Installed my new halyards earlier this week, took the time to calculate the size I should be installing as opposed to just guessing. built a excel sheet to calulate the percentage of working load on the line based on the square footage of the sail, and the tensile strength of the line.
according to the American Cordage Institute the Safe Working Load of a line is 90% of the tensile strength, minus a safety margin of at least 5 times the load you are expecting to put on the line gives you the Maximum Safe Load. Put simply, take 90% of the tensile strength, and divide by 5, and that gives you the Maximum Safe Load you can use that line for.
then to calculate your wind loading of the sail, use the following formula (Square ft of the sail)*((wind speed in kts)^2)*0.00431
I created a Halyard Size Calculator in Excel to do all the number crunching for me… to use it, look up the tensile strength of the line you are considering using, enter the values in row 4, and enter the square feet of the sail you are looking to replace the halyard for in cell C7.
The table below will give you a percentage of the Safe Working Load the line will be at in a given wind speed. You then need to determine what the maximum wind speed you intend on having the sail hoisted in, before you reef it (for most people this will be around 15 or 20kts). Then you need to look down the wind speed row, and find the smallest line that is under 20%. This will be the smallest halyard (or sheet line) size you can safely use on that particular sail.
The other thing you will want to factor into your decision is the working elongation, or stretch of the line. Look at the line specs and see what the stretch specs are for the line you are considering. Most companies will give you percentage of stretch at a given load. so if you are operating the line at the maximum 20% load, and the line has 1.75% stretch (such as the New England Ropes Sta-Set X line that I just bought) you need to calculate the length of your halyard, for example I’ll use 100 feet, you would have 1.75 feet, or 21 inches of working elongation, or stretch of the halyard. You will probably want to minimize this as much as reasonable, and to do so, you will need to use a larger line, so you are working at a lower percentage of it’s safe working load.
For my halyards, to reduce stretch as much as possible, I went with the 3/8″ line, and assuming I am always reefing my 165sq ft main any time it is blowing over 20kts, that gives me a maximum load of 5.75% of the safe working load of the halyard, and roughly .35% working elongation, which over the roughly 40 feet my main halyard that is not coiled in the line bag in the cockpit when the sail is hoisted, gives me roughly 1 3/4″ of working elongation / stretch of the line when a gust of wind blows, thus keeping the sail shape as best as possible.