First of all, editing my own post: fixing the propulsion*
And then: thanks for your reply. To answer your questions:
1. I'm using two speed controllers, "Graupner Profi 40 7,2-12V BEC". Each one drives one engine, and now draws current from one battery each. I have also cut the positive wire from one of the controllers to the receiver, to eliminate the risk of the two BECs interfering with each other.
2. The engines are two Graupner Speed 700 Turbo C 9,6V. They are mounted as shown in some pictures at the first page of this thread. As shown, the caps of the motors are in connection with each other, electrically speaking, but should that be a problem?
3. The propellers are two Raboesch "C"-type propellers, 45 mm in diameter.
On each positive wire leading from the speed controllers to the engines, there are one 25 A fuse. Because these fuses constantly burst, I have set the "servo travel" function on my transmitter to 70 % for both channels driving the speed controllers/engines. This worked fine a few weeks ago, when I ran the boat for two hours. The instability puzzles me.
Now that it seems I'll have to change a speed controller again, I might as well buy some completely new ones of another kind or brand, but I don't know if that will help (ref. the fact that I've already changed from one kind to another). I have also bought a brand new receiver, and installed it, in case there was something wrong with it - but to no avail.