I have a 1984 O-Day 28. When I purchased the boat in April 2016 I replaced the frozen and old thru hulls (3 of them). One with brass and one with marelon and the 3rd I'm not of. I neglected to regularly exercise the fittings. I just tried and the brass one (that controls sea water intake for head operation) works fine. The marelon fitting is frozen open. It controls inbound sea water for cooling the diesel engine. I'm afraid to force it open and risk a leak. I had the bottom planted 2 months ago so am not planning on hauling her out anytime soon. I'll try to force it open when she is on the hard and will be ready to replace it if needed. What. is the risk in me leaving this frozen open fitting in place? I did change the impeller in the recent spring and shut the valve off then with no problem. Why else would I need to shut this valve? I live in eastern NC where is warm enough in the winter that we use boats year round and do not winterize. Also is there any safe technique to try to open this valve while the boat is in the water? Does a change in temperature help at all?