• Mobile App For Android Now Online!

    Download it here. The app is searchable in the Google Play Store under Sailboat Owners.

    Sorry iPhone/iPad users, we are still waiting on Apple. :(

    Click the X in the upper right corner to make this go away

Line diameter/ construction selection

Status
Not open for further replies.
Oct 25, 2005
735
Catalina 30 Banderas Bay, Mexico
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 pounds If 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.
 
Jun 2, 2004
649
Hunter 23.5 Calgary, Canada
Another angle

I've never thought or considered any of these stretch issues, but a question occurs to me. When sailing to windward and getting hit by a gust, the apparent winds swings aft, increasing the angle of attack on your sails. This is due to the vector addition of the now stronger true wind with the "boat wind" created by the boat moving forward. Conventional wisdom would have you ease the sheet in such a gust to keep the angle of attack as it was. It sounds like you're saying that we get a free 2" of sheet easing due to stretch, and therefore don't have to ease the sheet after all? Doesn't that mean low-stretch sheets would require more trim changes in gusts, not less? High-stretch sounds more forgiving, so-to-speak. For example, for cruisers on autopilot. ...RickM...
 
Oct 25, 2005
735
Catalina 30 Banderas Bay, Mexico
Good point :)

But the AWA change is small compared to the sheet stretch. For true wind between 6 and 20 knots, the change in optimum AWA is about 3 degrees for my boat. The stretch in the sheets is greater than the trim change required by the change in AWA. Just the change in speed as the boat accelerates after a tack requires a trim adjustment. It does not take much trim to move the leech of the sail 1 or 2" closer to the spreader tip. Say the sail is properly trimmed 1" off the spreader tip. If you trim it to the 1" point right after a tack, then check it again when the boat is back up to speed, you will find that the sail is no longer set 1" off the spreaders. It will need to be trimmed in. How far it needs to move is controlled by the stretch in the sail and the sheet. The less stretch the better.
 

Ross

.
Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Moody, in practical sailing situations

How much of that 252 sq. ft sail would you have hoist in 35 kts? That is just short of a gale.
 
Oct 25, 2005
735
Catalina 30 Banderas Bay, Mexico
Every boat is different

a std rig C30 carries a 150% up to about 18 knots true (over 20 apparent). The #3 blade (100% LP) on my boat is 270 sq ft. and is up in 30+ with the main reefed. The #4 is 220 sq ft and I've sailed the boat in "gusting 40" conditions with a double reef and the #4. The main point is that the sheet loads are more wind speed related than sail area related. Winches and sheets should be sized for the highest loads. On a race boat, that means 100% LP jib in 35-40 knots. If you go to Harken's CompuSpec site, you can get an idea of the loads on many boats. I always get a kick when I hear "My primaries are fine, I can sheet the 150 in 20 knots with no problem". That same system in 30-40 knots will be stressed much higher by the working jib. From Harken, these are the loads on a Hunter 34: 100% Foretriangle @ 30 Knots: 1227.5 lbs 556.8 kg 100% Foretriangle @ 35 Knots: 1670.8 lbs 6.1f kg 100% Foretriangle @ 40 Knots: 2182.3 lbs 989.9 kg 135% Foretriangle @ 20 Knots: 736.5 lbs 334.1 kg 135% Foretriangle @ 25 Knots: 1148.9 lbs 521.2 kg 135% Foretriangle @ 30 Knots: 1657.2 lbs 751.7 kg 150% Foretriangle @ 5 Knots: 51.1 lbs 23.2 kg 150% Foretriangle @ 10 Knots: 204.6 lbs 92.8 kg 150% Foretriangle @ 15 Knots: 460.3 lbs 208.8 kg 150% Foretriangle @ 20 Knots: 818.4 lbs 371.2 kg Note that the 150 in 20 knots is about half the load of the 100 in 35 knots.
 

Rick D

.
Jun 14, 2008
6,965
Hunter Legend 40.5 Shoreline Marina Long Beach CA
Interesting Re: Outhauls

At a recent SoCal Hunter Rendevous, a speaker suggested using a Spectra or other such low-stretch line for Hunter outhauls, saying the difference was significant, with puffs translating into power rather than line stretch. I THINK he was talking about roller-furling mains as that is what most of the new Hunters are shipped with and they are loose-footed of course, but I suppose the same would be true for anyone with a loose-footed main too. Anyhow, since I have some 3/8 Spectra, I am going to replace my outhaul. And, I am looking suspiciously at the jib sheets too. Rick D.
 

Ross

.
Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Moody, Thanks for the numbers. Interestingly

at twenty knots the 150% sheet is applying at least 15 horse power to your boat, I don't know how much of the jib's load is carried by the forestay and we haven't calculated for the power of the mainsail. I allowed 6 knots boat speed for my calculations just because that is 10 feet per second
 
Dec 1, 1999
2,391
Hunter 28.5 Chesapeake Bay
My two cents....

From my perspective, I can't ever recall anyone I know either having a headsail, or headsail sheet, that was in good condition, fail due to wind strength alone. Sure, old and rotten sheets fail. Same thing with tired sails. But sheets and sails in good condition can usually stand up to more wind than the boats they are on and the captains at their helms can. When windspeed increases past a certain point, we all reduce sail area (and pressure on the sail, rig, boat, crew) in order to stand the boat up and keep on sailing. The bottom line (pun intended) is that sheet size seems more dependent on handling characteristics (in your hand and your winch) than on the amount of lbs it can take before it will break. While smaller lines can often handle the loads, they don't handle and feel as good as larger ones and so they are often chosen for that reason alone.
 

Ross

.
Jun 15, 2004
14,693
Islander/Wayfairer 30 sail number 25 Perryville,Md.
Warren, In one of the books by the Pardeys

They talk about their 1000 sq. ft. light air spinaker. Lyn says they fly it on a 1/4 inch sheet and take it down when the sheet starts to get skinny. I believe their boat weighs about 8 or 9 tons.
 
Oct 25, 2005
735
Catalina 30 Banderas Bay, Mexico
Breakage not an issue ...

Of course nothing that is well maintained and in good condition should ever fail, that was not the point. The fact is that our lines loose strength with age, how much is open to debate, but the loss is a fact. Just as old sails that stretch and cause excess heel, weather helm and rob performance, the wrong choice of sheets can add insult to injury. Since most of us don't replace the cordage on our boats even every 5 years, all the more reason to choose well. It will be on the boat a long time.
 
Status
Not open for further replies.