- Oct 25, 2005
The thread on halyards got me thinking about line sizes and construction for various uses.A common question is what size line to use for Genoa sheets. How do you choose? I'm a geek, so I don't like to guess when I make choices.The formula in the Harken Catalog for Genoa sheet loading is pretty widely accepted: Sheet Load (in pounds) = Sail Area (in square feet) x V^2 (wind speed in knots squared) x .00431 (a constant)For a typical 30ft boat the 150% Genoa has about 375 sq ft of area.At 20 knots the sheet load is 375 x 20^2 x .00431 = 646.5 poundsIf you use a 5:1 safety factor, this sail needs to have a sheet that has a rated strength of 3232.5 pounds. That would be between 5/16 Double Braid (3000 lbs) and 3/8 (5500).The same sheet on the #3 (252 sq ft) in 35 knots of wind will see 1330.5 pounds of load. For the same 5:1 safety factor the sheet needs to be rated 6652 pounds, calling for 1/2" double braid (7500 pounds).That makes 1/2" double braid look like a good choice for sheets for a typical 30ft boat.What happens to that sheet while the boat is sailing?With the 150 up in a gusty 15 knot breeze the sheet load goes from 525 in the gusts to 233 in the lulls. In the gusts the sheet is stretched about 3.5" and in the lulls about 1.5", a 2" range. A 2" trim change is very noticeable on most sails. The sail needs to be trimmed in the gusts and eased in the lulls, exactly the opposite of what the line stretch is doing.The situation is worse with the #3 up in a breeze. In a gusty wind, the wind speed changes from average - 20% to average +20% every 45 to 90 seconds. An average 30knots is a really a range between 24 and 36.The sheet load goes from 625 in the lulls to 1407 in the gusts. The sheet is stretched 5.75" in the lulls and over 11.25" in the gusts, 5.5" change in trim. 5.5" of trim is huge on a #3.There is a very good case to be made for using Spectra or Vectran for sheets.The same 1/2" diameter sheet in Vectran or Spectra is rated about 20,000 pounds, the 1400 pound load in the 36knot gust is only a 7% load. The sheet stretches just over 1.5" instead of 11.25". For cruisers, low stretch sheets mean the sails work better more of the time (better boat speed, fewer adjustments, and more miles per day) and for racers it means less work to maintain proper trim.