HR Hero plays with g'father with dementia

HR. Hero's grandfather or maybe uncle has dementia/mental health issues. The H takes the h to meet him and they end up reenacting a naval battle scene in the library, or it might be pirate ship scene. I remember them using the drapes like waves and moving chairs around. I think the old man was a retired navy officer.  The h respects the H more for being able to put aside his dignity and play with the old man instead of trying to correct his delusions.  That's all I can remember of the book, but I would like to read it again.