I also have a couple kites that I use. One is 15 feet by 3 feet, with two strings. With this the boat moves fast, and can tack back and forth like a sailboat to go upwind. This kite needs to be launched from land.
The smaller kite is 4 feet by 4 feet, but I have added a tail that is about 3 feet wide and 15 feet long. This kite is easily hand launched, so works as a backup kite. It pulls the boat, but only downwind or about 30 degrees off of downwind. So for this backup to work I have to stay generally upwind of home, which is what I do.
The electric motor is only enough to go about 2 or 3 MPH when out in the waves and wind (about 4 downwind). But I have used it to go to a small island upwind about 7 miles (Anguilla on top, St Marten below, small island in lower right) (with 2 batteries) and then use a kite to get back. But the motor is also good as a backup, and for navagating back into the harbor after taking down the kite when coming back.
I made a harness for the small single string kite that actually let me control the direction the boat was going. It was just a rope from each side of the boat going up about 10 feet and then tied together and to the kite string. By pulling in on one side or the other I could turn the boat.
I have also attached a small aluminum plate to the bottom of the motor. I can tilt the motor so the prop is out of the water and the plate is still in the water, and use that as my rudder.
I am getting another aluminum plate to make a keel down the middle of the boat. Initial experiments with the kites indicate that something in the way of a daggerboard or keel is needed. Also, with the big kite the existing rudder might not be enough.
If you like this sort of thing, check out cobratkite.com, and kiteski, and upski.