How much pull is on the rope? The strain on the rope should be directly proportional to the Life/Drag of the foil.
For example a Lift/Drag ratio of 10:1 would be a relatively low performance Airfoil. This would suggest 85 KG of lift producing 8.5KG of drag or 18.5 lbs of pull on the rope.
Given that 'you' appeared to be using 1 hand this would seem accurate. (excluding form drag which might still be low at 3 - 8 knots).
I'd say more like 10-12kg+ with the new foil, mainly cause it is horrendously unfair on the underside and has a leading edge like a golf ball... i like to structurally test things before getting carried away with finishing
the old thin foil at around 8mm thick had next to no drag on the rope once you got up at maybe 7-8kts, the tow line was slack most of the time even behind a 10ft dinghy with no wake to speak of.
in light of petrol prices here in NZ we are now conducting tests under sail, much better with no prop wash