Backstay adjustment ideas?

[prv]

Inspired by the other thread...

Ariam currently has a system similar to the second one ("12:1 Cascaded") on this page: http://www.harken.co.uk/content.aspx?id=3896 . Same wire strop and sheave, but the tackle is only 4:1 and uses fiddle blocks.

It doesn't work very well, because as the backstay is tightened it rotates. I guess this is the twist in the wire straightening out a little. The result is that the sheave on the end of it also revolves, and it wraps the strop around itself and the tackle.

The obvious solution is a swivel between the wire and the sheave, but in practice I'm not convinced this will work. The strop and tackle are secured very close together at the bottom, so there isn't much providing an anti-twisting force, and I would expect a swivel to bind to some extent under the load.

I guess my options are a single 8:1 tackle (if I can find 4-sheave blocks, and if that doesn't just end up revolving and twisting as well) or some sort of mechanical adjuster (the wheel or folding-arms kind). But maybe there's something else I haven't thought of, or a cunning way of stopping the twist on the existing arrangement?

To be clear, this is a fractional rig and I do adjust the backstay tension while sailing, but not with great accuracy or urgency. It's also not vital to holding the mast up (the aft-swept cap shrouds do that) although there would be a risk of inverting the bend if the masthead was released while sailing. The backstay chainplate is on the centreline and there isn't really anywhere to attach the legs of a Y-shaped system.

Cheers,

Pete

 

[smeaks]

In line wheel tensioner.....

Inspired by the other thread...

Ariam currently has a system similar to the second one ("12:1 Cascaded") on this page: http://www.harken.co.uk/content.aspx?id=3896 . Same wire strop and sheave, but the tackle is only 4:1 and uses fiddle blocks.

It doesn't work very well, because as the backstay is tightened it rotates. I guess this is the twist in the wire straightening out a little. The result is that the sheave on the end of it also revolves, and it wraps the strop around itself and the tackle.

The obvious solution is a swivel between the wire and the sheave, but in practice I'm not convinced this will work. The strop and tackle are secured very close together at the bottom, so there isn't much providing an anti-twisting force, and I would expect a swivel to bind to some extent under the load.

I guess my options are a single 8:1 tackle (if I can find 4-sheave blocks, and if that doesn't just end up revolving and twisting as well) or some sort of mechanical adjuster (the wheel or folding-arms kind). But maybe there's something else I haven't thought of, or a cunning way of stopping the twist on the existing arrangement?

To be clear, this is a fractional rig and I do adjust the backstay tension while sailing, but not with great accuracy or urgency. It's also not vital to holding the mast up (the aft-swept cap shrouds do that) although there would be a risk of inverting the bend if the masthead was released while sailing. The backstay chainplate is on the centreline and there isn't really anywhere to attach the legs of a Y-shaped system.

Cheers,

Pete

 

[wklein]

In line wheel tensioner.....

There is no reason for the tackles to twist if they are working correctly, The wire will not twist to this degree just by loading and unloading. Check that all the sheaves are turning as this can cause twists, also check the wire or rope doesn't have twists already in it. None of the blocks need to swivel do if you can lock any of them this will help too.

 

[Rigger Mortice]

I've got a similar system on my boat. The tackle is fixed to one quarter, the strop is fixed to the other quarter.
I've never had a problem with any twisting.

 

[dom]

I've had a problem like this before. There can be a tendency for multiple tackle blocks to twist in the direction of of the lay of the rope. If you have a laid rope, that could be the problem. Braided ropes, however, are torque neutral but you may have accidentally introduced some twist to the rope. Have you tried re-reeving the system after shaking down your rope? Here's how Harken suggest to do it:
http://www.harken.co.uk/article.aspx?id=12855

If that doesn't work, I'd look at your backstay. Wire rope of a laid construction is not torque neutral at all loads and is therefore balanced to be roughly neutral in its working range. If this is the problem (unlikely I'd say) it will not go away no matter what system you fit!

 

[prv]

I've got a similar system on my boat. The tackle is fixed to one quarter, the strop is fixed to the other quarter.

If the tackle and strop are fitted at opposite quarters then you don't have a similar system

Pete

 

[prv]

I've had a problem like this before. There can be a tendency for multiple tackle blocks to twist in the direction of of the lay of the rope. If you have a laid rope, that could be the problem. Braided ropes, however, are torque neutral but you may have accidentally introduced some twist to the rope.

The tackle doesn't revolve, and doesn't twist. It's the sheave above it, on the end of the stay, that turns.

Pete

 

[prv]

None of the blocks need to swivel do if you can lock any of them this will help too.

Certainly the lower block in the tackle doesn't rotate, and the sheave on the end of the wire doesn't have a swivel either. The top block in the tackle might have a swivel, I can't remember, but in any case it doesn't rotate relative to the bottom block.

Pete

 

[Javelin]

Have done this a number of times on various boats.
My fav is a cascade, simple, cheap and works really well.
See simple diag below.
Grey gives you 2:1
Red = 4:1
Blue = 8:1
Green = 16:1 if you really need it.
Just four high load ss blocks like these
4mm dyneema for reduced friction on the power side of the cascade (grey, red, blue)
And something nice to pull and cleat for the final line (green)

 

[prv]

Have done this a number of times on various boats.
My fav is a cascade, simple, cheap and works really well.

Did you read the original post?

Pete

 

[Rigger Mortice]

If the tackle and strop are fitted at opposite quarters then you don't have a similar system

Pete

It's similar, but it isn't the same. On the basis of your wisecrack, any system that does not perform as yours does currently is, by definition, not similar.

However, if you want to be a smartarse when peeps try to make helpful comments, I'll let you sort it out yourself.

Out.

 

[dom]

The tackle doesn't revolve, and doesn't twist. It's the sheave above it, on the end of the stay, that turns.

Pete

I can't imagine you'd get that much twist from the wire strop, which would make me suspect that the backstay is not torque neutral at its working load. You might want to attach your tackle to the main halyard instead of the backstay and tighten it to see what happens (being careful not to overload naturally).

If your backstay is the problem it may be because it is not constructed to resist torque-induced loads Rotational resistant wire rope consists of two or more layers of strands with an equal number of strands wound left and right. Under tension the rotational force induced by one layer is counteracted by the force in the opposite direction within the other layer. I can't imagine any rigger using the wrong wire, though if it is, you'll need to check your entire rig.

If it turns out to be the correct wire, the problem could be down to how it was taken off the spool, but you'd have to ask a rigger how to proceed if the wire has been tensioned post a mishandling induced twist.

 

[prv]

I can't imagine you'd get that much twist from the wire strop, which would make me suspect that the backstay is not torque neutral at its working load.

Indeed, that's certainly what it looks like to me. I don't know all that much about wire so I assumed it was an inevitable property of stranded rope. Apparently not.

You might want to attach your tackle to the main halyard instead of the backstay and tighten it to see what happens (being careful not to overload naturally).

Thanks - never would have thought of that . Test whether the problem is the cascade / tackle, or the stay. Good thought. I'm probably going to the boat tomorrow, so I'll try that and report back.

If your backstay is the problem it may be because it is not constructed to resist torque-induced loads Rotational resistant wire rope consists of two or more layers of strands with an equal number of strands wound left and right. Under tension the rotational force induced by one layer is counteracted by the force in the opposite direction within the other layer. I can't imagine any rigger using the wrong wire, though if it is, you'll need to check your entire rig.

Since the backstay is largely a trimming control, it's quite thin. Never measured it, but I'd guess maybe 3mm. No more than 4, anyway. So I can imagine it being a single twist of strands rather than multiple layers like 1x19. It feels like a Wayfarer shroud - do you know offhand what they'd use?

Thanks for the helpful answer

Pete

 

[prv]

However, if you want to be a smartarse when peeps try to make helpful comments, I'll let you sort it out yourself.

But your system is Y-shaped, so it's obvious that it won't suffer twist in this way. So telling me that you've never had a problem with twisting isn't a "helpful comment". And it also means that, for the purpose at hand, your system really isn't all that similar.

That said, I wasn't trying to be rude, hence the smiley in my post. I'm sorry you chose to take offence at it.

Pete

 

[Javelin]

If you use 12 strand dyneema and loose the 7/19 wire or whatever it is I think you'll find it won't twist at all.
In fact there is absolutely no reason on a frac rig not to use all dyneema, and save a load up top weight at the same time.

Prv - Er yes I did. A cascade with wound wire is always likely to twist hence in my earlier post saying use dyneema on all the power cascades.

 

[lustyd]

Your boat is quite fast enough. Leave the current solution and never ever tweak it, problem solved. Additionally, this will solve the problem of other people keeping up on forum trips

Slightly off topic, do you have a plan for the CIs next week assuming you're still going? Alan and I are looking at a Saturday AM start and Sunday crossing from West Solent somewhere if you're interested in company.

 

[dom]

It feels like a Wayfarer shroud - do you know offhand what they'd use?

Pete

Almost certainly 1x19 as torque induced rotational effects are really only a problem with longer runs.

But hang on! If you are running a 1x19 strop (which ain't flexible) over a sheave you will squash it and it will almost certainly induce twist. If that's the problem just switch the strop to Dyneema and you're done!!!

 

[prv]

Almost certainly 1x19 as torque induced rotational effects are really only a problem with longer runs.

But hang on! If you are running a 1x19 strop (which ain't flexible) over a sheave you will squash it and it will almost certainly induce twist. If that's the problem just switch the strop to Dyneema and you're done!!!

I haven't closely inspected the construction of either the strop or the stay - but I'm fairly sure the strop is a flexible type as it should be. It rolls over the sheave ok.

I'll do the halyard test tomorrow and that should indicate whether it's the tensioner inducing the twist or the stay itself.

Pete

 

[prv]

Prv - Er yes I did. A cascade with wound wire is always likely to twist hence in my earlier post saying use dyneema on all the power cascades.

Ok, sorry. I took the point of your post to be "use a cascade" (with mention of dyneema being incidental). Since it's already a cascade, this seemed to rather miss the point

If the main point was that using dyneema instead of the wire strop is less likely to induce twist, then it makes more sense.

Pete

 

[prv]

Your boat is quite fast enough. Leave the current solution and never ever tweak it, problem solved.

It's not just about speed, it's also about flattening the sail as the wind pipes up so that we're not being dragged over sideways. Also (especially if we're reefed, so the backwards pull of the mainsail is lower down) it's about guarding against inversion of the mast bend, ultimately leading to dismasting.

Slightly off topic, do you have a plan for the CIs next week assuming you're still going? Alan and I are looking at a Saturday AM start and Sunday crossing from West Solent somewhere if you're interested in company.

That roughly matches our plan. I've given people an approximate departure time from Southampton of "after lunch", and we'll head down the western Solent on Saturday afternoon. Where we stop depends on the wind - if it's westerly we'll tuck in behind Hurst Castle, if it's easterly I've had a pleasant night in the past in Alum bay. Straight southerlies Alum should also be sheltered, but of course we're hoping not to have those. Northerly I don't know - maybe just inside the Keyhaven entrance, but I've actually never been in there before. We'll see where is sheltered when we get down there.

Tide through Hurst changes a little before 7am the following morning; I don't like seeing 06:xx on clocks so we'll probably set off against the first gurglings of the flood at 7 . We're planning to break the journey at Braye unless it's northeasterlies, and aiming to get in in time for dinner. I suspect Chris will probably prefer to eat on board rather than worry about times and faffing about getting ashore - and since he is now the official cook on Ariam, his word on this is law

Afraid we're not going to sail in convoy en route if it means slowing down, but very happy to meet up in port or at anchor. I have Chris and Steph on board this trip, both of whom I think you met last year on the Poole trip.

I will have the cheese-on-toast on standby

Pete