[C320-list] Shroud tension

[Charles Holbrook via C320-list]

Hi All:
I just stepped the mast on my 95 Cat 320 and was wondering what the tension setting should be on the shrouds?
Charlie Holbrook"Nautigirl"Catawba Island, Ohio

[Mark Seyler via C320-list]

Charlie,

There is an article on the C320IA website that many have used for a starting point for rig tuning. That article is at https://c320.org/article.php/20040828173431380 .

The article on the website assumes that you would have a backstay tension adjuster. (An article on that is on the website here: https://c320.org/article.php/20120828164453754 . Especially if you race, you may want to consider adding one.

I don't race my 320, and because of the design of my Bimini (which is crucial in sunny and warm New Orleans), I cannot use the type of backstay adjuster that is recommended. So I had to estimate a fixed set of tension readings, based partly on the percentage of breaking strength for the rigging wires. Here is what I came up with:

Cap Shrouds PT3-19.5 950lb. 8%
Mid Shrouds PT2-27.5 660lb. 8%
Fwd Lwr Shroud PT2-32 840lb. 10%
Aft Lwr Shroud PT2-22 450lb. 5%
Backstay (each) PT2-28 550lb. 7% (X2 = 1100lb., 14.5%)
Forestay (est.) PT3 -26 1500lb. 14% (Possibly 1375lb. or 12.5%)

The tension numbers are based on the Association's Loos PT2 and PT3 tension gauges, which I borrowed through the tool loan program. The forestay tension is estimated because you can't put the Loos tester on the forestay itself when you have a jib furler. And the alternative estimates are the result of attempting to consider the difference in angle between the forestay and the backstay, despite never having taken trigonometry in school.

I hope that the above is helpful. Good luck with your rig tuning.

Mark Seyler
S/V Reality,
Catalina 320, #232
New Orleans, LA

[Charles Holbrook via C320-list]

Thanks Mark.  I have an adjustable back stay even though I don't race.
Charlie Holbrook

[Charles Holbrook via C320-list]

Mark, Thanks for the break down on tension settings for my shrouds.  Silly question..... is the number after the gauge (ie Cap Shroud PT3-19.5, Mid Shroud PT2-27.5) the setting I should be adjusting for on the gauge?  If so, the aft lower shrouds feel loose to me so I tightened them to 28 instead of 22.  Everything I read says I won't break anything but too tight could cause bending of the mast.
Charles Holbrook"Nautigirl" 95 Cat 320.   

[Mark Seyler via C320-list]

Yes, the number shown is the tension reading I adjusted for on the Loos gauge in question.

Regarding the aft lower tension: the setting I used was loosely based on the article from the Association's website, which aimed to put "pre-bend" into the mast by letting the forward lower shrouds pull that part of the mast forward a bit. I don't think there's any great downside to going with a tighter aft lower tension, as long as it's not tighter than the forward lower. Tighter aft lowers might reduce the flattening effect from increasing backstay tension on your mains'l because the middle of the mast wouldn't be able to move forward as much. But I'm not convinced that the very sturdy masts on our 320s bend that much anyway. I think the more important effect of adjustable backstay tension is to reduce sag in the forestay.

Fair winds and smooth sailing,