I have a Catalina 16.5 Capri. I purchased a model without a keel, so it has a centerboard that I can swing up and down. My problem is that this model seems kind of tipsy and so I was wondering if you could add water ballast. There is a plastic threaded plug that screws in at the rear end of the boat that could be opened to allow water to enter the base. I'm just not sure if that is what this was designed for. Does anyone know for sure?