This page aims to offer some simple formulae and information to help you make the most from your rope
ROPE STRENGTHS AND WEIGHTS
Rope strengths are tested according to Marlow’s QA25 and 26 quality procedures. Generally these procedures are in line with BS EN ISO 2307, however, a number of other internationally recognised test standards are used including EN 1891, EN 892 and EN 564
Marlow Ropes generally specify a Minimum Breaking Load (or sometimes an Average Breaking Load). It is the responsibility of the user to determine an appropriate factor of safety and Safe Working Load. This factor of safety must be determined after considering all the risks, the strength reducing factors, and the expected life of the rope. The table on the left shows some of the factors that may affect the determination of the factor of safety.
Most rope strengths in this catalogue are given in kilograms (kg). However, the correct measure of force or breaking strength is Kilonewtons (kN). Conversion factors from one to the other are:
Kg to kN x 0.00981
kN to kg x 101.972
Rope mass is determined be weighing a sample of rope whose length has been measured at a reference load. For most ropes this load is calculated as:
Reference Load (kg) = D2/8
Where: D is the rope nominal diameter (mm)
| STATIC LOAD | FATIGUE OVER EXPECTED LIFE OF ROPE |
| DYNAMIC LOAD | FREQUENCY OF INSPECTION |
| STRENGTH REDUCTION DUE TO SPLICES OR KNOTS | EXPOSURE TO CHEMICALS |
| STRENGTH REDUCTION DUE TO SHEAVES | EXPOSURE TO UV |
| STENGTH REDUCTION DUE TO BENDING | INTENDED LIFE OF ROPE |
| EXPERIENCE / TRAINING OF OPERATORS | ABRASION |
| EXPOSURE TO HIGH TEMPERATURES | CONSEQUENCES OF ROPE FAILURE |
ELASTICITY AND EXTENSION
Rope extension consists of several components.
Elastic extension: This is the recoverable component of the rope’s extension and is immediately realised upon release of the load
Visco-elastic extension: The contraction of a rope does not follow the same path as the rope’s extension. This results in an element of extension that is not immediately recoverable but will recover if relaxed for sufficient time. If the load on the rope is cycled, a hysteresis loop is formed which will exacerbate this element of stretch
Permanent extension: This is non-recoverable. When the rope is initially loaded all the plaits, strands, and yarns become “bedded in”. This results in a small permanent extension. Most of these constructional effects occur within the first few loadings and have little effect on the rope after this time. In addition to this there are some permanent molecular changes that occur to the material that result in creep.
READ MORE ABOUT ELONGATION AND CREEP IN DYNEEMA ROPES
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- Elastic extension
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- Visco-elastic extension
SHEEVES
Matching sheave profiles and diameters to ropes is essential and there are a number of different criteria that need to be taken into account. As an aid, the following guidelines can be used:
Sheave diameter:
Ropes used round tight radii can be adversely affected by compression and flex fatigue. In order minimise these, it is important that the correct sheave diameter is chosen.
Sheave Profile:
The correct sheave profile is essential to ensure the sheave rotes freely and to eliminate any unnecessary abrasion and chafing. The profile of the groove in a sheave should support the entire rope. Normally a semicircle of 10% greater diameter than that of the rope is appropriate. ‘V’ groove sheaves should be avoided since they compress the rope and have points of local friction reducing the life of the rope. Sheaves should be maintained so that they rotate freely in use.
| ROPE TYPE | SLEEVE DIAMETER |
|---|---|
| Braided Ropes | 8 x Rope Diameter |
| 3 Strand Ropes | 10 x Rope Diameter |
| Multi-Plait Ropes | 10 x Rope Diameter |
| Aramid Ropes | 20 x Rope Diameter |
Aramid ropes are very susceptible to bend fatigue and therefore may require a much larger sheave diameter.
Please note: these sheave calculations may be affected by sheave design and application. If in any doubt, always consult your Marlow Rigging specialist or contact our Technical Department directly.
GENOA AND MAINSHEET LOAD CALCULATIONS
The Genoa and Mainsheet formulas shown here are intended as a guide to calculate sheet loads based on known sail areas. To gain accurate information for your craft it is advisable to consult with a Marlow Professional Rigger, Sailmaker or Navel Architect.
Note: Wind speeds should be the maximum apparent wind speed recommended for the sail.
SL = SA x V² x 0.02104
Where:
SL = Sheet Load in kilograms
SA = Sail Area in square meters
V = wind speed in knots
SL = SA x V² x 0.00431
Where:
SL = Sheet Load in pounds
SA = Sail Area in square feet
V = wind speed in knots
ML = E² x P² x 0.02104 x V² √(P² + E²) x (E - X)
Where:
ML = Mainsheet Load in kilograms
E = Foot length on main in meters
P = Luff length of main in Meters
V = Wind speed in knots
X = Distance from aft end of boom to mainsheet attachment point
ML = E² x P² x 0.00431 x V² √(P² + E²) x (E - X)
Where:
ML = Mainsheet Load in pounds
E = Foot length on main in feet
P = Luff length of main in feet
V = Wind speed in knots
X = Distance from aft end of boom to mainsheet attachment point
