Backstay Tensioner - overkill?

[tome]

I have a very nice Navtec backstay tensioner which I haven't yet fitted. My rigging is 10mm which suggests that I should have the -22 model which is indeed the model I have, see short handbook.

Problem is that I've wound back the max pressure as far as I can and it still allows it to be pumped up to 3000 psi which equates to a tension of 8400lbs. The breaking tension on my rigging is 19,425 lbs. So almost 45% rig tension is possible given a careless crew or a gadget fiddler.

My main worry is on the tension on the forestay if someone should do this. Since the backstay makes a wider angle with the mast, the forestay load could be 1/3 more or 60% if this happens.

Is this an accident waiting to happen, and if so how could I prevent it? The safest model appears to be the -12 which I could limit to a safe load, but this specifies a max wire size of 8mm in the handbook.

Will talk more to Navtec but any thoughts welcome.

 

[gianenrico]

if the boat is small enough, it could probably curve, raisng bow and stern and assuming a more bananish look.
Cheers,
Gianenrico

 

[iangrant]

Firstly, don't nick me rigger till 'e's finished wiv my boat, secondly you won't catch me with that much weed dragging along whatever you do to your rig and thirdly you've yet to confess you got it on the cheap.
Accident waiting to happen - ha - I'm not surprised your worried 'bout getting the sticky up thingy broke with your hairy arsed Emsworth crew, blimey --- except for John, and i've a plan to have 'im jump ship to Asterie anyway...

Thoughts ? Keep the new tensioner an fit it to the 44 footer, you know it makes sense.

Ian

Ian

 

[woody001]

Or you could fit running back stays like proper yachts!

The backstay tensioner should be 30-40% of the stays breaking load at MAXIMUM tension, most boats should be designed like this i think. This will ensure it will be ok when it get rough (but you should release the back stay to 2/3 of breaking load if it does) and wil allow for shock loads.

It also depends if your rig is mast head or fractional if fractional the maximum backstay tension should be limited by the maximum mast bend. I think ya baby was mast head?
I believe you will also have to check the mast rake after you have worked out the maximum tension as the rake should be 1-3%. (time to get a backet of water and a heavy object on the main halyard.) the % is the distance from the halyard with the heavy object in a backet of water and to the mast at boom level.
If mast head rig you want 0.5% to 1%. If fractional you want the 1% to 3%. It should be the % of maximum mainsail luff length.

thats my homework done for 2nite!

woody

 

[iangrant]

A running backstay could never manage the purchase required on a rig to tension the forestay..you do need a proper tensioner - it's just that the one Tome's fitting is the one he nicked off that big boat he visited called Mirebella or something?

Ian

 

[woody001]

I thought you could achieve higher mast bend with runners on a fractional rig than you can on a mast head rig with a backstay tensioner?
Therefore Runners can and do tension the forestay i thought?

or should i shutup and make some tea?

 

[tome]

Your rigger tells me he'll never be done cos you're an onwards meticulous sort. He was at the top of your mast when I called, admiring the view between split pins.

You're welcome to try to poach my cook & steward, but you'll have to get through his interview which is tough and thorough. Expect an inclining experiment at the very least.

So it was a bargain. Just didn't think Mirabella V needed 2 of them so did them a vanishing favour.

 

[tome]

Cheers Woody, that's about the same as my calcs. Masthead rig so don't have runners. Rake's about right (helm) but my worry is overkill- not yet ready to do the French canals. Thanks for your input.

Salut
Tom

 

[ashanta]

Tome,
Please accept my apologies if Iv'e got this wrong!

I am assuming by what you describe it's a modern fractional rig you have and the tension required is exceptional. the mast will stay erect without the backstay but would collapse without the forestay, unlike a mast head rig that can stay erect without either fore or back stays.
the modern fractional rig needs tremendous tensionon the fore and side stay's, the tension placed on the back stay depends on the point your point of sail. e.g. running = backstay slack, beating backstay high tension creating the mast above the forestay to bend even more to give a better sail shape. Whatever point you are sailing between the run and beating can be guaged as a clock

 

[tome]

No, it's a single-spreader masthead rig with forestay, babystay, inline capshrouds, aft swept lowers and backstay. Mast is keel stepped. I also have a removable inner forestay without runners (it goes almost to the top of the mast).

I'm happy about when I need tension and when to relax it, but concerned about the power of the tensioner and whether it could result in loss of the forestay.

 

[brianhumber]

Until a couple of years ago I would have advised you not to worry too much. However my forestay mast fitting ( fractional mast ) weld failed off the Normandy beaches in a gentle F4/5. I saved the mast thereby giving loads of grief with the arsehole surveyor appointed by the insurers who claimed it failed due to old age. It was only having a smattering of engineering that enabled me to prove the weld on the builtup mast fitting was 'chickenshit' with no penitration, hence this was a latent defect waiting to happen. Client 1, Surveyor 0
However this incident has dented my confidence in mast fittings somewhat and would suggest you get your forestay mast attachment checked over before you start winding up the backstay pressure.

Brian

 

[ex-Gladys]

Back in nineteen hundred and frozen to death, I went into Millbay docks at Plymouth to see the Whitbread round the world fleet on arrival. Whilst looking at GBII I overheard crew talking to family on the dock pointing out wrinkling in the side decks where the crew got over enthusiastic with the hydraulic backkstay adjuster....

 

[ashanta]

I have a mast head rig and I recently bought a new gennie from Chris Scanes at Topsham. He came aboard to measure (While on the hard) and we discussed the rig. I have a slight prebend built into my mast but he was keen to see how much forward bend I was getting from the babystay which he found acceptable. I have a large turn buckle adjuster on the backstay and I have to take many many turns to get good tension on the forestay and flattening of the main. I was told that we underestimate the amount of tension required and therefore we often sail with a poorly tuned rigging??
To me I just seem to keep onlearning and trying things out.

Regards.

Peter.

 

[uforea]

Tom, as your boat is masthead rigged the tension on the forestay will be the same as the back stay as the fulcrum is in the same place. On a fractional the tension on the forestay will be greater because of the position of the lower fulcrum point. I won't have thought that this would be much greater on a 7/8ths rig but not sure of the maths.
Ted.

 

[William_H]

High tome. The concept of loading a stay to a fraction of the max working strain horrifies me. If some keen soul hass fitted or specified oversized cable for the rigging then as already stated you can overstress the mast or hull. The backstay tension should be enough and only enough to get the effect you need. Yes you could easily destroy the hull or mast. I appreciate your problem of needing a suitable limit for the tension if it is a hydraulic system with remote pump you might investigate a presure relief valve adjustable by yourself only (hidden way)
Meanwhile you might, if going to the extreme of a powerfull backstay tensioner consider this or similar on the baby forestay as beiong more usefull. regards olewill

 

[tome]

The tension in the forestay will be considerably more. The backstay angle is around 24 degs whilst the forestay is about 17 so forestay tension will be approx 1.4x backstay (sin 24 / sin 17).

 

[tome]

The rigging size is as specified by Holman & Pye so not worried about overstress on mast or hull. However, you make a good point about limiting the tension. I think the easiest way of doing this may be to fit a collar to the ram to limit travel and prevent it from putting too much tension on the rig.

Also will consider your suggestion for a tensioner on the inner stay.

 

[DJE]

In pre-stressed concrete construction we regularly pre-tension steel strand to 70% of its breaking load. This is brand new quality assured strand with a dirty great lump of concrete to push against. Still nobody stands behind the jack and they do, very occasionally, go BANG.
60% of breaking load for stainless wire of uncertain vintage is asking for trouble IMHO.

 

[tome]

Agree this is asking for trouble. By my calculations, the backstay is 15.6m so at normal tension can expect around 22mm elongation. I have a rigging tension meter so will put this on the forestay and wind up the hydraulics to max tension (40%) and measure the ram position. I can then limit the travel to this mark by making up a spacer to fit the ram. Can't think of a better way.