I am in the market for a C30. We just completed a survey on a boat today and found a mysterious crack when the boat was hauled. The crack was a hairline crack 18" below the water line, just even with the starboard locker. The crack had a slight radius in the 11 to 1 o'clock position. Ultimately, we didn't know what to make of it. We were told that the only way to know for sure is to grind and see how deep, repair as necessary. We are just trying to figure out how serious this is, how common this is, and whether we will see it again. Has anyone seen this before? If we knew what the root cause was, we would feel more comfortable going forward. We could repair and not worry. Without understanding root cause, we'd just be throwing money at it. Two more observations... 1) The boat had several very large blisters 4 to 6 inches. Theory - maybe related to delamination, 2) Owner installed an AC in the V berth himself. In the process of doing so he added an oak plank to close in that storage area under the v-berth. Theory - this oak plank acted as a stiffner and didn't allow the boat to flex. Any thoughts would be appreciated. Thanks in advance!!!