Welcome to the Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines - Enjoy!
Don't have any great ideas how to tether the Swallow Bruce but I do have an inexpensive radio controlled Vac-U-Boat tug with about ten foot of fishing line and a bobber that I use as a rescue boat (Jim Dandy to the rescue!). Do a couple of circles around the errant boat and you're ready for the pull back to safety.