Backstay tension?

You can use the amount of strech as a guide to tension. Irrespective of the wire diameter 1mm of stretch in 2metres is equivalent to 5% of the breaking load. So for a tension equal to 15% of the wires breaking load you need 3mm of stretch measured over 2m.

<hr width=100% size=1><font color=purple>Ne te confundant illegitimi.</font color=purple>
 
Hello Peter I am guessing that your boat has a masthead rig ie forestay to the top of the mast meeting the backstay. All the boats around here with masthead rig have an adjustable backstay. Usually the backstay divides into 2 to an attachment at each stern quarter. A steel plate with a pulley at each end runs on the two wires to the backstay such that when they a re pulled down by a small tackle it squeezes the wires together shortening the backstay. With this device adding no extra tension The backstay is adjusted only enough to take out the slack but without any real load on the rig and just enough to tension the foresty straight in a light breeze.. The inner forestay is adjusted to give some tension on the rig ie the inner shrouds but not enough to bend the mast. When the wind strenthens you increase the backstay tension to counteract the tendency of the foresty to sag to leeward with the jib pressure and to pull against the inner forestay to bend the mast middle forward to take some of the camber out of the mainsail.
Many sailors ague about how much tension should be on a standing rig. The fact that so many experts disagree means to me it is not significant except that a large load must strain the hull when the boat is not in use. Some guys at my club actually release the forestay tension on a fractional rig after sailing and retension using a highfield lever before sailing. Certainly the strain is vastly more when sailing than ever you would set it up static. You just need to make sure that the leeward shrouds or other stays don't become so slack that pins can fall out or turnscrews loosen. Hopefully they are all locked anyway.
My guess is that someone has discarded the backstay tensioner and that by adding one and adjusting it to the wind strength you may get better performance and less heeling in strong winds.

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Unless you are racing, there's no need to be too scientific about backstay tension. Its there to stop the rig going over the bow when you fly the Spini. If its a masthead rig, the general rule is to set it up so that, sighting up the mast, the jib sheave is no further aft than the gooseneck (i.e. Mast Top front should not be aft of bottom rear)
From there you can set it up under way to adjust the forestay tension (not too much sag when beating) and to stop the mast pumping (oscillating fore and aft) in a seaway.

If its a fractional rig, then its a bit more complicated, but you can basically wind it up until the mainsail is the right shape



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I'm afraid that I disagree with some of this. Have sent you a pm Poter.

<hr width=100% size=1>my opinion is complete rubbish, probably.<P ID="edit"><FONT SIZE=-1>Edited by boatless on 16/03/2004 12:32 (server time).</FONT></P>
 
I would be interested in why you disagree, as William_H's explanation seems good to me, especially as my backstay arrangement is exactly as he described.


<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by lumphammer on 16/03/2004 16:07 (server time).</FONT></P>
 
Well, it's probably just hair splitting in the main, and a typo in the post which may have changed it's meaning.


"With this device adding no extra tension The backstay is adjusted only enough to take out the slack but without any real load on the rig and just enough to tension the foresty straight in a light breeze"

Beacause of the lack of a fullstop this might be interpreted as 'when the adjuster is slack this does not increase tension over and above a single backstay' or, the way I read it at first, without a capital T, implying that the device puts no extra tension into the system while increasing forestay tension somehow.

A normal masthead rig has inline cap shrouds and inline lowers if fitted. Putting tension on an inner forestay will not increase tension on the lower shrouds. A larger or perhaps more upmarket masthead rig will have fore and aft lowers to properly position and secure the mid-height mast.

The effect of backstay tension is to tighten the forestay. It has little to do with the inner forestay, which is mainly used to put a slight positive bend in the mast to keep it from 'panting' - pumping back and forth in waves. Typically this bend is from half to one mast diameter, and is left in at all times. Your sailmaker cuts the sail to reflect this.

Many sailors argue about rig tensions. True, but I'm not sure that most experts do.

Most owners will release the backstay in order to reduce overall fore and aft tension. You might release forestay tension on a swept back spreader fractional rig, but I've never met anyone that does it

Lastly, an unwired or pinned clevis pin could fall out if a lee shroud was slightly slack. Sailing with slackish lee shrouds is perfectly accaptable. While the post does not imply otherwise, a reader might think that a tight lee shroud is possibly desirable.

Overall, maybe I was a bit pedantic.

I could tell you a good story about an owner who tightened his lee shrouds, tacked etc... but he was kind of embarassed when we found the stretched bottlescrews he'd thrown away before claiming that his chainplates were faulty. But I won't!


<hr width=100% size=1>my opinion is complete rubbish, probably.<P ID="edit"><FONT SIZE=-1>Edited by boatless on 16/03/2004 17:20 (server time).</FONT></P>
 
Thanks boatless. Is this discussion still going if so ..... I appologise for the bad wording and I don't disagree with you. The comments about pins falling out should have been left out. Lee shrouds are always slack in a blow but there is something wrong with havingg them too slack but I am not sure what.
There can be no doubt that pulling on the backstay will tension stretch the forestay and jib luff if the inner forestay is tight then the stretch of the forestay will seffectively give more pressure on the inner forestay to bend the mast just what you want. Most boats around here using this configuration will have an adjustable inner forestay to add more bend.
I disagree that adjustment is only if you are racing. When the wind rises a flat mainsail will enable you to spill air pressure more easily have less heeling pressure and get you pointing closer to the wind and ultimately home sooner. Again applogies for the confusing wording if nothing else we can get some usefull discussion going.
PS I advocate fractional rigs but have fiddled with a lot of masthead rigs.

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Don't forget backstay tension increases compression which increases bend.... Also bending the topmast aft induces more twist in the sail, and in high winds this helps. Sails (particularly Dacron) have the draft move aft as breeze picks up resulting in the leech hooking to windward. All this does is increase heeling and slow you down....

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"
 
William

Yes, that's the way I set up an inner forestay if it's the bottlescrew type. Put a bit of bend in and then do the backstay. I used to race a 40 with a 4:1 tackle on the babystay, good for letting it off to use the spi pole, but even on a winch, a b*gger to get the bend in again afterwards!

It's certainly not just for racing - as you rightly say - balanced boats should be for eveyone, although you'd never know it looking at most of them!

I too prefer fractional.

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
After some very interesting discussion I do not think that either William H or Boatless have given the originator an answer to his question, i.e. how much back stay tension should he use.

Excuse what may be my ignorance but it appears that mast head and fraction rigs may have a different method for and different degrees of back stay tension. Is this correct and what are the different procedures.

My experience is with a fractional rig for an offshore 20' racing dinghy without a back stay and slight swept back spreaders. Here we generally use light jib halyard tension in light winds, increasing this as the wind increases (approx 2 - 4/5) and then releasing as the wind increases further.

Jib halyard tension bends the centre mast section forward and the release of this allows the top of the mast to fall-off to leeward and spills power from the main. This also opens and closes the slot and determines performance.

Mainsail halyard tension and clew outhaul are also used to power up or flatten and de-power the main.

I would be interested to hear of any thoughts on the benefit of a backstay for this rig. Although not presently in the class rules, if of benefit it would be worth investigating and pursuing.

Thanks.

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Merlin, Hi

I did actually give Poter a full reply, but did it as a pm.

You're quite right; there are differences between m/head and fractional rigs. Trouble is that there are sub-categories. Most m/heads have shrouds inline (athwartships) with the mast. Most fractionals (these days) have swept back spreaders and chainplates and a standing backstay. A few exceptions have inline shrouds and running backstays.

So, typical m/head uses backstay only to achieve forestay tension. Mast bend (semi-permanent - used to put the mast forward of 'in column' and hence stop panting in waves) usually achieved with inner forestay (aka babystay), which may be quickly adjustable, or bottle screw. Also achieved using forward pair of double lower shrouds to pull mast fwd, resisted by the aft lowers.

The more common fractional uses cap shrouds to pull back against forestay AND to push mast fwd via the spreaders - the latter resisted by the swept back lowers. Backstay tension, on a boat with no runners particularly, will increase forestay tension at the same time as increasing mast bend (the mast is really rotating around the forestay, with the lowers desperately trying to hold everything straight - which can get out of hand). Running backstays attach at forestay height and directly tension the forestay without affecting bend. On these boats you'll find a lighter section topmast, which can be more easily bent by the standing backstay to depower leech. This gives independence between forestay tension and mast bend. Nice, but you've heard all the stories about forgetting the runners...

Going on to your boat, increasing jib halyard tension has the same effect as increasing shroud tensions. Much simpler system, but on larger boats a wire luff is not common, so it's not possible. This is because having a separate forestay allows you to control the tension in that only, to control luff sag. By then having a stretch luff, controlled by halyard tension means that you then have the draft depth/position control independently.

So would a separate standing backstay help your rig settings? And note, everything you said about tensioning the sails themselves is agreed 100%, I'm just dealing with the rig itself here.

Answer: depends on the topmast section. If it's tapered by 25% or more, I'd give it a go, great way to dump the leech. If you do try it, be careful because (unless I've missed something) you have no lower shrouds to resist mid height bend - not normally a problem until you bear away onto a reach with everything up tight and pulling! Otherwise you'll really just tensioning the forestay - which you've already done on it's halyard.

So I've typed all this and suspect you're going to leave it as it is!

John

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
Boatless,

Thanks for your response. You are right in that I will not be adding a backstay, but this has been an education and good advise on the subject.

I do not understand the term runners and I assume by cap shrouds you mean those fixed above the fraction rig forstay/jib halyard.

In addition why de-power the main sail leech with mast bend. There are other methods to control power here.

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Thanks for the thanks!

Cap shrouds are those that go to the top of the mast on a m/head rig, or the height of the f/stay on a fractional; more specifically to roughly the same height as the forestay attachment on any type of rig. Tallest pair of shrouds anyway.

To complicate, should add that the caps often attach slightly higher than the forestay on a fractional rig in order to pull the masthead back, rotating around the forestay. If lower they would only tension the forestay, and lower than the forestay they would pull the lower mast back - not desirable.

Standard runners aka 'running backstays' are a pair of backstays taken from the stern quarters to the approx height of the forestay. On a fractional rig it is not possible to have fixed backstays to this point because the mainsail needs to be free to travel. So with a running backstay, the weather one is tensioned to directly tension the forestay. During a tack or gybe, it is released and the old leeward/new windward runner is tensioned. A shockcord is usually rigged to pull the 'old' runner back clear of the boom/sail as it crosses the boat. There will probably also be a standing backstay to the masthead crane, as usual, for security. Lots of stories abound regarding not doing runners on time, or failing to do them at all- not funny in a gybe - which are important, but suspect most are urban myths. Much like spinnaker scare stories!

An optional supplement to these are the checkstays, same as a running backstay but don't go as high, usually about 2/3 forestay height. Gives more control over lower mast bend. Operated in the same way.

Both runners and checkstays are normally single part for the main length, terminating in a 4:1 tackle and thence to a winch or lever.

The swept back spreader/chainplate/cap shroud elimimnates runners, but does not give such good ultimate control. But obviously a d**n sight simpler for most boaters.

As to why de-power the leech via backstay, well, it's just because you can! Do you remember the bendy glassfibre topmast "Lionheart" rig? That was an extreme example, (which also made the tip of the mainsail more useful by allowing a slightly elliptical top). It's a very effective way or twisting off the top of the main without affecting the lower part as much. Brings the centre of effort down quickly with just one string.

Given that a fractional rig tends to be a much more 'mainsail driven' boat, the more cotrol you have over it's shape the better.

Hope that's clear, please say if not.

John

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
Too late to edit... para 4

"There will probably also be a standing backstay to the masthead crane, as usual, for security"

should read

There will probably also be a standing backstay to the masthead crane, to control topmast bend, and, as usual, for security.

<hr width=100% size=1>my opinion is complete rubbish, probably.
 
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