I've had the privilege of seeing a few of these skutjes races live. Very spectacular, they sling them around like dinghies. The spectator fleets are huge.
It’s not only in those conditions that they sail well. I once anchored in the Sneekemeer on a quiet evening and watching a barge sail across the meer with very little wind and was amazed at the speed it managed to achieve with very little wind. I remember reading an article about sail shape and it made the point that the Dutch learned a long time ago how to cut the luff of the jib to account for the inevitable sag and this maintain an efficient shape.
The video says
"Amazing Dutch barge sailing in the late 1960's, extracted from the Bert Haanstra documentary 'Stem van het water / The Voice of the Water' - too amazing not to share! "