The impeller don't drive the boats as fast as I wished. So I looked for another way to reach more speed.
An ice sled cause less viscous drag and indeed, it runs so fast that I hardly could control it. Full throttle was only possible for a few seconds. If you give throttle in curves it comes to an about-turn, though it is a perfect emergency break ;-)
Anyway, the indoor ice rink was far to small to make full speed, or I have a lot to train yet...
It is absolutely necessary to use the bumper, otherwise it can damage the transmission gear of the servo. This happened to me as the sled skid uncontrolled against the edging. I recommend to use a bigger servo, they are cheaper anyway.
I know, the winter has already gone, but if you find an ice rink and ask politely, there may be still a way ;-)
I want to give thanks to the very friendly and helpfully management of the Eissporthalle Frankfurt/M http://www.eissporthalle-ffm.de/
Here you can find the files for the impeller: https://cults3d.com/en/game/impeller-boat-rc-spiral-vase-printable
10.04.2015 Today we made another try.
This one was followed by a quadrocopter