How to create 'inner-tube being towed by boat" physics effect I have a rough terrain (plane, displacement mod) that serves as a LAKE. I have an INNER-TUBE (a torus) that I want to tow around on top of the LAKE.

My initial method (that doesn't work, or is incomplete):


Rig: Create a curve. Parent an empty ("BOAT") to the curve in a 'Follow path' relationship. Add a couple of Limit Distance constraints to the INNER-TUBE to create a range of distances the INNER-TIRE can move around the BOAT.
Physics: Create a Rigid Body World. Set LAKE as passive, INNER-TUBE as active. Add some dampening and friction to the LAKE and INNER-TUBE .
Animation: Set frames and negative-start evaluation time for BOAT path (negative start time allows INNER-TUBE to settle on the LAKE before movement). Mark start/finish keyframes for BOAT.


The result is: Not Much. Once BOAT starts moving INNER-TIRE moves with it... but INNER-TUBE is completely unreactive with the TERRAIN, and doesn't swing wide after BOAT turns corners. INNER-TUBE may as well be directly parented to BOAT : INNER-TUBE just stays in the same angle to TRUCK as at the initial orientation.

In short: physics ain't happening.

The scene's enormous so I won't provide a blend file. But if someone could provide a simple .blend that illustrates how to do this boat-tows-inner-tube effect at its simplest level, I'd be grateful :)
I'm not an expert on rigid body physics in blender, so I can provide a full example, but it sounds like you have used an object constraint to attach the tube to the boat. If you want physics you should use a rigid body constraint instead. See: https://www.blender.org/manual/physics/rigid_body/constraints/types/index.html