Tight Strings

Philip Donegan of Kemah, Texas asks:

"My 35ft boat has a 379 ft2 mainsail. I’m using 10mm braid for my reefing lines, but I am thinking about replacing them with smaller Dyneema lines on the theory that the smaller diameter line will reduce the friction on the blocks. Am I correct, and if so, what is the minimum line strength I can use?"

Win Fowler replies:

Loading on a sail is a function of sail area and the apparent wind speed squared. The formula is L (load in lbs) = SA (sail area in ft2) x AWS2 (apparent wind speed in knots squared) x C (coefficient). The value for the coefficient is determined by a number of variables that I won’t go into here, but I like to use 0.00431. To compute the maximum load on your mainsail, I’m assuming the entire load is being taken either by the clew or the leech end of the reef. With that in mind, first determine the apparent wind speed when you will reef and then plug that number into the formula. For example, let’s assume your boat is very stiff and you don’t have to reef until the apparent wind is blowing 30 knots. In this case the computation would be as follows: Load = 379 x 302 x 0.00431=1,470lbs. The mainsail should never experience a higher load, because in winds over 30 knots you would either be taking in another reef or lowering the sail completely.

So on your boat, if your system is a single-part line, it must have a breaking strength of around 1,500lb. Remember, though, that you must also apply a safety factor of two, and preferably three times that amount to accommodate for things like shock loading, aging, chafe, knots, splices and small–radius sheaves in your reefing hardware. These factors all reduce a line’s strength.

This means that your 10mm line, which I assume is polyester double-braid with a breaking strength of about 4,000lb, is adequate. If you’ve got a two-part reefing system, the load can be half that amount.

Although you could replace your polyester line with 5mm Dyneema single braid—its breaking strength is over 5,000lb—I don’t think you’ll find it any easier to pull the 5mm Dyneema through the blocks than the 10mm double-braid unless the latter is rubbing against the cheeks of the blocks. If the sheaves are turning freely, the friction in the blocks is proportional to the load on the block, the radius of the sheave and the quality of the bearings and lubrication. Line diameter has no effect. Although the 5mm Dyneema line might be easier to pull through the sail’s reef cringle, it probably will be harder for you to winch in, and it certainly will be harder on your hands.

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