Formulae
Winches and Reel Capacities
Length of rope that can be held on a winch drum or reel can be calculated as follows:
Length (m) =710542 x T(F²-D²)
d²
Where:
T= Traverse in metres
F= Flange diameter in metres
D= Drum diameter in metres
d= Rope diameter in millimetres
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.
Genoa Sheet Load Formula - Metric
SL = SA x V² x 0.02104
Where:
SL = Sheet Load in kilograms
SA = Sail Area in square meters
V = wind speed in knots
Genoa Sheet Load Formula - Imperial
SL = SA x V² x 0.00431
Where:
SL = Sheet Load in pounds
SA = Sail Area in square feet
V = wind speed in knots
Mainsheet Load Formula - Metric
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
Mainsheet Load Formula - Imperial
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
Sheave diameters and sizes
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:
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.
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.
- Braided ropes = 8 times rope diameter
- 3 Strand ropes = 10 times rope diameter
- Multiplait ropes = 10 times rope diameter
- Aramid ropes are very susceptible to bend fatigue and therefore may require a sheave diameter of up to 20 times rope 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.
Rope Mass
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)
Conversion Table
To Convert |
|
Multiply by |
|
To Convert |
|
Multiply by |
Inches to Centimetres |
|
2.54 |
|
Centimeters to Inches |
|
0.3937 |
Feet to Metres |
|
0.3048 |
|
Metres to Feet |
|
3.208 |
Yards to Metres |
|
1.0936 |
|
Metres to Yards |
|
1.09361 |
Miles to Kilometres |
|
1.6093 |
|
Kilometres to Miles |
|
0.62137 |
Pounds to Grams |
|
453.592 |
|
Grams to Pounds |
|
0.002205 |
Pounds to Kilograms |
|
0.4536 |
|
Kilograms to Pounds |
|
2.20462 |
Tons to Kilograms |
|
1016.05 |
|
Kilograms to Tons |
|
0.0009842 |
Gallons to Litres |
|
4.546 |
|
Litres to Gallons |
|
0.22 |
Square Feet to Square Metres |
|
0.0929 |
|
Square Metres to Square Feet |
|
10.7639 |
Square Yards to Square Metre |
|
0.8361 |
|
Square Metres to Square Yards |
|
1.19599 |
Cubic Inches to Cubic Centimetres |
|
16.387 |
|
Cubic Centimetres to Cubic Inches |
|
0.06102 |
Cubic Feet to Cubic Metres |
|
0.02832 |
|
Cubic Meteres to Cubic Feet |
|
35.315 |
Rope Circumferance (Inches) to Diametre (mm) |
|
8 |
|
Rope Diametre (mm) to Circumferance (Inches) |
|
0.125 |
Kilograms to Kilonewtons |
|
0.00981 |
|
Kilonewtons to Kilograms |
|
101.972 |