oars over a motor?
I used to row my 25 foot, 4200 lb wooden Folkboat around. It did not have an engine. I used a single 14''or so foot long sweep that I built. It is not the weight or length that governs the length of the sweepbut the freeboard and beam. Because of the freeboard of the Folkboat I needed roughly 10 feet outboard of the rail and roughly 4 or so feet inboard. I had constructed the sweep myself using a clear 12 foot hem-fir 2 x 4 and some 1/4" plywood for the blade along with some odds and ends salvaged along the waterfront. The blade was somewhere around 8" - 9" wide and 3 feet or so long. I had cut a gentle curve in the end of the 2x4 and used closely spaced ring nails (and one small bolt at each end of the contact area of the blade) and epoxy to fasten the blades. I used a winch as a thole pole and loop of line that I dropped over a winch as the oarlock. I had glued long wedges on forward face of the 2 x 4 which kept the oar from sliding inboard unless rotated 90 degrees and I radiused the face of the oars where they bore on the winch so that the blade could be feathered. I had planed a long grip and had bolted a large salvaged zinc plate to the inboard part of the shaft to balance the weight of the extended part of the shaft and blade. A counterballanced oar weight makes rowing much easier. I cut up a piece of old carpet that I hung on the coaming, and on the shaft of the oar to protect the boat and oar from damage when I got careless. I hung the butt end of oar from the flag halyard on the spreaders and used a piece of shock cord at the shrouds to secure the blade end at the deck.
I rowed standing up, facing forward with the helm lashed to counteract the tendancy of the boat to turn due to having only a single oar in the water. If I rowed harder the boat would turn away from the direction of my strokes and if I rowed more slowly the boat would turn toward the oar. It took a bit of work and time to get the boat up to speed but once she was going she moved pretty easily at something I would guess to be roughly a knot (I did not have a knotmeter).
Whenever I could I sailed her in or out of where ever she needed to go and so did not have to row very often.