Adjustable backstay anchor point

[Irmascherkid]

Hi All,
I have a question regarding the loading that the adjustable backstay anchor point on a bav 30 cruiser is subjected to in "normal " circumstances,  the only reason I ask this is that the last time I was leaving the boat I noticed that the shackle that the lower block is secured to is sum what out of shape and will definitely be getting replaced before next time out, but having looked through different chandlers I can't find the same type with a SWL or WLL on them? I have one that is spare but slightly smaller in size and have checked online ( apparently this should have a WLL of 800 kg ) in you honest opinion would this be sufficient for the loads that run through the backstay? The shackles that I have are 316 stainless steel.
Thanks in advance for any guidance on this.

[Salty]

  It's a good point Irmascherkid, and I'd say that 800kg would be well below the requirement for a
Bav 30. On all of the Bavarias I'm familiar with, the backstay divides to two fixing points on the transom, and it is the thickness of the wire going to those two points that should, in my opinion, be the guideline to the size of shackle you need. As a "rule of thumb" and without information on the breaking strain of the shackle, or of the wire, I would opt for a "D" shackle where the thickness of metal the shackle is constructed from is the same as or thicker than the wire it will be attached to. If you know the breaking strain of the wire, then look for a shackle with at least the same breaking strain. On eBay many of the stainless shackles sold have this information within the product details.

[Irmascherkid]

Hi salty, you are correct about the backstay splitting into 2 and being anchored on both sides of the transom, having looked into it further the spare shackles that I've got have a 8mm body and 10mm bolt and if going by ringing and lifting sites then I should have 0.75T SWL at a factor of 4-1 thus giving a total failure point of 3T, if I remember correctly the wire on the backstay should be 6mm? ( now before anyone and everyone puts their heads in their hands with dismay I will make it clear now that I do not condone the use of under sized shackles and thus being the reason that I ask the question, also as for the comment above regarding the 3T at 4-1, as I have no information regarding the manufacture of the said item then I will use the appropriate 0.75T margin and not use it on my backstay) would you say that a SWL of 3T would be ok? I'm just unsure regarding the total stress that would be on the equipment.
Thanks.

[Salty]

Short of measuring the stress on your rig with a suitable tension gauge, I don't know offhand how much stress the wire has to withstand, and hence the suggestion of using a shackle where it is constructed from metal of a thickness not less than the diameter of the backstay wires. "Loos" make a suitable tension gauge that would be ideal for measuring the tension on your rig. I have one and will take it with me when I go to my boat on Saturday in order to measure the tension on my rig. I guess the tension on your rig may be somewhat similar, though much will depend on how much you pull up the adjusting tackle. Other than that I'd say your 8mm shackles would be fine.

[Irmascherkid]

Thanks for the feedback salty, will compare the two for size but will agree that a tension gauge will give the most accurate range of load. At least then I can mark off on the tackle maximum pull for the equipment.

[Yngmar]

Your 6mm backstay has a BL of approximately 3100 kg. Typical working load for a backstay on a fractional rig is up to about 20% (numbers from the Selden "Hints and Advice" book).

So you need a shackle with a minimum of 3100 kg BL and at least 620kg SWL. Wichard makes forged shackles rated for high loads, but they're not cheap.

[Irmascherkid]

Thanks for putting me in the right direction yngmar, will be consulting the hints & tips booklet a lot more now.
Yes you are right about the price being more expensive for a wichard shackle but for £30 a shackle with a BL of 4300kg puts my mind at rest.

[Salty]

The backstay on my B36 divides to a 6mm wire bridle leading down to 7.3mm "U" bolts through the transom. Don't know why 7.3 mm was used unless that is the metric measurement of an imperial fitting. The wire is connected to the "U" bolts with 6mm shackles, and the same size shackles are used on port side backstay block and tackle. Note, these fittings were on the boat when I bought it in 2010, and have not been changed.
Measuring the tension on the 6mm bridle wire with my Loos tension gauge gave a reading on each leg at point number 8 on the gauge. Scale 8 is well below the minimum shown for a 6mm wire where the scale for that size starts at point 18. Point 18 if reached shows a tension equivalent to 3% of the breaking strain for the wire, or around 100kg, so point 8 is well below 100 kg. because the wire forms a bight, the tension on the single wire above the bight and leading to the mast head will be more or less double that reading, but it is also a thicker wire, and is well out of reach for anyone at deck level to be able to measure the tension on it.
So maybe I don't have sufficient tension on my back stay, but I'm thinking your 8mm shackles would be well able to take care of the job.

[Irmascherkid]

Thanks for the info salty, I will be heading down to the boat on Tuesday so will measure everything up and make sure, You mentioned that your backstay is attached with "u bolts" I'm pretty sure that mine is attached to stainless steel brackets or that could just be my imagination playing tricks with me but I will definitely know on Tuesday.

[Symphony]

The backstay does very little, if anything to support the rig. It is only there to enable you to bend the mast. The same backstay is fitted to boats with in mast where the mast cannot be bent, so the tension is never changed. If you look inside your transom you will see that there is very little reinforcement around the anchor point, be it either a U bolt or the stainless tang and no attempt to spread any load to the hull rather than just the transom.

[Kibo]

I agree it is there for tensioning (bending) the mast but I think it also does a good job of helping to hold up my rig when off the wind, especially flying a spinnaker going down wind..... If you have ever lost a forestay in a sudden gear failure (I have) a good way of saving the mast is to fall off the wind really quickly so the backstay is holding the mast up and gives you time to sort out a temporary forestay using a spare halyard (we did).

[Salty]

If tension on the backstay causes the mast to bend, obviously aft, then when there is no tension on the backstay or very little, then at those times there will be similarly less tension on the forestay. This then allows the upper part of the mast to move back and forth in strong winds resulting in a pumping action at the foot of the mast. At the foot of the mast there is a cast aluminium plate which takes the weight and the thrust from the mast. If the backstay is not tight enough, then the pumping action can damage the cast aluminium plate to the point where it begins to crack and break up. This is particularly noticeable when your boat is moored and with no sails up. So if you notice a heavy vibration occurring while moored, it could well be that your backstay needs more tension. And if that  cast aluminium plate is already cracked . . . . . . . . . . !!  When wuz the last time you checked it?

[Kibo]

Well Salty, completely agree.......and.....I check my rig a lot since we liveaboard about 4 months a year.... and I make sure my backstay is appropriately tensioned for the conditions. I have an in mast furling system but it does not stop me from adjusting my backstay according to sailing conditions. If I want to furl my main.... I let off the backstay to flatten the mast.