Wooden seat, mounted on runners, which allows the oarsman to slide more efficiently through the arc traced by the end of his oar.
The longer the distance from the handle of an oar to the pivot point of the rowlock the more force that can be applied. A long oar could not be pulled through a stroke unless the oarsman moved in his seat. The pivot point is placed on outriggers for the same reason. The holes in the seat allow water to drain from the convex surfaces.