YachtingRopes.com

In the past, we have presented you with a couple of articles on how to select a sheet, a halyard or a mooring line and what the various properties of the ropes mean. We did not only address the materials, but also items like cutches. 

Various people however have asked us what the actual load on sheets and halyards is. In this article we would like to present to you in more detail how this can be calculated and what it means for selecting a sheet.

Jibsheet

Calculating the load on a sheet is as follows:

SL = SA x V² X 0.02104

SL = Load on the sheet (Kg)
SA = Sail area of the Jib(m2)
V = Wind speed in knots(Kn)

The load of a sheet for an (unreefed) jib - sized 18m² is about 436kg at a wind of about 35 knots. With a 30m² and a wind of 20 knotsthis would be 252kg. Of course the accuracy of these values is limited, but is gives you a good idea of the forces on a jib sheet. This diagram displays the relation for a sail area between 15m²and 50m² and a wind speed between 5 and 40 knots:

The relative increase in force on the sheet is decreasing with increasing wind. An increase of wind from 5 to 10 knots with a 20 m² gib results in a 4 fould increase on the sheetload. An increase from 20 to 25 knots results in an increase of 60%.

When selecting a sheet, keep in mind what the avarage weather conditions are that you most likely will sail. Most of us will not sail in conditions over 30 knots. Take into consideration the various types of jibyou migth be using.

Mainsail sheet

The calculation for the load on the mainsail sheet is a bit more complicated:

E² X P² X 0.02104 X V²
ML = --------------------------
SQRT(P² + E²) x (E - X)

ML = Load on the sheet (Kg)
V = Wind speed in knots (Kn)
E = foot length of the main (m)
P =Luff length of the main(m)
V = Schijnbare windsnelheid (Kn)
X = Distance of the aft end of the boom to the mainsheet attachement point (m)

We give you an example based on the following data:

E = 3 meter
P = 10 meter
V =20 knots
X = 1 meter

This means that a boat with a mainsail sized at 16m², the load on the sheet will be 362 kg when the wind is about 20 knots.The accuracy of this figure is alsolimited. Below is an comparison of a boat with the above data and one with a luff of 15m, a foot length for the main of 5m and X being 1m:

The relative increase of force on the sheet is decreasing with increasing wind. An increase of wind from 5 to 10 knots with a 20 m² gib results in a 4 fould increase on the sheet load. An increase from 20 to 25 knots results in an increase of 55%.

When selecting a sheet, keep in mind what the avarage weather conditions are that you most likely will sail. Most of us will not sail in conditions over 30 knots. When using a reefable mainsail, all loads have to be calculated for all reefs. The sheet system that most boats are installed with, also influence the load on the sheet. Some reduce the load with a factor to 25% of the load measured with the formula above.