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Shroud Tension

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1981capri
Navigator

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USA
175 Posts

Initially Posted - 03/10/2008 : 16:22:32

Does anyone have what the upper and lower shroud tensions, as measured with a Loos Tension Gauge should to be set at?

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Ericson33
Admiral

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USA
892 Posts

Response Posted - 03/10/2008 : 18:29:17

Forward lowers- hand tight
aft lowers- 32
shrouds - 38
tighten forward lowers when wind is above 20 true

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my_tahoe_too
1st Mate

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USA
32 Posts

Response Posted - 04/22/2008 : 18:32:50

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Capri25
 Forward lowers- hand tight
aft lowers- 32
shrouds - 38
tighten forward lowers when wind is above 20 true
<hr height="1" noshade id="quote"></blockquote id="quote">

R U still sticking with these #'s Chris?

//doghouse

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Ericson33
Admiral

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USA
892 Posts

Response Posted - 04/22/2008 : 19:21:53

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by my_tahoe_too
 <blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Capri25
 Forward lowers- hand tight
aft lowers- 32
shrouds - 38
tighten forward lowers when wind is above 20 true
<hr height="1" noshade id="quote"></blockquote id="quote">

R U still sticking with these #'s Chris?

//doghouse
<hr height="1" noshade id="quote"></blockquote id="quote">

Yes, I read your thread, I think you are going to scream down wind but suffer up, depending on the cut of the main. My main is cut for 8-12" of rake with 3/4" prebend using the aft lowers. Forward lowers are kept loose to allow for more of a slot between the genoa and main. The biggest help I have had is extending the forstay to 31'-10". In light wind I get really good forstay sag and I use the sheet and backstay to adjust the luff.

Every year is different and I need to tune the boat again this year as I allow slack over the winter to release compression on the cabin top

Eom

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my_tahoe_too
1st Mate

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USA
32 Posts

Response Posted - 04/23/2008 : 12:18:02

Thank u Chris - will advice and report after race. . . upwind is the test.

//doghouse
Capri 25 #34
My Tahoe Too!

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my_tahoe_too
1st Mate

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USA
32 Posts

Response Posted - 04/25/2008 : 13:17:39

Great! Thats just what Im looking for. Will report Post race results in the 'morning. thank you all. . .

//doghouse

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SailCO26
Captain

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USA
457 Posts

Response Posted - 05/07/2008 : 09:20:46

In another thread in this Forum regarding rig tune, Harry Pattison of EP Sails was nice enough to chime in on the discussion. I'm posting his info here as well, since in a search for rig tune info a thread titled "Shroud Tension" might get more attention than a race results thread (and I'm sure I'll be one of those searching again next spring!).

Here's what Harry had to say:
<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by epsails ((http://www.catalina-capri-25s.org/forum/topic.asp?TOPIC_ID=17540))
 Basically your headstay length sets the amount of mast rake you have, the backstay controls how much tension you have on the headstay and how much you bend the mast, the cap shrouds keep the top of the mast centered side to side. In a rig like the Capri 25 that doesn’t have in-line lower shrouds it is the combination of the tension on the forward and aft lowers keep the middle of the mast from sagging to leeward and help to control mast bend.

Pre-bend in a mast does two things; it helps set the mast to fit the mainsail you have, and it makes the mast more stable by reducing mast pumping and giving it increased lateral stiffness. While you can induce some pre-bend with just shroud tension that is really not an option unless the rig has swept aft shrouds and spreaders. In a Capri 25 you would have to have the cap shrouds extremely tight to really induce pre-bend and that is not recommended because having the caps that tight would then require having both lowers set up very tight to keep the middle of the mast from sagging to leeward as the wind increases; and having them that tight would greatly limit how much control you have of mast bend.

With in-line cap shrouds pre-bend is induced by having a fulcrum that the mast is bent around, either chocks in the mast partner with a keel stepped mast or the forward lower shrouds with a deck stepped mast. To measure pre-bend you have to pull on the back stay just enough to take the slack out of the headstay. With the backstay just tensioned you can make the forward lowers tighter until you get the amount of pre-bend you want. After setting the forward lowers the backstay is used to control both headstay tension and mast bend, and the aft lowers are used to control how much mast bend you will get. As the wind increases your headstay will sag more and make the genoa fuller. This is countered by pulling the backstay tighter which increases the headstay tension and decreases the headstay sag. At the same time it bends the mast more which flattens the mainsail. However you will quickly reach the point where the mast is bent as far as it can go without distorting the mainsail and making it too flat, and that will limit how much tension you can get on the headstay. The aft lower shrouds counter this bending component by limiting how far the middle of the mast can bend forward. They need to be set up tight enough so that they keep the mast from over bending for you mainsail when the backstay is pulled on to its maximum.

Just a note on pre-bend; class rules limit the mid-girth of the main, how wide it can be across the middle. If you build a sail for more pre-bend the sail has to have more luff curve to fit the bend. For every inch of luff curve you add to the front of the sail you have to move the leech roach in an inch to keep the mid-girth the same. Since leech area adds much more performance in helm balance, pointing, and off the wind exposed sail area we build our mains with a minimum of luff curve and as much roach as we can. Our sails are designed to have only ½” to ¾” of pre-bend, just enough to stop the mast from pumping too much.

Harry Pattison
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