32.881678,-111.854982,Francisco Grande, Casa Grande, Arizona,
USA

Mark writes:

My calculations and test suggest the 200 lb. is right for me. I
weigh 185 pounds, fly a T2 154 (75 lbs), and my harness fully loaded is 35
pounds. Total all up flying weight is 185 + 75+ 35 = 295 lbs. Times 1.4 = 413. I
use Pro tow barrel release so divide by 2 = 206.5. I feel the White and purple
is a good fit for me.

His tug's weaklink is 330 pounds (see chart above) also on half
the load, so it is 50% stronger than his weaklink.

He also writes:

We tried to simulate how the rope is pulled while on aero-tow.

My experience in weak links breaking during lock out is that it does not happen
very fast. The line is pulling and pulling and pulling and then it snaps. It
takes about 2 or 3 seconds. I have induced a lockout purposely and not on
purpose several times with the tandem glider and students above me in the
harness. It may seem like it's very fast but if you analyze the whole thing from
start to finish it takes a few seconds.

So with the come-a-long I would put the tension on the loop over about a 2 to 4
seconds before it would break. The weak link material really stretches a lot
before it breaks. If you stop the test before it breaks, you will see your loop
has now stretched and will be longer than it was originally. If you then do the
test again it will break much sooner than the rated breaking strengths. What
this would suggest to me is, if you have a close to lock out or old link that
has been stretched many times, replace it with a new one.

I really like the links being sold at towmeup.com. They test every spool at
different areas to assure consistent product is being used. The
http://www.towmeup.com/weaklink.html website has good information. I
recommend everyone go read it.

Regarding knots he writes:

The knots do not matter so much for just one tow to how they break
or the breaking strengths. A double fisherman's knot or a Tandem multi-wrap knot
will not give better strength but wears less on the weak link and therefore last
longer.

Double knots put less wear on the weak link material. Doing 20 to 50 tandems in
a week, you see the weak link material starting to fray. More were it makes
contact with the carabineer than were its tied, but never the less a wider
contact point were its tied gives you a little better durability of the knot.

From: 
http://www.towmeup.com/weaklink.html

A popular misconception uses a very common 135 pound test green spot trolling
line. In fact the myths associated with this product are so strong we no longer
carry it. If you wish to obtain some, it can be easily found at any decent
fishing store as Dacron Trolling Line made by the Green Spot company. Here's how
the myth goes - Look at the knot tied link shown in the picture above.

Many, many pilots believe that if you take a 135 pound weaklink line and tie it
in a loop that it will break at 270 pounds. The logic is that there are 2
parallel load paths sharing the load, each one capable of withstanding 135
pounds. They miss the fact that any properly tied weaklink will ALWAYS fail at
the knot. The line is pulled tighter and tighter at the knot and should break
right at the knot every time.

Depending on the knot used the line will break at 40-60% of its rated strength.
Typically a weaklink tied with a double grapevine will break at slightly over
the rated strength of the line. The only way to tell for sure is to break test
it with a calibrated tester. There have been many inadvertent weak link breaks
caused simply because pilots ASSUMED that the link was stronger than it was.
This has led to the asinine belief that if one loop is good 2, 3, or 4 is even
better.

In use the weaklink material will stretch well over 60% in length before it
fails at the knot. Adding a multitude of loops makes it impossible to spread the
load evenly, and the link will likely bind up causing it to fail in an often
unpredictable and unrepeatable manner. To ensure consistent breaking strengths,
stacking equal length weak links works, using a single appropriate strength link
is the best, and multiple wraps should never be used.