split backstay tensioning

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TimStewart

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Hi

Can anybody tell me the answer to two questions.

1 How do you measure and set the tension in a split backstay. I know the tension in the backstay should be 15% - 20% of the wires breaking strength. But at the position where the single backstay splits into two bridle legs, it is about 12 feet up and I can't reach to get the tension gauge on. If I placed the correct tension gauge on the smaller wires of the split backstay, would the readings I require be half of the single wire 12 feet up?

2 The rig is mast head with single swept back spreaders. The lower shrouds are tensioned to 10%, and the cap shrouds are tensioned to 15%. The forestay has got a roller reefing system. There is a slight bend in the mast athwart ships between the spreaders and the mast head. the middle of the bow is over to the port side ever so slightly ( top of the mast over to stbd slightly). With all the shrouds tensioned equally, do I need to tension the split backstays to different settings.

Can someone advise me please.

Thanks
 
The tension in the backstay is a funstion of the tension in the split stays, (obvious really). To get the backstay tension you will need to measure the tension in the split backstay, they will both be equal near enough. You will then want to apply a little AS physics and add them vectorially to get the backstay tension. QED. As for the correct way to tension google SELDEN and download their guidance booklet on rigs.
 
Firstly I suggest the bend in the middle of the top half of the mast is a bend inherent in the mast ie from an accident. It can not be straightened by rig tension. If you feel it is significant then you need to fit diamond stays ie a wire from the middle of the mast over small spreaders mid span up to the top. Repeated on the other side. The diamond is tensioned (when the mast is down is OK) with more tension on the starboard side so the pressure on the small spreader base presses the middle of the mast toward straight. The other side balances the pressure to a lesser extent.

Now regarding rig tension I have always been disparaging toward the idea of tension being set to a % of max strain of the wire. So I warn you many disagree with me however....
Most boats around here have adjustable backstays. This is more appropriate to fractional rig boats but also to a less degree appropriate for mast head rig. I take it from your description you don't have an inner forestay and this fact may change the theory. If this is the case then your cap shrouds pressing on swept back spreaders provide the force forward for the middle of the mast. If an inner forestay is fitted that force is provided by that inner forestay. Which is also often adjustable.
The idea is that in strong winds you apply more backstay tension to force a bend in the mast (fore and aft) to take some of the camber (baginess) out of the mainsail. In light winds you ease the backstay to straighten the mast and return the camber of the mainsail.

Spllit backstays can have their tension adjusted by having one leg on a 4 purchase tackle with a jamb cleat. The junction of the single backstay to the split is at a pulley with the split part being one continuous wire.
The other method is to fit a plate with 2 pulleys spaced about 40mm with the two legs of the split backstay passing on the inside of the pulleys. The plate and pulleys are pulled down by a tackle to tension the backstay by forcing the split parts together to the desired degree.

If you don't want an adjustable backstay then in any case the backstay tension should be set to give the desired results as seen by the rig performance under pressure. Backstay should be tight enough to remove as much forestay sag as possible. Certainly don't exceed about 8% max strain. ie about half of the total strain on the single backstay part. Don't bother about trigonometry here. (that is just obsessive)
Check you mast for straightness when static and when hard pressed and adjust all the side stays acordingly. I reckon all you need for static tension is enough tension to hold everything firm. There is no point in putting the hull under permanent long term stress. Some people have a highfield lever in the forestay of fractional rig so stress is released when the boat is not in use.
good luck olewill.
 
The tension on each leg of the split backstay is half of the total tension multiplied by the cosine of half the angle between the split. Example, if the total tension is to be 1000 lbs and the angle between the split is 50 degrees, the tension on each leg should be 500 x cosine 25. Does that make sense ?
Good luck
 
Thanks for the answers

I can find out the tensions in the split backstay bridle because I can reach them with the tension gauge. What would be the formula then for calculating the tension in the 'out of reach' upper bacckstay
 
Ooops, my brain was out of gear for my last post. The half total tension is DIVIDED by cosine half the angle, not multiplied. So, when you measure the tension in the split section, you can calculate the tension in the upper, single part as follows - tension in one leg of the split multiplied by the cosine of half the angle equals half the tension in the upper, single part, therefore double this number for the total tension. Sorry about the screw-up
 
Really need a bit more info on boat size, rig, and whether you have inner forestay, runners and checks.
Disagree with your basic premise that tension in backstay should be 15-20% of bs - this is used as a starting point for static rig tune, but more so for shrouds than stays. Your backstay tension has direct affect on forestay tension, so I'd expect BS unlikely to exceed 12% or so (backstay typically has greater angle to masthead than forestay unless mast unusually far aft). I'd go back to your tuning guide, and look to:
(a) caps at 15-18%
(b) lowers at 12-15%, but keep mast straight even if shroud tensions different
(c) backstay <10%
Now sail in light to moderate breeze, and check mast bend, straight in the boat, and forstay sag. Adjust caps to straighten mast, lowers to manage bend, and backstay to control forestay sag.
 
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