Rig tension gauge

[VicS]

On my rig, the backstay splits just above headheight going to 2 anchor points on the deck. None of the guides I've seen show this arrangement. I've always assumed I tighten the two halves at 10% (aiming for a 20% stretch), is this assumption correct?
Mike
Apologies for slight thread drift.

The splay between the two lower sections of the back stay will mean that you will have slightly less tension than the equivalent of 20% in the upper section but other wise yes you are correct.

A little bit of simple maths will calculate the actual tension required in the lower sections if you really want to be precise about it but since it's all "rule of thumb" stuff anyway hardly worth the bother

 

[Skylark]

I've not seen them that cheap in the UK.

Try our very own Salty John of this parish. And you'll get 50p change from the £200.

 

[Ceirwan]

Its been a long time since I did physics. But I'm fairy certain you can't just add the two tensions to get the answer. The geometry of the split is important (the angles the legs intersect at) You'd need to work out the vectors of force.

Much simpler if you can find a way to reach the single stay.

Edit: There are new Loos gauges on eBay for £99. For the 5,6,7mm wire version.

 

[ghostlymoron]

You need a structural engineer.

 

[VicS]

Its been a long time since I did physics. But I'm fairy certain you can't just add the two tensions to get the answer. The geometry of the split is important (the angles the legs intersect at) You'd need to work out the vectors of force.

Much simpler if you can find a way to reach the single stay.
.

Or as I said in #21 a little bit of simple maths.

55 years since my last maths and physics lessons but I think I could still manage that given the lengths of the lower sections and the distance apart of the deck attachments.

 

[rob2]

With split backstays it will depend on whether you have a bottlescrew for the upper span. Assuming you do, then the lower screws are used to balance the length of the lower spans (sighting up the mast to ensure it is not pulling the masthead out of line).Of course, as the adjustment approaches working tension, you can check that the tension is equal between the two lower spans, but achieving the final tension is ideally done with the bottlescrew for the upper span. Without a screw for the upper span, you've got to keep checking tensions and alignment of the lower pair until you achieve the rig setting you want whilst staying within acceptable parameters for all three spans.

The extra complication justifies the use of a guage as you can measure any element of the rig at choice rather than having a rule taped to every single element - once the rule is removed you're pretty much back to square one! It is possible to reattach the rule if a stay's tension is known and calculate the changes from there, but if you've adjusted several others in between can you be sure that the tension has remained unchanged?

Of course, all this is in part theoretical as adjustments are often made on the fly with no measurement at all. My friends boat with twin backstays was usually set up by eye and then balanced by tapping each stay and fine tuning to the same note as its opposite number. This proved more difficult with the backstays as the lower tension resulted in a less than musical "thwang". Perhaps that explained why she sailed better on one tack than the other...

Rob.

 

[GHA]

With split backstays it will depend on whether you have a bottlescrew for the upper span. Assuming you do, then the lower screws are used to balance the length of the lower spans (sighting up the mast to ensure it is not pulling the masthead out of line).Of course, as the adjustment approaches working tension, you can check that the tension is equal between the two lower spans, but achieving the final tension is ideally done with the bottlescrew for the upper span. Without a screw for the upper span, you've got to keep checking tensions and alignment of the lower pair until you achieve the rig setting you want whilst staying within acceptable parameters for all three spans

Not strictly necessary to keep checking the tensions, if the single upper stay is pointing at a point in the middle if a line between the 2 bottom attachment points the the tension on each lower split backstay will be the same. It can't not be.

Anyway, why bother getting so hung up on exact tensions of each and every stay, once they're close isn't the most important part getting the mast the right shape?

 

[steve yates]

Excuse my ignorance, but how do recognise if your boat is fractional whatever or masthead?
Mine is a bradwell18, it has two sets of shrouds and a split back stay.

 

[VicS]

Excuse my ignorance, but how do recognise if your boat is fractional whatever or masthead?
Mine is a bradwell18, it has two sets of shrouds and a split back stay.

The forestay goes to the top if its a mast head

If it's fractional it only goes part way up. 3/4, 7/8 or whatever

Unless non standard your Bradwell 18 is a masthead rig.

It looks like the same set up as my Seawych. Masthead rig with aft swept spreaders and cap shrouds with a single set of lowers in line with the mast.

http://sailboatdata.com/viewrecord.asp?class_id=6040

Common on small boats from that era

After over 35 years I still dont know how best to set it up!

 

[ghostlymoron]

A masthead rig has the forestay and backstay both attached to the top of the mast. On a fractional rig the forestay is attached slightly lower down either 3/4 or 7/8ths usually.

 

[KellysEye]

>That's really bad advice!!

Really. Please will you and others look things up before posting you will find sites sites say slack rigging can actually fatigue relatively quickly reducing its working life significantly.

 

[VicS]

>That's really bad advice!!

Really. Please will you and others look things up before posting you will find sites sites say slack rigging can actually fatigue relatively quickly reducing its working life significantly.

Yes we know that but it should be set to the correct tension, not left slack nor over tensioned as you are suggesting.

A good guide of the correct tension ( for a conventional mast head rig) is that when sailed to windward in a F4 the leeward caps should just start to go slack at 20° of heel

 

[30boat]

>That's really bad advice!!

Really. Please will you and others look things up before posting you will find sites sites say slack rigging can actually fatigue relatively quickly reducing its working life significantly.

My rig is tuned to over 20% of the breaking strain of the wire.Who said anything about slack rigging?

 

[VicS]

My rig is tuned to over 20% of the breaking strain of the wire.Who said anything about slack rigging?

Westerly Fulmar ?

Fractional rig IIRC

 

[GHA]

>That's really bad advice!!

Really. Please will you and others look things up before posting you will find sites sites say slack rigging can actually fatigue relatively quickly reducing its working life significantly.

you don't need a gauge just tighten it as much as you can with a spanner,

That's the really bad advice!!!!!

 

[30boat]

Westerly Fulmar ?

Fractional rig IIRC

Very fractional....

 

[Ceirwan]

Or as I said in #21 a little bit of simple maths.

55 years since my last maths and physics lessons but I think I could still manage that given the lengths of the lower sections and the distance apart of the deck attachments.

You had yet to post that when I started my reply.
Evidently you have a better memory for it then me because I've forgotten how to do it.

 

[gasdave]

Thanks for all these comments, folks. I'm still not sure how common the Loos gauge is as an owner's tool which is really what I was trying to get a feel for! I'm not about following the herd necessarily but benchmarking oneself against others is part of the learning journey.

It is a bit pricey at £200 and I'm still undecided whether that's worth the expense. Though I guess in the context of owning a sailing boat some might argue that £200 was cheap compared with a lot of the other gear we can be tempted to "invest in". The ease of use is a big attraction, not just for initial set-up but also for intermittent checks/fettling.

 

[VicS]

Thanks for all these comments, folks. I'm still not sure how common the Loos gauge is as an owner's tool which is really what I was trying to get a feel for! I'm not about following the herd necessarily but benchmarking oneself against others is part of the learning journey.

It is a bit pricey at £200 and I'm still undecided whether that's worth the expense. Though I guess in the context of owning a sailing boat some might argue that £200 was cheap compared with a lot of the other gear we can be tempted to "invest in". The ease of use is a big attraction, not just for initial set-up but also for intermittent checks/fettling.

I have never seen one but then I move in circles where people mostly own old sub 26ft tubs, not big flash 37 footer's and are happy for rigging to be anywhere between slack and "feels about right".
A club I once belonged to had one I believe. (checked the website .. they do and a moisture meter!)

It's peanuts compared with the value of your boat, the cost of replacing any rigging, and your annual expenditure........... You know you want one .......... get it on order tonight! http://www.saltyjohn.co.uk/riggingtensiongauge.htm

 

[Concerto]

Like 30Boat I also have a Fulmar. To get the 20% cap shroud tension I found the smaller PT-2M Loos guide did not have sufficient deflection, after I bought one secondhand from ebay - even though it should have been within its capability. So I ended up hunting for the larger PT-3M Loos model and luckily I found one on ebay a few weeks later. So I am in an enviable position of having two Loos guages onboard, so can tension rigging from 5mm to 10mm!

Do I think it was money well spent - yes. The two cost less than a new PT-3M.
Would I set a rig up without a gauge? No.
Should Gasdave invest in a gauge? Definitely.