Thanks Prairie Tuber for the info. Yes, it is part c that I can't turn. I'll try as you suggest and see if I can pull it up off D to get some lithium grease or other recommended lubricant underneath. Great illustration, by the way. This makes talking about the issue much easier. It also gives me an idea of how I can illustrate technical matters in the future.
I was very happy to hear from your reply that the valves function as a pressure relief point. I was given the idea earlier on that the boat didn't have them.
Thanks for the likely name of my boat. It has always been a mystery to me. The model number, which I don't have at the moment, is a 4 digit number that begins with the number 2 I think. It's 12' 6" in length with a soft bottom, inflatable keel, and a hard deck.
Thanks again for your help. This is a great boat and I hope to use it for a few more years.