One of my favourite boats on the great HBBR row down the Thames was Tony Waller's Isabella III, built to Joel White's Shearwater design. The picture shows him easing through Radcot Bridge heading for the pub.
Later, Tony was pushing against a lock wall with his oar and it slipped into a crack, breaking off the end.
Continuing to row, he expected to feel the boat tipping over due to the imbalanced thrust of the oars, but interestingly the break made very little difference. He lashed up a new end to the blade with duct tape (what would we do without it?), not to restore performance but to protect the ends so the bits could be epoxied back in place when he got home. Here he is by Wallingford Bridge with the ghastly evidence.Just goes to show that the length and size of oar does not really matter, within limits. What really makes the difference is a comfortable rowing position and the right gearing (except when racing, of course, when milliseconds can make the difference between glory and goathood).