Ged,
We’ve always used them as sweeps rather than sculls, being concerned that using such big oars with overlaps on the handles, although feasible, risks finger-crushing misery.
One of the reasons why we were so successful in the rowing legs (and windless sailing legs) of Sail Caledonia was that we were prepared to row for hour after hour. And that was only possible because these oars were so comfortable and effective to use. During one windless Loch Ness Challenge (which we won), we rowed nearly the whole length of Loch Ness and half way back - about 30nm.