Breaking strength
Typically the diameter of halyards and sheets is determined by clutches, cleats and blocks on board of your ship. If you are into yacht racing or rigging a new boat, it can be good to calculate the minimum required breaking load of sheets and halyards in order to reduce weight of ropes onboard. Calculating the required breaking load is a more precise approach for determining the diameter of sheets and halyards.
Safety factor
It is a good idea to build in some safety margin in your calculations. Often a safety factor of 4 is used, which means that a rope will receive a workload of only 25% of the breaking load. This is so called Safe Working Load. In practice, however, sails are usually reefed or the boat is in a safe harbour before a rope is loaded to its maximum. That is why we use a safety ratio for cruising yachts of 2 and even 1.5 for racing yachts.
Bear in mind that the weakest link counts. A splice typically reduces the strength by 5-10% but a knot can take off 50% of the strength. For these calculations we assume that the ropes will be spliced.
Breaking strength genoa sheets and halyards
The calculation for sheets and halyards is the same, with the exception of main sheets (see below).
For halyards and genoa sheets you can easily calculate the breaking strength by multiplying your sheet size in square meters with 30 (for spinnakers take 13). This allows you to handle your sails still at 7 Bft (41-47knots). We have done the maths for you in the table below. Loads can vary slightly for e.g. catamaran, but variations are usually covered by the safety factor.
For halyards with a 1:2 purchase, you can divide the breaking strength by two. Please note that clutches, cleats, blocks and shackles still bear the full load though!
| Size sail area (m2) | Minimum breaking strength (kg) Regatta (safety ratio 1.5) | Minimum breaking strength (kg) Cruising (safety ratio 2) |
|---|---|---|
|
20 m2 - 30 m2 |
900 kg - 1,350 kg |
1,200 kg - 1,800 kg |
|
30 m2 - 40 m2 |
1,350 kg - 1,800 kg |
1,800 kg - 2,400 kg |
|
40 m2 - 50 m2 |
1,800 kg - 2,250 kg |
2,400 kg - 3,000 kg |
| 50 m2 - 60 m2 | 2,250 kg - 2,700 kg |
3,000 kg - 3,600 kg |
| 60 m2 - 70 m2 |
2,700 kg - 3,150 kg |
3,600 kg - 4,200 kg |
Breaking strength main sheets
For mainsheets one important factor needs to be added to the calculations: being the point where the main sheet is attached to the boom. This then results in the formula below with these factors:
(A) = surface area of the sail in m2
(V) = wind speed in knots
(X) = the distance (meters) from the boom end to the point where the mainsheet is attached
(E) = boom length (meters)
Mainsheet load = (A) x (V)2 x 0.021 x (E)/(E-X)
If the purchase on the mainsheet is four, the load on the mainsheet is divided by four. But note that the forces applied on the mainsheet travellers and blocks will stay the same!
