Sad to hear your news about your knee and your decision to sell the boat.
A close rowing friend, 6'5" and with enough medals to form body armour, chose to commute by bike daily for speed and economy, but alas suffered an accident which nearly lost him his right leg and left him with a right arm permanently bent right angles at the destroyed elbow. To get a pint to his mouth he has to use his left hand.
I forbade all four of my offsprings to indulge in motorbikes. 4-wheeled Metal crumples better. I also forbade both my boys to play Rugby, thus eliminating two very frequent sources of serious injury.
Knees can be treated and some are very successful indeed. An old Aussie rowing friend visited Ireland in 2015 and one day we climbed the steps to the top of the Skellig (700ft). He did it despite a knee he had sustained doing something else. He has since had some job or other done to it (not replacement) and latest reports are that he is back doing galley slaving in a boat on the Yarra (Melbourne)
Since 2012 I have been yotless, in that I was bought out of the Moody 40 I had shared with others since '03, so all my sailing, apart from dinghy-ing round our bay in Kerry, has been by invitation and I get more of those than I can avail of, so the sale of a boat isn't the End of the World.
Cheers Tom, the knees are probably just from carrying round a considerably overweight body, not bike related. I think I am going to part with the boat, but with any luck I will buy something similar when I retire so I can indulge in some longish cruises, nothing expensive, tatty but working will do for me! preferably with a bit more headroom inside!