Backstay tension.

[Allan]

I have moved from a fractional rig to a masthead rig and need to understand the backstay tensioner a little better. One of the main worries I have is over tightening. I have not made any changes yet. I have a nearly new mainsail which heads up very well. The genoa starts to luff long before the main, so I think increasing the forstay tension with the backstay tensioner would be a help but I don't know how much to turn it. I have an inner stay. Are there any simple ways to tell when I have the correct tension?
Allan

 

[Norman_E]

A Loos tension gauge is the dogs doo-dahs for the job. (Saltyjohn on this forum sells them)

Because the forestay and backstay are at different angles, with the backstay being the longer of the two you have a situation where the tensions are not equal, and forestay tension will be higher than the backstay tension. As you normally cannot put a gauge on the forestay because of the furling gear, you need to make an allowance when tensioning the backstay, and do not tension it too much.

Remember that tensioning the backstay pulls the mast backwards, altering the shroud tensions. This is another reason why a rig tension gauge is a very useful tool if you are going to try and tune your rig.

EDIT. As you have an inner forestay, you might need to slacken that before tightening the backstay, and re-tension it afterwards, as it may be preventing the forestay achieving full tension.

 

[johnalison]

In an ideal world one would like the forestay tension to be enough to give a straight forestay, but as this is impossible some compromise is necessary. How much forestay sag you get will depend on this tension and this depends on what boat you have. Even a considerable sag can be accommodated for by cutting the genoa to the right shape, and this has been known to sailmakers for traditional Dutch craft for centuries.

Another possibility comes to mind that your main is fairly flat and you are bringing it up too far to windward or with too little twist. With a moderate cruising boat it may be better to sheet the sails to match and drive the boat more effectively a little less closely than with a racing boat.

 

[jonny_impulse]

How much mast bend do you have at the moment? You need some, but not as much as on a fractional rig. Also, how much can you tighten forestay? Is the foresail on a furling system? A furling genoa will never point as high as one cut for racing, for example. The inner forestay will help to provide stability and get mast bend in the top half of the mast, which is where you need it. What's the boat - traditional cruiser or sportier model?

 

[Allan]

My boat is a Westerly 33, very much a cruising yacht. I have yet to check the rake and bend ect of the mast. The genoa is radial cut No1 and on a furler.
Allan

 

[matelot]

[ QUOTE ]
As you normally cannot put a gauge on the forestay because of the furling gear, you need to make an allowance when tensioning the backstay, and do not tension it too much.



[/ QUOTE ]

usually the designer allows for this by making the forestay diameter greater than the rest so that the elongation is similar. That was certainly the case on my Moody

 

[PeterGibbs]

Quote: I have an inner stay. Are there any simple ways to tell when I have the correct tension?
______________________________
I suggest the correct answer is - when you have acheived the max bend in the mast (usually 5 degress) it is safe and productive to apply.

Yours is a Westerly so is well built and the chainplates can take some tension, but it is the result, I think, that should determine how much pressure you apply. Winding on until the doors jam in the bulkheads would not be advisable!

I offer one further caution: I do understand the benefit of shaping sails for windward work, but wince when I see sailors applying the same tension to their sails when running off the wind - such practices serve no sailing purpose and shorten the useful life of the sail.

PWG

 

[Twister_Ken]

What sort of backstay adjuster do you have?

If it's a rope tackle and falls, or a 'squeeze twin backstays together' sort, you're unlikely to be able to load it up enough to do any harm. Even the 'stick-in-a-winch-handle' sorts are difficult to overload without getting very Arnold Shwarzenegger. The only ones where you really need to worry are the giant bottlescrew and the hydraulic sorts. On a 40 footer I used to race we'd pump the hydraulic adjuster up to 5 tons pressure going upwind in a breeze.

Going upwind, try pulling it down until the forestay is more or less straight. This'll take less sweat in light airs than in heavy. Don't forget you can let a lot of tension off when running to move the rig forwards a bit.

Unless your boat is frail, you're unlikely to bend it with backstay tension.

-----

Edit - backing up what Peter says above, halyards, backstay, outhaul, etc should be tight going upwind in any sort of half decent breeze, but all can be slacked off advantageously when you move onto a reach or run and need more fullness in the sail.

 

[Norman_E]

Not so on all boats, my forestay is 10mm with the upper part of the backstay also 10mm, with two 8mm lower parts. That arrangement, with the 10mm part of the backstay out of reach, makes calculating the forestay tension a bit of a pain.

 

[Norman_E]

You amaze me! 5 tons?

On a 40 footer the backstay was probably 10mm, which (if it was 1x19 wire) has a minimum breaking load of about 8.5 tons, so if the backstay adjuster really did put 5 tons tension on, it was a much bigger percentage of break load than I would consider safe, and would still be high (in the 40% range) for 12mm wire, and would be imposing an even more severe load factor on the forestay.

 

[Allan]

Ken, I'm not sure of the name for my sort of tensioner, it has a wheel (approx 10-12") which works on a threaded section at the bottom of the single backstay. I think I need to try going the whole hog and tightnen everything when going upwind.
Allan
PS, I take your point about slackening off to go downwind.

 

[Twister_Ken]

Yup 5 tons. A Swan 411, pretty hefty beast.

 

[Mudplugger]

Alan, on my 33, mast bend was not more than 3" at rest, and forestay tension as tight as I could get with standard type hand tools, probably a 9" screwdriver and a 6" spanner on the bottle screw /forums/images/graemlins/smile.gif As I had twin Backstays, I had an 8-1 double block purchase twixt the two....fully loaded, both stays would pull in no more than 1-1.5".....advice given a long time ago.....33 is built like brick outhouse, and unless close relative to Godzilla, unlikely to break anything! /forums/images/graemlins/grin.gif

 

[matelot]

5 tonnes pressure in the adjuster isnt necessarily 5 tonnes load in the wire

 

[RJJ]

There is a bit of good advice here, and some total nonsense. From a racer's perspective, lots of cruising guys go sailing for 25 years and still don't understand their rig works. It's worth learning, even on a cruising yacht, for 2 reasons
- it's more fun, and you get there faster
- you rely on the same principles if caught out in genuinely foul weather.

You asked how much backstay tension to use. The answer is:
1 - not so much that you break the boat - but as suggested, this is virtually impossible!
2 - not so much that you rip out the chainplates - also highly unlikely
3 - not so much that you overbend the mast - possible
4 - enough to straighten the forestay.

You need to trade off 3 against 4. More forestay tension = more pointing, because it flattens the jib. That's why the AC guys keep breaking their boats in half - but don't worry, they have powerful equipment that helps them do so! Straightening the forestay has exactly the same effect on the headsail as bending the mast does on the mainsail, because it drags forward the leading edge halfway up.

If you overdo it (3) then you overbend the mast through compression. This results in loss of sideways tension, and isn't really what your mast is probably designed for. Most cruising masts are designed to be ultra-strong rather then ultra-adjustable.

The optimal amount of mast bend is not dictated by some "5 inches is about right". It is determined by the cut of your mainsail. If your mainsail is cut for a pre-bent mast, then setting a straight mast will make it very full - it will backwind easily and overpower too early. If your sail is cut very flat, then a pre-bent mast will give you "luff-starvation" which is very ugly, slow, and bad for the sail. Luff starvation is a crease running from the clew to 1/3way up the mast.

I'd suggest the following steps to get it right - and your sailmaker should help you
- decide how much pre-bend your mainsail needs, and the max it can handle
- wind on the backstay to tension the forestay, to the point where you reach excessive mast-bend
- tension the upper and lower shrouds as necessary (downwind side, tacking each time) to preserve rig tension and help keep the mast straight.
- the more bent the mast is, the more effect the lower shrouds have in straightening it
- if your mast is then significantly within your "bend" limit, tighten the backstay some more and repeat.

Eventually you hit a point where either
- you physically can't wind on any more
- you can't straighten the mast any more by winding the shrouds up.

Your inner forestay is primarily there not to bend the mast, but to stop it from "panting" in chop. Sure you can use it to apply pre-bend in very light airs - but if you really want to fiddle it's better to adjust the lower and cap shrouds.

Good luck

 

[gandy]

I'd just like to add that a number of people told me "if you're using hand tools its virtually impossible to over-tighten". Taking that literally, I stripped one of the rigging screws on our split backstay during our first season with the boat. Now I use a tension guage. 15% breaking strain on 6mm doesn't actually feel that tight.

 

[Mrnotming]

Forestay Tension

All good mostly good sound advice. Sailing in say 15/17knots upwind with full Genoa tighten away till sighting up the forestay produces no difference, then back off fractionally and mark the position on something permanent.Beware that on some boats of good standing the relationship of the shaft to the engine may change and the tension should always be released prior to using the engine for propulsion.Listening to the engine note during the process of tightening (after the first setting up with it off) will give an indication of distress or misalignment of the shaft occurring. Many boats do not have any form of flexible disc at the rear of the gearbox and these are the ones I speak of. Others have enough flex built in iin the drive train to make this test unnecessary.The process certainly tests the fittings involved especially if overdone. As another poster mentions, this is highly unlikely unless a hydraulic cylinder is involved at the backstay or forestay.Good sailing to all!

 

[photodog]

"There is a bit of good advice here, and some total nonsense. "


Always a good way to introduce yourself!

Looks like you will make a excellent addition to the gang, who on the whole are a bunch of shrinking violets.......

Welcome aboard!

 

[jason -and the arguenauts]

- decide how much pre-bend your mainsail needs, and the max it can handle

Could do with a bit of guidance oon this. I've been told that 2 to 4 inches would be reasonable on a masthead rig 36 ft cruiser but I can find no designer spec and have no idea what the sail was cut for.

- wind on the backstay to tension the forestay, to the point where you reach excessive mast-bend
- tension the upper and lower shrouds as necessary (downwind side, tacking each time) to preserve rig tension and help keep the mast straight.
- the more bent the mast is, the more effect the lower shrouds have in straightening it
- if your mast is then significantly within your "bend" limit, tighten the backstay some more and repeat.

Where do the intermediates fit into this. I always thought that the sequence was 1 tighten the caps, then the forward lowers put the curve into the mast, the intermediates just keep the mast in column and the aft lowers stop the mast going too far forwards and so are slacker than the forward lowers.

and I still dont understand why they put a backstay adjuster on a masthead boat like mine. its easy to see the use of one on a fractional rig, but why adjust back stay tension on a masthead once you have it right?

P.S. And how do you measure mast rake ( mine should be 46 cm) when you cant know if the boat is floating as designed?

 

[RJJ]

Could do with a bit of guidance oon this. I've been told that 2 to 4 inches would be reasonable on a masthead rig 36 ft cruiser but I can find no designer spec and have no idea what the sail was cut for.

If you can recognise the symptoms of an overly-full or a luff-starved mainsail then you'll work it out. If your mainsail never, ever backwinds even as you ease it down the track in a blow, it may be too flat. If it's impossible to stop it backwinding even in moderate winds, it may be too full. Of course, you can't judge anything in isolation (which is the joy of sailing), because incorrect jibsheet/car settings/outhaul/mainsheet can all affect the slot.

The amount of bend is measured as: the distance between the aft-edge of the mainsail track and an imaginary line running from the top of the track to the gooseneck. 2-4 inches could be right, but this depends on the cut of your mainsail - might want 1 inch, might want 5-6 inches.

Where do the intermediates fit into this. I always thought that the sequence was 1 tighten the caps, then the forward lowers put the curve into the mast, the intermediates just keep the mast in column and the aft lowers stop the mast going too far forwards and so are slacker than the forward lowers.

That sounds about right to me! Just go ahead and experiment. If the mast bends too easily (fore-and-aft) when you pull on the backstay, you need more tension on whichever combination of shrouds helps keep the mast straight (fore-and-aft). Subject to the limit of what your boat/bottlescrews/chainplates can handle.

I hadn't thought of the interesting point made above about the prop-shaft. Clearly factors like this place an operating limit on what you can do - although it seems only to be an issue if motor-sailing.

and I still dont understand why they put a backstay adjuster on a masthead boat like mine. its easy to see the use of one on a fractional rig, but why adjust back stay tension on a masthead once you have it right?

There's a little performance advantage from relaxing the backstay progressively as you bear away onto a fine reach, particularly in F2-3 when you won't be up to waterline speed - gives more fullness in the headsail, when you no longer need it flat for pointing. Essentially this makes the headsail work a little more like a spinnaker.

There's further advantage in letting the rig forward offwind. Moving the Centre of Effort forward helps make the boat stay offwind - an aft-raked rig has the tendency to make the boat round up. Take look at a photo of a Bembridge Redwing downwind and you'll see what I mean. Admittedly, with a cruising rig the extent of adjustment means you get less benefit.

Or you can just decide that you have it right for upwind (where it makes the most difference to your all-round performance), you can't be bothered to think about it again, so set-and-forget!

P.S. And how do you measure mast rake ( mine should be 46 cm) when you cant know if the boat is floating as designed?

46 feet surely? ;-) I would measure the mast rake to a fixed object such as the aft end of the cockpit - but if you need to make comparisons with other boats, you have to replicate whatever measurement they've used. If that includes replicating other external factors such as ballast, unfortunately that's what you have to do.