On a Fractional rig how much tension should be on the lower shrouds.
When I'm hard on the wind in about 15kts the leeward lower shroud is very loose (movement visible)
Is it due the cap shrouds not being tight enough allowing the mast to move to leeward or is it just the lower shrouds not being tight enough.
Same issue occurs on both tacks
Well reading your post again perhaps not "very loose" would be the best adjustment as long as it's even slack on both tacks.
If the hull is made of snot & bandages, I mean fibreglass, it could be the hull flexing that is causing the lower shrouds to go slack when the rig is loaded. Tightening these shrouds to remove the slackness when under load, may result in the rig being under load 24/7/365 and not just when the wind blows a bit harder. Could be wrong again. Yeah I know, steel hulls also flex.
Your caps should not be slack to leeward. They should be pretensioned to approx 20% of SWL of the rigging wire. Ie the rig should be under load 24/7/365. The rig pretension is significantly greater than the loading that the windward shroud experiences and similarly much greater than the unloading of the leeward shroud.
A few on this forum could do with some professional advice IMHO.
I had the rigging replaced 12 months ago but I've only notice this problem recently. Can the wire stretch?
I think I'll get the rigger back to check tension on all stays.
Once the mast is strait on each windward tack , what ever slackness in the lower leeward shroud should be normal.
No. Swaged rigging will not stretch. It is possible that the old thimble and eye rigging can elongate if the eye deforms under sailing shock loads. It is possible that on old hull cannot maintain rig tension and is permanently deformed resulting in loss of rig pretension. The strain (delta"l"/"l") for stainless steel rigging is approx 1.5mm per metre to achieve correct rig tension. However the difference in thermal expansion between aluminium c/f stainless steel is such that rig tension is significantly effected by ambient temperature and rig tension should be checked (for racing performance) with a Loos gauge for optimum results.
After replacing standing rigging it is wise to sail the boat hard with plenty of sail and heel. This will pull any irregularity out of the wire that may be there simply from being coiled in a roll, then dock or anchor up and go through the re/rigging procedure again, and you will most likely tension up. Loos gauges are wonderful for giving you a reference for how tight you have got your rig and will indicate any adjustments you make. If you are inclined to tinker with your rig or you tend to worry about tensions they are well worth the money.
Southace,
With all due respect frant is very correct. The wire doesn't tend to stretch over time because it is never or shouldn't be tension beyond its elasticity. Normal rigging parameters/ practices prevent this. If you continually tension a wire beyond a elasticity point, and any give is purely in the wire, you will eventually stretch and snap it. When wire is placed under load when sailing it will stretch, if you don't exceed an elasticity point the wire will return to a preset tension/length when the load comes off. This is why slack shrouds can often reduce rigging life. The shock loading from suddenly going slack, tight, slack, tight often exceeds the elasticity point of the wire and rapidly causes work hardening/fatigue, stretching it beyond no return slightly every cycle.
Some rigs have different sized wire on different shrouds, This is so that stretch can be balanced to keep a rig uniform through varying pressure loads.
Like a guitar string when you change them, you work them until they stay in tune , then a good guitar will look after them. A poor guitar and you are forever tuning, the strings always break and you wear out the machine heads.
both sort of right.....
www.spaceagecontrol.com/calcstre.htm
What Is Cable Stretch?
Two kinds of stretch occur in cable based on wire rope: constructional stretch and elastic stretch. This stretch is due to two different causes.
1. Constructional Stretch - When cable is made, the load at the closing head is light. Therefore, there are small clearances between the wires and strands, and between the strand and the core. The application of initial load causes wires and strands to seat properly and a slight overall elongation of the strand or cable accompanies this section. The amount of constructional stretch is not constant for all cables - it depends on such variables as type of construction, length of lay, and other factors, including the load applied.
2. Elastic Stretch - Elastic stretch is the actual elongation of the wires of a strand or a cable. This is caused by the application of a load up to the yield point of the metal. The stretch is approximately proportional to the load applied. When the load is released, cable subjected to elastic stretch returns to its approximate original length, providing the stretch has not reached the yield point of the metal.When the elimination of as much stretch as possible is important, the cables or assemblies can be proof loaded to remove most of the constructional stretch. For assemblies, this process also verifies the holding power of the terminals. Proof loading is usually done by applying a 60% load to the cable or assemblies. This load is based on the minimum breaking strength of the cable or fittings, whichever is lower. Handling the cable as little as possible after prestretching helps eliminate putting constructional stretch back in.
Good info nods,
The constructional stretch is what I was referring to in a previous post
Just a follow up to my original post.
Had the riggers who replaced the rig 12 months ago return to re-tension the it.
It has fixed the problem, boat sails better with less weather helm and .5kt faster (in 12-15kts wind)
No slack in the lower shroud now.
So the message is make sure your rig is tensioned correctly, it makes a considerable difference.
I think I must have missed something, what was the recommended tension for the lower shrouds?
Is it just tension them until the mast is straight after the caps are tensioned or is there a recommended tension setting?
I have noticed on my Delphia that there is some slack in the leeward side lowers.
Lowers stop the middle of the mast bending fwd or sideways which can result in a slack forestay. It also depends on the angle of the lowers, eg. on my Etchell they are in-line so only stop the mast going sideways.