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  #31  
Old 10-19-2011
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Quote:
Originally Posted by RichH View Post
To set forestay tension .... forestay reacts, principally, with the backstay and if the angles that forestay and backstay attach to the masthead are 'approximately the same, then the forestay tension will be approximately the same as the backstay .... you adjust backstay tension to get the correct forestay tension.
Hi Rich

I too have a T-37 and have used the selden folding rule method on the backstay to supposely set tension on the furling headstay. I'm still not certain if that tension gets transmitted to the headstay if the mast is beefy. For example undo the headstay and then look up the mast. I can detect no bending what so ever with the headstay undone and with 20% on that backstay. Not sure what spars you have, but mine are keel steped New Zealand spars. Best Regards.
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Quote:
Originally Posted by ccriders View Post
For what its worth, a bicycle wheel with thin gauge spokes is tensioned higher that the same wheel with thicker spokes and a longer spoke would be at greater tension than a shorter one (though that would make a strange wheel, like the one I made for a clown's bicycle)
When tensioned correctly (and given no damage has been done to the rim) the rim will be round and true and the tension on the spokes will be equal. Since the wheel receives dynamic rotational (radially and laterally) loading any unequal tension will walk around the wheel until the spokes loosen to the same tension.
While we don't sail bicycle wheels, our sailboat rigs share some engineering factors. So thinner gauge wire will be at a higher tension that heavier ones and opposing wires should be of equal tension and gauge. Also stress risers like odd bends at fittings must be relieved or the wire/fitting will fail from fatigue prematurely.
Keep on pedaling
John
Except for one very important consideration .... you dont have sails mounted to bicycle spokes. If you did you'd also have to consider the distributive loading on all the spokes by the sails.

Rigging does TWO functions: Holds the mast up AND provides a consistent 'catenary' platform from which to fly 'jibs/genoas, etc.'.
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  #33  
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Quote:
Originally Posted by casey1999 View Post
John,
Little of topic, but how does a bicycle wheel work? With the weight on the axel of the wheel, which spokes are holding this weight?
You know, this question has never been satisfactorily answered and there is no single simple answer. There are times the hub is suspended from the rim's arc at the top of the wheel (static mode) and there are times the top spokes and opposite side down spokes are supporting the rim (going around a corner at speed). The rim goes a little flat at the contact patch and those spokes go out of tension when on a straight line. That reduced tension allows the opposite spokes to also go out of tension, therefore none of the vertical spokes are supporting the bike for a split second.
For sure it is not like a wagon wheel but a dynamic entity of several parts. Which is why I think of a bicycle wheel when I think about tuning the rig of a sailboat. An unstayed mast would be like a wagon wheel. Modern rigs are like bicycle wheels.
John
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Quote:
Originally Posted by RichH View Post
Except for one very important consideration .... you dont have sails mounted to bicycle spokes.
Except when you put fairings over the wheels to reduce air turbulance and improve aerodynamics. The sail effect can be huge and that is why UCI mass start events don't allow disc wheels.

Also, when you say hold the "mast up" you are simplifing considerably the dynamics of a modern sail rig.

John
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Quote:
Originally Posted by ccriders View Post
You know, this question has never been satisfactorily answered and there is no single simple answer. There are times the hub is suspended from the rim's arc at the top of the wheel (static mode) and there are times the top spokes and opposite side down spokes are supporting the rim (going around a corner at speed). The rim goes a little flat at the contact patch and those spokes go out of tension when on a straight line. That reduced tension allows the opposite spokes to also go out of tension, therefore none of the vertical spokes are supporting the bike for a split second.
For sure it is not like a wagon wheel but a dynamic entity of several parts. Which is why I think of a bicycle wheel when I think about tuning the rig of a sailboat. An unstayed mast would be like a wagon wheel. Modern rigs are like bicycle wheels.
John
Yea that bicycle wheel is pretty complex for somthing that on the surface looks so simple. I once saw a bike wheel where the spokes had been replaced by long rubber bands (wheel not on a bike) to see how the thing works when forces are applied to the axel.
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  #36  
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Quote:
Originally Posted by casey1999 View Post
Thanks, there is a lot of good info here, I will need to read many times to understand all of this.

One question, what do you mean by "Normal fore/aft 'prebend' is defined as ~3/4" forward bow for a single spreader rig ( I have single spreader). Also, how would I measure the forestay tension if the forestay is covered by roller furler? Also, the Selden manual (down load hints and advice from below link)

Seldén Mast AB

states tension forestay up to 40% breaking strength. Based on you statement that anything above 30% could lead to fatigue breaking, why would Selden say 40%.
Regards
I've found that the forestay tension is critical to both sail shape and operation of the furler. If your furler starts to get cranky, the first thing to check is sag in the forestay. The right tension here is the first step in getting the rig balanced. On deck-stepped masts like mine, it's important to get it just right to avoid too much down-pressure on the deck. (A note: I've completely done away with any balsa core here and have made the entire area around the mast step solid glass. I don't know why these builders even thought about cored decks under mast steps because it seems like none of them ever hold up over time.) As far as the 40%, I bet it's to eliminate as much metal fatigue as possible when the headsail is moving about. The amount of abuse the forestay takes what with the furler foils rotating around and the genoa movement always scares me. My headstay is 9/32, wish it were even larger but there's no way to change to 5/16 without changing the furler.
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Quote:
Originally Posted by lancelot9898 View Post
Hi Rich
I too have a T-37 and have used the selden folding rule method on the backstay to supposely set tension on the furling headstay. I'm still not certain if that tension gets transmitted to the headstay if the mast is beefy. For example undo the headstay and then look up the mast. I can detect no bending what so ever with the headstay undone and with 20% on that backstay. Not sure what spars you have, but mine are keel steped New Zealand spars. Best Regards.
Caution, the following discussion is 'mostly' for CUTTER RIGS, not for 'sloops', although the 'link' described at the end is applicable to 'both'.

Cutter rigs with TWO stays in front of the mast are an ENTIRELY different 'animal' from a simple rigged sloop with one stay in front of the mast. The problem is with a cutter you have TWO stays reacting to a SINGLE backstay, Intermediate stays to react with the forestay (that stay thats immediately in front of the mast) .... and worse the sails in the foretriangle are of different areas and differing aerodynamic functions and reactions TO the stays on which they are mounted.

The FUNCTIONAL problem with a cutter is: if the larger headsail becomes windloaded it reacts by 'transferring' part of it load to the smaller forestay (in proportion to the aerodynamic forces generated by each sail ... and the staysail 'under' a 'topsail' isnt going to have the same aerodynamic forces being generated as if it were flying 'alone') .... and the headstay becomes 'more slack' as a result. With a slack headstay a cutter rig will not 'point', will heel over aggressively and will be slow as hell and start 'slipping off to leeward' when going upwind. The net result is an extremely complicated rig tension problem ... and with MANY 'variables' acting at the same time.

Soooo ... I first set up the rig like it were a sloop 15% in caps, 4 lowers and backstay/headstay ..... intermediates and forestay at 0%. then readjust forward and aft lowers to get at least 1/2" mast prebend.
THEN 'work-up' in increments the forestay and intermediates to ~10-12%. You DO NOT want more than 10-12% on an intermediate or you risk pulling the chainplate off of its knee attachment. Final 'tune' of my 'basic' setting is to put the backstay at ~20% and 'leave it' (If I were circumnavigating, that backstay would be at ~15%).

With the 'basic' cutter rig tune as above ........
When needing to "point"/beat, I watch how the headstay sag to leeward is matching the luff hollow curve (where the luff hollow curve 'should' be . see 'article' listed below) and either slack off the forestay and/or when in 'normal' ≤15kts. apply running-backstay tension!!!!! .... alternatively its easier just to run forward and slack the forestay down to 3-5% (a good guess) !!!!!!! with such 'adjustments' the boat will 'stand up' with less heel, take off like a rocket, and point like a banshee.
With the 'normally unloading' slack headstay you will be tacking through 95-105°, with a tight headstay / loose forestay you can easily sail to well over 35-40° apparent ... (and sheet the yankee 'inside' the cap shrouds if you want and without 'pinching' or 'barberhaul' the headsail/yankee !!!!!). Slacking off the forestay will 'transfer' most of the backstay (15-20%) to the headstay.
When going downwind, I keep a taught forestay ... as downwind it makes NO difference (relatively) how much the headstay goes slack.
When sailing in BLAMMO conditions ... with no headsail, a staysail + deep reefed main ... I bring the forestay full TIGHT, upwind or down.

Cutters really need to be able to continuously adjust FORESTAY tension ... according to the present wind and seastate condition .... but of course while watching the 'rest of the rig' so that nothing goes much over 30% tension for accelerating 'fatigue' considerations.

Bob Perry, I think, has addressed all the 'tension', etc. complexity issues on cutters with: "Simply sail them like a sloop" .... which means to me get rid of the forestay/staysail/intermediates when sailing in normal conditions (slacking/removing) ... and only use the 'excess' when in Blammo or survival conditions.
I still occasionally race my Ty37, am a 'fanatic' even when 'cruising' and therefore dont mind doing all the on-the-fly rigging and sail changes.

Here's the link that refers to 'matching the 'cut' of the luff to the sag of the headstay .... applies to ALL sails that are attached to 'stays' ... cutter or sloop, make no difference. If anyone cant read this 'article' give me a PM and Ill reply with an email copy. http://i1086.photobucket.com/albums/...LuffHollow.gif

If you want to get a cutter rig to 'point' that headstay HAS TO BE taught .... and I think that the forestay has to be SLACK as the 'easiest' way to auto-tighten the headstay. Headstay + forestay does NOT equal one backstay (in tension-speak) but enough load for TWO bacskstays. 15% + 15% = 30% ..... and thats at or near the 'fatigue endurance limit' for 300 series stainless.
MOST times you dont 'need' 15% in a forestay so why do it as it only automatically slacks off the headstay?

hope this helps. ;-)

Last edited by RichH; 10-19-2011 at 06:03 PM.
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Quote:
Originally Posted by smurphny View Post
I've found that the forestay tension is critical to both sail shape and operation of the furler. If your furler starts to get cranky, the first thing to check is sag in the forestay. The right tension here is the first step in getting the rig balanced. On deck-stepped masts like mine, it's important to get it just right to avoid too much down-pressure on the deck. (A note: I've completely done away with any balsa core here and have made the entire area around the mast step solid glass. I don't know why these builders even thought about cored decks under mast steps because it seems like none of them ever hold up over time.) As far as the 40%, I bet it's to eliminate as much metal fatigue as possible when the headsail is moving about. The amount of abuse the forestay takes what with the furler foils rotating around and the genoa movement always scares me. My headstay is 9/32, wish it were even larger but there's no way to change to 5/16 without changing the furler.
Yes, I also have a deck stepped mast. The mast and step are very strong. Reason for deck step is that boat (S&S34) originally from Fremantle Austrailia where they need to lower the mast to go under some bridges to return to port. I will check my tension on the fore stay. The thing that surprises me with the deck step is that I figure I will have about 13,000 lbs down ward force on a 5inch x 5 inch wood copression post.- That seems like a lot- I surprised these compresion post do not fail.
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Old 10-20-2011
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Quote:
Originally Posted by casey1999 View Post
Yes, I also have a deck stepped mast. The mast and step are very strong. Reason for deck step is that boat (S&S34) originally from Fremantle Austrailia where they need to lower the mast to go under some bridges to return to port. I will check my tension on the fore stay. The thing that surprises me with the deck step is that I figure I will have about 13,000 lbs down ward force on a 5inch x 5 inch wood copression post.- That seems like a lot- I surprised these compresion post do not fail.
Yes, indeed, a lot of force being set on the deck. It amazes me that the deck takes this kind of weight. It MUST deflect at times. On my old A35, there is a large header which unloads the weight to the bulkhead/forward cabin doorway below. I have thought about making some kind of removable post to go right under where the mast sits and down right to the keel...a lolly column of sorts...for long, rough passages.
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Originally Posted by smurphny View Post
Yes, indeed, a lot of force being set on the deck. It amazes me that the deck takes this kind of weight. It MUST deflect at times. On my old A35, there is a large header which unloads the weight to the bulkhead/forward cabin doorway below. I have thought about making some kind of removable post to go right under where the mast sits and down right to the keel...a lolly column of sorts...for long, rough passages.
I think that is a good idea. I hear of a lot of deck stepped mast boats with no compression post do have problems with the deck defect and eventually bending in permanently. Once you get some deflection, then all your shroud and stay tension goes very low, as discussed in this thread- not good.
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