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SailCO26
Captain

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USA
457 Posts

Response Posted - 05/07/2008 : 09:13:09

Harry: THANKS MUCH for the very articulate and informative post. That's very much in line with how I've been tuning the rig in the past, and will return to that setup post-haste. Unfortunately tonight we have RC duty, so wont know the results until next week.

I'll also cross-post your info in the "Shroud Tension" thread, since I'd probably go back to that thread for info before a race results thread.

Edited by - on

SailCO26
Captain

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USA
457 Posts

Response Posted - 05/07/2008 : 12:47:23

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Capri25
 Taken from the WYC Pages:
by Ken Seider
Mainsheet, Spring 1983 issue

The next step is to set up how much the middle of the mast bends. This is controlled by the forward and aft lowers along with the backstay. If you were to tighten the backstay without any tension on the aft lower shrouds, the first part of the tension would be transmitted to the forestay and straighten it. But as a point, the tension no longer goes into keeping the forestay tight; instead, it starts to press the masthead down towards the deck. This is "compression loading," which happens to all rigs to some extent. Because the mast section is "bendy", it will bow forward in the middle, as much as 8" – 10". This is fine if you have a really full sail and need to flatten it, but I like a medium-full sail with lots of power to get the boat moving and pointing. Therefore, I tighten my aft lower shrouds as tight as I can, and then put just enough tension on the forward lowers to keep them snug so the mast doesn’t "pump" in a seaway. The tension on the aft lowers keeps the middle of the mast from bending too far. Three to five inches of bend when the backstay is applied is good. Also, by preventing the mast from bending too much in the middle, the rig tension is transferred back into the forestay, where the tighter it is, the better pointing you will obtain.<hr height="1" noshade id="quote"></blockquote id="quote">Going back and reviewing this, I think some of the confusion is arising from the statement in bold. I think the issue is that the context in this case is that the backstay is full on at this point, which is not exactly clear in the transcript. I think the key is in the statement <blockquote id="quote">quote:<hr height="1" noshade id="quote">The tension on the aft lowers keeps the middle of the mast from bending too far.<hr height="1" noshade id="quote"></blockquote id="quote">If the aft lowers are tensioned with the backstay OFF, the function of "keeps the middle of the mast from bending too far" would keep the mast from bending at all (or very little), and the backstay becomes strictly a headstay tension control. However, if the aft lower tension is set with the backstay ON, then the condition "keeps the middle of the mast from bending too far" will be met. Then once the backstay is eased the aft lower tension (and the mast straightening out) will upright the spar, until the fwd lowers kick back in, to the limit of the desired pre-bend setting.

Thoughts?

Edited by - on

my_tahoe_too
1st Mate

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USA
32 Posts

Response Posted - 05/07/2008 : 19:27:03

Looks like very sound analysis and I think that is how I perceive MT2 now. Harrys' comments were very instructive, especially regarding the tensioning of the backstay FIRST and then adjust the aft lowers. I still subscribe to "straight stick" with 38 on the caps - in column - and minimal sag (i.e., between sagging and snug) for the aft lowers with 0% backstay, and just sagging when "hard on" @ anything beyond 50% BS tension and the wind pipes up.

Anyone else?

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The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.