I think there have been some mighty fine ideas "floated" in this thread. All of which may solve the Bad Knee syndrome many (almost all of us considering our mean age of 60+ on this site)(the "mean" pun was intended) have to endure.
I'm about to offer an alternative to the above: Don't make any alterations or build contraptions to relieve the pain issue while sailing. The knees aren't any better onshore, are they? If you have trouble with knee pain while sailing, you probably have trouble walking to the head at 2:30am as well.
The Admiral, Buddy the Seadog and I were attending the Strictly Sail show in Annapolis last October. I was hobbling around with my "bad Knee syndrome", not sure how long I could stay as a result, when a voice from my left said, "Knees hurt?" I thought "DUUH, how could you tell", but said "yup! they sure do".
Now, I was scheduled for knee replacement surgery in January, He asked if I could spare 20 minutes, pointing to a nice welcoming chair in his booth. I said' What for?" already taking a seat. He proceeded to show me a type of electro-massage device the size of a in-pod, said it may help relieve the pain in my knee. I was doubtful, but was willing to try anything that offered relief.
He put the electrodes on either side of my knee and handed me the device saying " turn up the intensity as high as possible til it hurts, then back it down one setting. So I did. Up, Up, Up, Up, Up, Up, Up, Up, then Down one to a setting of Eight. It felt wonderful! He came to check on my progress and was startled at the setting I was on, and said" you must really be in pain! Most people would be jumping out of their skin on that setting!"
Well, this has been a long post, I know, but I have to tell you I got up and spent the next four hours walking around that show in no pain whatsoever! I went back to his booth an bought one of the machines. I used it two times a day for the next two months and once a day for the third.
I went to my Orthopedist for a pre-surgery consultation without limping, and jogged in place for him on the spot. He was stunned! Asked me what I had been doing for my knee, so I showed it to him. He said if it's working, keep doing it. He cancelled my surgery, saying when I can't stand the pain again, come back.
I use it two-three times a week now, still no pain compared to last year. Because of that my activity level is back where it should be and I have dropped the thirty pounds I put on when the knee hurt like hell.
The unit wasn't cheap, and you can probably find less expensive ones, but you can check it out at www.utopiagear.com
Just my 2 cents.