Mirco,
I split this topic off from the SI Settings topic since it was a totally different question.
The chart speed does not affect the recording. It is only a playback or realtime feature on the unit itself.
I am not sure if HumViewer uses the boat speed in its distance calculation or not. I dont think so. If you are recording at higher speeds, the objects will be smaller than actual size since the number of pings per foot is less. And conversely, if you are going too slow, the images will be elongated and will measure longer than normal.
Hope that helps,
Robert