It has been annoying me for more than a decade.  

It's historical romance - set in the US.  It begins just pre-Civil War.  I think it might have been set in New England, but I couldn't swear to it - definitely costal area though.  

Plot - there's a young girl who grows up having a big crush on a guy she knows.  She has several older brothers and I think the guy might possibly have been a friend of the brothers.  Regardless, she makes a spectacle of herself trying to get his attention at various times while she's growing up.  Once she gets older, she (I think) still has a crush on him, but has learned not to draw attention to it.  The hero is a ship designer, and he is designing an innovative ship (I think for the Union, but maybe just to design it and I think that its an ironsided ship, since that's when the first one appeared) and his plans get stolen by the Confederates.  He "borrows" the heroine's boat to chase after the thieves, not realizing that she is on the boat.  They chase after the plans, which I don't think they ever catch up to, but I do know that by the time they get the boat back to the town they live in she's thoroughly compromised.  Her brothers force them to get married, to avoid a scandal, and the hero goes off to recreate his ship designs for the Union.  They really don't interact through the entire war.  She has a son as a result of their encounter on the boat, but I don't remember if she tells the guy or not.  The war ends, and he comes home, only to realize that the girl never lived in his home.  Once he left, she left and went to live with her grandfather - who lives in a lighthouse (I believe).  The grandfather eventually takes a boat out to sea to die, and the hero gets his leg crushed trying to get the grandfather to come back, or trying to figure out what happened to him, something.  

Does this ring any bells with anyone?  I've done a bunch of searches looking for the book, but since I don't know the name of the book or the author, and the identifiable details haven't turned up anything, I've never turned anything up.