Backstay tensioner

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gianenrico

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My question to all and any of the forumites with sound experience to share:
I'm planning to rig a backstay tensioner on the double backstay of my 40footer; actually they are backstays, as the 2 run separately from masthead. I'll put an horizontal 4:1 purchase some 4 or 5 meters from deck and lead down the control line to the base of one of the backstays.
PROBLEM: /forums/images/graemlins/confused.gifwhich would be, in your opinion, the best way to fix the 2 fiddle blocks to the s/steel backstays? I was thinking to use Jubilee clips below and above the fiddle blocks but may be there are more elegant ways.
Thank for any advice,
Cheers
 
Because I feel they will tend to slide toward the top of the stay, unless there is a very strong tension downward; your suggestion is perfectly vible if I could put the control line amidship in the stern: but this would clutter too much and, besides, there not a strong point avalable. The boat is a cruiser, so comfort has to be taken into account together with performance.
 
I have this arrangement and the blocks are fixed to the backstays with nylon whipping. It was hemp whipping when I bought her and the previous owner told me it was there for as long as he had the boat (20 years). It seems to work very well and is unobtrusive.
 
3 blocks? One around each leg of the backstay and a 3rd centrally positioned to accept a line/wire from one backstay chainplate, through the 3rd block and down to where the 4:1 tackle awaits, secured to the other backstay chaiplate. If my maths is right, that will give you an 8:1 downhaul. Alternatively, Barton make a fitting to achieve much the same.
 
Goldie, the mast is 15 meters high and the stern is 3,5 meters wide, so at 5 meters from deck he stays are still 1,5 meters apart (thus barton hardware not practical.
Having the line running from one chainplate to the block and then back to the other chainplate will make life at the stern a little more complicated, but I greatly appreciate your suggestions.
Cheers
 
The problem you've got is that if you don't attach the tensioning arrangement "symmetrically" on the stern (either to the centre or to the bottom of both backstays) then whatever arrangement you come up with will not be able to apply equal tension to both stays
 
If the fiddles are blocked in a given height along the stays, when I pull a line running out of the system, I don't see why different forces would be applied to the individual backstays. Or am I missing something? /forums/images/graemlins/confused.gif
 
Do you know what type of stainless wire your backstays are? You will know that wire basically comes in two different types. One which is designed to flex and pass through and around blocks and one that isn't so designed. If you have two fixed backstays both coming from the top of the mast down to their own chainplate ( one on each side ) then it is quite likely that they are of the non flexible type.

In such circumstances I think you should proceed with caution because where the wire is bent around the two tightening blocks, you may cause a permament kink which will be a weak spot.

Hope this makes sense.
 
[ QUOTE ]
The problem you've got is that if you don't attach the tensioning arrangement "symmetrically" on the stern (either to the centre or to the bottom of both backstays) then whatever arrangement you come up with will not be able to apply equal tension to both stays

[/ QUOTE ]This isn't so. The tension is horizontal between the stays - it cannot be greater on one side than it is on the other.

I think Pinnacle may have an incorrect image of the arrangement - the stays remain masthead to chainplate. The tackle just pulls them a little closer together. I've done the same with a Spanish windlass.
 
The geometry is quite complex but the fact that you've got the end leads down to only one of the attachment points suggests that the tension in the backstays will not be balanced. (The key factor is the tension in the control line - that will be significant and will cause imbalance somewhere in the system)

On reflection I think you would be well advised to convert it to a conventional split-backstay - I can't imagine that that will be much more cost / effort than what you suggest and would certainly be more satisfactory
 
[ QUOTE ]
This isn't so. The tension is horizontal between the stays - it cannot be greater on one side than it is on the other.

I think Pinnacle may have an incorrect image of the arrangement - the stays remain masthead to chainplate. The tackle just pulls them a little closer together. I've done the same with a Spanish windlass.

[/ QUOTE ]
The factor you are missing is that the control line for the purchase is itself led down to the point of attachment of one of the backstays - that is very different to the Spanish windlass which I agree does lead to a balanced.

In fact it equates to there being a signficant downwards force on the point at which the block/purchase is attached to the backstay on the side to which the control line is run.

The odd corollary is that if there is significant load on that line then the purchase would not work anyway because you would not be able to apply sufficient tension.
 
Tension in backstays is never balanced. It is normal for the windward stay to be under high tension and the leeward to sag. Pulling the stays together by the proposed mechanism will try to lengthen the run of the stays by pulling them off the straight (shortest) line - which will pull the mast back and increase the tension in both stays. The horizontal pull between the stay must be the same on both stays – you can’t beat the physics – but it won’t act equally however or wherever you change the direction of the control line.. The stay with less tension will move less but that doesn’t matter. The mechanism works just fine – I have used it.
 
My concern is not wih the horizontal forces but with the vertical. However on reflection the maximum tension in the control line is going to be only about 50kg - provided that you don't have any mechanical advantage at the point of attachment - so that is insignificant.

The other thing that concerns me is the degree of movement required - how much do you reckon you have to bring the two backstays together to increase the tension in the backstay by say one tonne?
 
Ah but it is the horizontal forces which are doing the work by moving the stays from the direct run.

I'm afraid that I couldn't begin to say how much movement was required to increase tension by any given value - this would depend on so many variables (rigidity of mast, rigging type, gauge and elasticity, hull bend etc). However is isn't really a question of the measure of tension it is movement of mast which is required. That can be guestimated from Pythagoras. It wont be completely accurate but will give an idea. Alternatively – just try it!
 
I did some basic pythag, which is why I am a little sceptical as to whether this will work. On my masthead set up I tend to use about 10cm of my backstay adjustment - According to my calculation the mechanism you describe would need to bring each backstay 140cms closer to the centre line - i.e. to bring them 280 cms closer - to achieve a similar affect. That does not sound feasible.

Fractional rigs use the adjustment in different ways but I would have thought would require an even greater adjustment range.
 
As I said - try it. I have just replaced all my standing rigging (after 40 years it seemed time) and I stood at the stern with the rigger trying the effect of this mechanism. We both felt there was ample movement in the masthead without anything excessive in the backstay. So back went the blocks, secured now with nylon whipping rather than hemp.
 
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