I'm a dimwit

[alanporter]

I\'m a dimwit

at physics, maths, and all that stuff, so I have a problem. My aft stay needs retensioning to 1400 pounds strain. I have a tension guage but, unfortunately, my back stay splits into two, like an inverted Y. I don't know whether each leg should measure 1400 pounds or whether the load should be shared between them at 700 pounds each. A further complication is probably that the angle between the splits is about 60 degrees, and I wonder if this affects the final number. It would be nice to be smart. Can anyone help ? Ta.

<hr width=100% size=1>Alan Porter

 

[boatless]

Re: I\'m a dimwit

Sir Oliver's horse came ambling home to Oliver's Aunt.

Or Sin = opposite / hypotenuse

Cos = adjacent / hypotenuse

Tan = opposite / adjacent

Or Tension in your wire = 700 / cos 30

T = 700 / 0.866

T = 808.3 lbs

But, that assumes that the tension given (1,400) is for the single part of the backstay. Sounds about right though.

<hr width=100% size=1>my opinion is complete rubbish, probably.

 

[Rick]

Re: I\'m a dimwit

My aft stay needs retensioning to 1400 pounds strain. I have a tension guage but, unfortunately, my back stay splits into two, like an inverted Y. <snip> A further complication is probably that the angle between the splits is about 60 degrees, and I wonder if this affects the final number.

I'm a dimwit too, as I've forgotten, but it seems that each leg carries half, and the angle does affect the load. I think I've got it, and to allow for the off centre effort, it'll be in the order of 808 pounds each leg - COS(30) * 700. You'll need to measure the angle and verify my calcs.

Two other comments - firstly are you sure you need 1400 pounds? My understanding is that the standing rigging should be tuned with a static load of 10 - 15% of the breaking strain of the wire for the shrouds, and less for the stays - something like 8% - 12% - is your wire greater than say 3/8" 1x19?

Secondly, on my boat (masthead 30 footer) I've got the same arrangement as you, and have added a pair of blocks on the legs, shackled together, and to which I've added a 4 to 1 tackle - I can induce an extra 2 inches of mast bend by pulling the legs together - nice cheap adjustable backstay for less than $100 AUS.

<hr width=100% size=1>

 

[alanporter]

Re: I\'m a dimwit

Thanks for your replies gentlemen. My stays are 1/4 inch 1 x 19. According to Donald Street's book the breaking strain is about 8000 lbs, so my 1400 lbs is somewhere between 15 and 20%. Nigel Calder's book recommends 20% 0f breaking strain for back stays and 15% for cap shrouds.

<hr width=100% size=1>Alan Porter

 

[nordic]

Re: I\'m a dimwit

Or to keep it nautical:

Sailors Often Have Curly Auburn Hair Till Old Age!

I was useless in school at maths, but I always remebered that one!

<hr width=100% size=1>

 

[William_H]

Re: I\'m a dimwit

Hello Alan I am with rick perhaps that is an OZ thing (some doubt that Queenslanders can be considered Aus but thats another story) I see you have a ketch which probably negates all the theories about adjusting the backstay to induce mast bend etc as on a sloop.
I am hardly an expert but I just can't see how any rigging should be tensioned as a percentage of the max working load of the wire. As an extreme if I used oversized wire on my rigging I would end up banana bending the boat. (Ask Rick about bending bananas.) When we tension a cylinder head bolt we stretch it to some 20% of the max working load of the bolt in tension. This reduces the susceptibility to fatigue failure.
From 20% stretch to 40% stretch at each detonation is far less likely to fatigue than from0% to 20%. However here we are pulling against very rigid metal of the head and block. In our boat we have a hull and mast that is anything but rigid considering the lengths involved. So you may have difficulty getting the required tension. It is just not necessary because depending on the length of the stay you will get elongation of the wire and secondly we are not concerned about fatigue failure of the wire. The wire is more likely to fail from corrosion induced crystaline failure. Finally I hope we have not chosen stay size so that working loads will aproach that of the safe working load opf the wire. We need more size to cope with deterioration of the wire over time. Can anyone explain why I am wrong and the experts are right I just can't see it.
Lastly if you do get 1400 lbs on your back stay check your engine stern tube alignment. You may have heard of a J boat Australia 2 I believe it was called Americas cup challenge of some years back in USA. Through a combination of factors the boat hull parted near the keel and sunk in seconds. The banana bent beyond breaking. Don't let me scare you but take more advice on 1400lbs. Regards will

<hr width=100% size=1>

 

[William_H]

Re: I\'m a dimwit

Hello Alan I am with rick perhaps that is an OZ thing I see you have a ketch which probably negates all the theories about adjusting the backstay to induce mast bend etc as on a sloop.
I am hardly an expert but I just can't see how any rigging should be tensioned as a percentage of the max working load of the wire. As an extreme if I used oversized wire on my rigging I would end up banana bending the boat. (Ask Rick about bending bananas.) When we tension a cylinder head bolt we stretch it to some 20% of the max working load of the bolt in tension. This reduces the susceptibility to fatigue failure.
From 20% stretch to 40% stretch at each detonation is far less likely to fatigue than from0% to 20%. However here we are pulling against very rigid metal of the head and block. In our boat we have a hull and mast that is anything but rigid considering the lengths involved. So you may have difficulty getting the required tension. It is just not necessary because depending on the length of the stay you will get elongation of the wire and secondly we are not concerned about fatigue failure of the wire. The wire is more likely to fail from corrosion induced crystaline failure. Finally I hope we have not chosen stay size so that working loads will aproach that of the safe working load opf the wire. We need more size to cope with deterioration of the wire over time. Can anyone explain why I am wrong and the experts are right I just can't see it.
Lastly if you do get 1400 lbs on your back stay check your engine stern tube alignment. You may have heard of a J boat Australia 2 I believe it was called Americas cup challenge of some years back in USA. Through a combination of factors the boat hull parted near the keel and sunk in seconds. The banana bent beyond breaking. Don't let me scare you but take more advice on 1400lbs. Regards will

<hr width=100% size=1>

 

[SteveGorst]

Re: I\'m a dimwit

This is most easily solved by doing a scale drawing.
Draw a vertical line 14cm long so 1cm = 100 pounds, for the sake of my explanation label the bottom of the line A and the top of the line B.
Draw a line at 30 degrees angle from the vertical from A upwards and to the left.
Draw another line at 30 degree angle from the vertical from B and downwards to the left so that the two lines meet at a point call it C.
You should now have a triangle, called the triangle of forces in Physics.
Measure the distances from A or B to C and multiply by 100.

This will give the force/strain needed.

It's easier to do than to explain!!

Cheers
Steve


<hr width=100% size=1><A target="_blank" HREF=http://"www.nwcc.info">North Wales Cruising Club</A>

 

[William_H]

Re: I\'m a dimwit

Hello Alan and all Firstly my apolgies for two replies and also to ric for those unkind comments about Queenslanders. Discussing this queastion of stay tension as a function of working load. I think many have taken this as the need for 20% of Max working load of the wire. This figure comes from table of wire strength for size. I suspect what is really intended is 20% of working load of the wire. This is a function of the sails and boat. The only way to find that figure is by puting strain gauges on the wires when sailing in the strongest wind, sail size combination that will give for side stays the max treasonable heel. ie over 45% For side stays this would be a function of the stiffness of the boat. The figure could be aproximated by a naval architect from beam hull form weight etc.
As for your backstay tension if my suspicions are correct you would need to take your strain gauge (wire tension gauge) out in a good wind put up all sail you can manage and check the working tension when running with spinnachers up if you use them. Then try for 15% of the result. You could check one side of the backstay and tension it so one side gives 15% of your result.
Any comment will

<hr width=100% size=1>