MMNETSEA, in regards to your questions... I'd think a proper way to consider this method would be to get the specs on the wire from the vendor and use that information to calculate your 5% load vs x distance. Of course, most vendors don't have a clue about the wire they're selling you and you'd probably have to go back to the maker--for example in the USA Loos publishes a lot of info about their wire rope.
However, boats are "bendy"...tension a rig, go for a sail in a stiff breeze and watch everything change. I'm not sure that anyone needs exact measurement beyond what this .pdf document advocates if all they're doing is tensioning a rig that is already installed on the boat.
When initially putting up the rig, there is a tremendous amount of construction stretch that must be taken out of the wires (especially if a tall rig) and one really does need to know the specs on the particular wire in order to figure out how much of that stretch needs to come out before fine tuning a rig. For example, in the case of our mainmast, I'm told that the construction stretch on the 3/8" 1x19 wire should be more than an inch and 7x19 about 3 inches. I'll be tracking down that "about" for an exact number shortly.
I just noticed that Calder recommends the same 5% load/stretch method in his Mechanical and Electrical Manual
. As many cruisers follow Calder as a good standard, I'd imagine many cruisers are using this exact method for rig tensioning.
On the other hand, Brion Toss's Rigger's Apprentice
gives a bit more detail which you might like. In a section on wire options he gives us a discussion of Elasticity and Elastic Limit. Here we learn that rigs are generally designed with the tightest wires tuned to at most 25% of the wire's strength. Now, the Rigger's Apprentice
takes the reader through a discussion of rig design including spars/masts and allows one to use the expected loading on the particular boat and the rig design to figure out which wire rope is right for the job. If we follow this section a bit further, we learn (on page 146) that elastic stretch is a function of local load and wire rope diameter relative to the inherent elastic properties of the metal used in making the rig. A formula is given for the approximate percentage of stretch for a given length of wire rope:
e=[P/D^2] x F, where
e= elastic stretch as percentage of length
P = load on wire rope in pounds
D= nominal wire rope diameter in inches
F= the reciprocal of (A x E x 100)
Th info that goes into F= 1/[A x E x100] is that A is the cross sectional area of the wire rope in square inches, E is the modulus of elasticity (which tells you how much a substance stretches per pound of tension applied) Carbon steel, for example has a lower E than stainless steel. In this particular book, there are a couple nice little graphics showing us inches of stretch per 50 ft of wire at 25% load for an example stay configuration.
I hope that you find this information somewhat useful and that other cruisers will as well. In several places I've advocated the book, Rigger's Apprentice
, as one that should be in a cruiser's library. Its the only place I've seen that a layman could read about rig and spar design and actually re-engineer their own rig on their own boat (unless they've got a really exotic 4 spreader set up with wires galore...).
Oh, I must make a disclaimer here....though I said I was shying away from rig tension gauges....we just bought the Loos PT3 gauge.
That's what you get when two engineers are married--the curiosity just takes over
I'll probably be verifying the 5% method with the gauge and I'll let you know how it goes.