On Monday, June 2, 2014 4:50:10 PM UTC+12, Andrew wrote:
My experience seems logical when one considers that after
liftoff, the tension on the rope should be close to the drag on the
glider, i.e. about weight divided by L/D, i.e. about 35lbs or less.

No, that's not the case unless you're not climbing.

With a tug flying at 65 knots and climbing at 6 knots (typical for our glass two seaters) somewhere around 9% of the weight of the glider (up to 600 kg or 1300 lb) is being borne by the rope. That's about 120 lbs in addition to the 35 lbs from drag.

With a 300 kg all up single seater (PW5, Libelle etc) flying a bit slower and climbing at over 1000 fpm there is actually even more strain on the rope.

I do agree that if it doesn't break on initial acceleration then it probably won't.

I'm not going to go into the turn back or not question again other than to say if you can land safely more or less straight ahead then of course do so, but you should also be competent to turn back if that's best.