How long is the boat out of the water each year, and how well sealed is the planking? You've ogt to give the boat a chance to stabilise. if the timber takes up and seals the gaps eventually each year, then the problem is caused by taking the boat out of the water each year. There's no way anyone is persuading me to have our hillyard out of the water for more than two weeks. She's well sealed, but all the loading is different, the hull will start to change shape and the planking will dry out. You can do various things to mitigate all these effects, but it will still happen to some degree. If she's dry and seaworthy by the end of the summer, can you leave her in over the winter?