Pulley, friction ring, Nub and load forces
Load forces as a function of deflection angle
To choose your fittings or your ropes for your deck plans, you must take into account that the effort exerted by the rope on a pulley, a friction ring or a Nub must take into account a coefficient which is determined by the angle of the efforts given by the rope. (see table below)
As an example, a low friction ring that returns a rope at 180° will be subjected to a load equal to 2 x its load, but when returning a rope at 30° with a Barber Hauler, a return pulley or a Hook, the efforts are only 52%.
At the foot of the mast, it is preferable to use Duralumin or Nub rings than resin rings because they are subjected to greater forces at 90°, i.e. x 140%.
You can remember that at the foot of the mast the maximum sail area values indicated in our tables must be reduced by 40% to choose your low friction rings.
Comparative table :
|Deflection angle||Load Factor|
The values of sail area given in our tables must integrate both the angle of the efforts given by the rope, but also the wind speed, little reminder.
Wind speed at which the canopy must be reduced :
- 25 knots for category A, 22 knots for category B, 19 knots for category C, and 16 knots for category D
ISO 12217-2 adopts slightly different wave height and wind force limits, thus aligning with the future EU Directive.
NOTE: The definitions of these design categories are in line with those of the European Union Recreational Craft Directive 94/25 EC as amended by Directive 2003/44.
|Wave height up to||+/- 7m significant||4m significant||2m significant||0.3m significant/ 0.5m max.|
|Wind force||Up to 10 Bft||Up to 8 Bft||Up to 6 Bft||Up to 4 Bft|
|Design wind speed (m/s)||28 (54 nodes)||21 (41 nodes)||17 (33 nodes)||13 (25 nodes)|
The standard adds the following examples (©ISO), which should not be confused with definitions :
"A boat given design category A is considered to have been designed to sail in winds of Beaufort force 10 or less and associated wave heights, and to survive in more severe conditions. Such conditions may be encountered during long crossings, for example, across oceans, or near coasts where the area is not protected from wind and waves for several hundred nautical miles. Winds are considered to be gusting up to 28 m/s.
A vessel given a design category of B is considered to have been designed to navigate in waves of significant height up to 4 m and winds of Beaufort force 8 or less. Such conditions may be encountered during offshore voyages of sufficient length, or near the coast where shelter may not be immediately available. These conditions may also be encountered on inland waters of sufficient size to generate the wave heights cited above. Winds are considered to be gusting up to 21 m/s.
A boat given a design category of C is considered to have been designed to operate in significant wave heights up to 2 m and winds of Beaufort force 6 or less. Such conditions may be encountered on exposed inland waters, estuaries, and coastal areas in moderate weather. Winds are considered to be gusting up to 17 m/s.
A vessel given a design category of D is considered to have been designed to navigate in significant wave heights up to 0.3 m and occasional waves up to 0.5 m high and winds of Beaufort force 4 or less. Such conditions may be encountered on sheltered inland waters, and coastal areas in good weather. Winds are considered to gust to 13 m/s."
Strength calculation of a rope (Simplified approach)
The calculation to determine the breaking load required for your simplified sheet is as follows
Sail area (m²) x Wind speed² (in kts) x 0.021 = Working load (in daN) x 5 = Breaking load
Quick formula below :
- Breaking strength of halyard or mainsheet required = Genoa area x 80
- Breaking strength of the halyard or mainsheet required = Mainsail area x 100
- Breaking strength of the Spinnaker halyard or sheet required = Spinnaker area x 30