Sylvain, I suspect that if you search back through the forum for “oars” and “rowlocks” you will find numerous very helpful discussions on these points. The basic answers are 1) “very long” – last time I calculated the ideal oar length it came out somewhere between 10 and 12 feet which is unfeasible. Most people compromise at 8ft. 2) there are reinforcing pads under the side decks where the rowlocks can be fitted. Again a read around old forum posts will give you more details. 3) serious sculling would require rowlock on the transom.
I am contemplating putting “rowing” on the cruising conference this year, or at least a topic called “what are the uses of oars on a wayfarer and where can I put them?!”
hope this helps
Boris. Cruising Sec. W6330.