OK, Ill take a shot at this one as I (currently) am far more a kite guy than a sailing guy.
Newt, I believe you will find that a simple sled like in your first post will be unwieldy once it gets big enough to generate the sort of pull one might need to actually pull anything.
If you want to use a kite for a downwind sail, you should look at the NASA wing design. Designed for just about exactly what you are looking for, and as close to what you are looking for as I think you are going to find. The design might be "fun" to try and launch from a boat with other
rigging, but they could be recovered from
water if it went in, and larger ones do make a ton of
power. People have sailed behind them before.
If not that, I think something in a larger parafoil might work. BUT... size does not equil pull on the line/lift. Most of the large parafoils do not pull/lift all that much -
single line kite folk like big impact in the sky, but are generally not looking to
lift their
anchor off the ground. My biggest kite is 250+ square feet, and can still be easily hand flown from its low
wind threshold of about 5 knots up to about 15-17. I have flown it in as much as ~25, but it took two
men to pull a bight in the line. Most any celled parafoil is going to be a problem over
water though, if they go in, they fill with water and become really effective sea anchors - to the point they can be difficult to recover.
High aspect traction kites (those designed to pull, and used as sails for folks that ride kite buggys like myself) come in a variety of sizes, and are designed to pull, but work best in a close/beam reach. While they can be flown up into the wind cone (close reach) or down wind in what would be a broad reach, they are not at their best running with the wind (they generate significant
lift from foreword speed, and function more like a wing than a sail at that point. Bummer with these is they are a soft foil and are really unsuitable for a launch at sea.
The
inflatable single skin kites the kite surf guys use are designed to be water re-launchable, and make a ton of power. I cant speak to their performance personally, as all my traction kiting is land based.
Low aspect ratio traction foils, (QuadTrac's from 20 or so years ago come to mind) did better running with the wind, but make less power out on a close/beam reach.