

Load Case 1 (Fig 10.3A)

(rig

Case 2 10.3B)

)

r %

• ■.v.vXv; >"-"-"'•:-;•*-*: r-i-.-c-z

W

f—0

T,

0

0

G

0

m— 1/f-1

0

T,

Th,+rbu

0

u

m-2/f-2 f

0 0

o 0

Ti r,

. . -

Wü 0

o	If	BD+0.6P	> l,+l2
2)	If	BD+0.6P	< >,+l2

Fig 1 1.4 Dimensioning forces for shrouds

Fig 11.5 Shroud load - rig F-0

Shroud Tension (D1)

Fj tF2 see Fig 11.4

Dimensioning Load (P#)

PD1 = 2.8-D1 [N] Single Lower Shrouds PD1 = 2.5 -D1 [N] Double Lower Shrouds PD2 = 3.0-D2 [N]

separately, and then compare the results and choose the worse case, ie the highest load for each shroud.

Fig 11.5 gives the dimensioning load of the shrouds on an F-0 type rig. As can be seen it is the shroud tension multiplied by 3, and the smallest permissible shroud angle is 9°.

Fig 11.6 shows the same thing for a single-spreader rig. Depending

Shroud Tension (D#, V#) D3 = F3 /sin fij V2 = Fj/(cosy2 tan§3) C2 = Fj - V2sin 72 D2 = (F2 +C2)/sin$2

VI = (F2 -hC2)/(cosy f tanp2 ) + V2cosJ 1/cos! 2 °7 " F2 + C2 + V2sinl2- Vlsinl 1 D1 = (F1 +C1)/sin^1 F1 ,F2 ,F3 see Fig 11.4

Dim ensioning Load (P#)

PD1	—	2.8 -D1 [N] Single Lower Shrouds
PDI	—	2.5 -D1 [N] Double Lower Shrouds
PD2	—	2.3-D2 [N]
PD3	—	3.0-D3 [N]
Pvi	—	3.2 -V1 [N]
PV2	—	3.0 • V 2 [N]

on whether we have single or double lower shrouds the dimensioning load is the shroud tension multiplied by 2.8 or 2.5. The upper shrouds are dimensioned from the shroud tension multiplied by 3 though, and the smallest permissible athwartship's angle is still 9°.

Fig 11.7 deals with the double-spreader rig. The method of calculation follows the same pattern as on the previous rigs. After calculating the shroud forces according to the formulae, we apply safety factors to the different parts and get the shroud loads. Basically, the safety factor distribution follows the one for the single-spreader rig. apart from the Vl-position shroud, where the safety factor is 3.2. If we have separately coupled intermediate and upper shrouds to a common lower shroud, this shroud has to take the combined pull from the intermediate and upper shroud, that is the reason for the increased factor of safety. If, on the other hand, the intermediates and uppers run all the way down to the deck, their combined strength must at least equal VI.

Forces on the stays The longitudinal loads are primarily dependent on what tensioning devices there are on the boat: winches, tackles, hydraulics etc. The NBS-standard recognizes six different types of rig. Each basic type, masthead or fractional, is divided into three sub groups, according to Fig 11.8. For the masthead rig they are: (1) Double lowers, (2) Single lowers with inner forestay and (3) Runners with inner forestay. The fractional rig is divided into (4) Runners with checkstay, (5) Single lowers with swept

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