Tensioning a fixed backstay on a crusier

[Salty John]

Many thanks for all of the replies, much appreciated.

The mast has two sets of swept back spreaders. The backstay is not designed to be adjustable while sailing. I'm not sure how I could easily check forestay tension as the furl mechanism hides the shroud.

To keep things simple, I've assumed the length of the two lower wires to be 10m and also 10m for the single section. Assumed distance between chainplates is 4m.

That suggests that putting a 10% load on the lowers will put pretty much all of that, almost 20% on the upper, single wire. That sounds like too much for a cruising boat.

I'm happy to read from Tranona that the backstay doesn't contribute much to mast stability.

Seems like I should try to put a good bit less than 10% stretch on the lower wires, depending upon how accurate / repeatable the Loos gauge is to use.

I'm particularly impressed how you guys can remember school boy trig.

All this trig depends on the wire being of the same diameter and breaking strength throughout. It probably isn't.

 

[ffiill]

Questions about tensioning wires always intrigue me.
My Dad as part of his RAF training WW2 was taught how to rig biplanes and he was told that the struts take the load wheras the wires keep the system in balance.
In other words the rigging should be tight but not pre stressed such that when the mast tries to move in any particular direction the wires then become under tension and opposing wires become slack.
I have never quite understood the point of pretensioning the back stay as when the mast is loaded an undue amount of energy is being transmitted down the mast which has then to be dealt with by the keel.
If the keel is weak then the mast at best breaks it and at worst goes through the bottom of the boat.

 

[GHA]

Would a dimensional analysis help, or would that muddy the water further?

Nope, caught up, /L converts from a length to a factor which you can use to convert the load down L into the vertical component, I should know better

 

[VicS]

All this trig depends on the wire being of the same diameter and breaking strength throughout. It probably isn't.

The diameter of the wire, or its breaking strain, does not come into the trigonometry.
The trig , (or Pythagoras used by GHA and bignick) relates the tension in the two lower sections to the tension in the single upper section.

If the OP knows what tension he requires in the upper section either of the two methods will allow him to calculate the tension needed in the lower sections to give that required tension.

 

[bignick]

The diameter of the wire, or its breaking strain, does not come into the trigonometry.
The trig , (or Pythagoras used by GHA and bignick) relates the tension in the two lower sections to the tension in the single upper section.

If the OP knows what tension he requires in the upper section either of the two methods will allow him to calculate the tension needed in the lower sections to give that required tension.

Exactly. Just be warned though, don't start tensioning your backstay to a fixed percentage of its breaking strain though as it could be way too much. Selden used to have a good starter guide to doing a "dock tune"? I'll see if I can find it for you.

// edit
http://www.riggingandsails.com/pdf/selden-tuning.pdf

Even with a permanent fixed wire backstay it's possible to add adjustment. It's easier to visualise than describe:-
http://www.harken.co.uk/content.aspx?id=3896
See the 8:1 split backstay system halfway down.

 

[VicS]

Exactly. Just be warned though, don't start tensioning your backstay to a fixed percentage of its breaking strain though as it could be way too much. Selden used to have a good starter guide to doing a "dock tune"? I'll see if I can find it for you.

Should be in their "Hints and Advice" pdf http://www.seldenmast.com/_download.cfm?id=6740&download=2956550&filename=595-540-E.pdf

 

[alan_d]

The single section is too high to reach so I will not be able to measure it.

I'm glad mine wasn't too high to reach, although I hadn't realised that the alternative to standing on the pushpit was trigonometry (which might have been safer).

 

[Salty John]

All the trig and Pythagoras in the world will not answer the OP's question:

"Given the Y form, if I tension the two lower sections to a nominal 10%, what will that give to the single section? The single section is too high to reach so I will not be able to measure it".

Unless the wire diameter/breaking strain is the same in all parts of the backstay. If the split legs are of smaller diameter than the main part of the stay, which is often the case, then the OP will need to know the breaking strain of the two wire sizes in order calculate the appropriate setting.

Personally, I'd get a ladder or other altitude enabling device and set the tension in the main part of the backstay to about 15% of its breaking strain.

 

[VicS]

All the trig and Pythagoras in the world will not answer the OP's question:

Of course it will, and much more safely than climbing a precariously balanced ladder on the back of a boat! Some people wont/cant climb ladders at all.

Suppose the angle between the lower wires is 40° and he tensions them to 5kN

The tension in the upper section will be = 2 x 5 x cos20 = 9.4 kN

All he has to do is look up the breaking strain for the wire diameters he has and use the actual figures for the angle and the tension applied to the lower sections.

 

[cliff]

I just tension mine until the bog door jams then slacken off until the bog door opens again - job done

 

[bignick]

The mast has two sets of swept back spreaders. The backstay is not designed to be adjustable while sailing.

The amount of backstay tension required will be dependent upon the spreaders' sweep angle. If its substantial (like my boat) then the shroud tension will generate most of the forestay tension. If its mild then the backstay is used to generate forestay tension.

I still much prefer an adjustable backstay - it allows you to control forestay sag as the wind increases without leaving the rig permanently wound up to the max.