Formulae

Winches and Capstans

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