Pete,
Please don't take my response as your final answer. There are people here who have vast experience and may give you better answers. This is (partly) just my personal experience.
Some drakes will become very aggressive if they feel they're competing for the hens. Much advice will tell you that you should have only one drake for every five hens. We have a higher ratio than that, and I've had only one issue with it, but it was with grown ducks, not the ones we raised ourselves.
We bought some year-old buffs last spring, three hens and a drake. Within two days, I returned the drake, which I was by then calling the "demon drake," to his previous owner. I possibly should have given it more time for things to shake out, but it was breaking my heart. He wouldn't allow all our ducks we'd hand-raised to do anything they wanted to do. He chased them around and pulled out their feathers. If they tried to go in one of their ponds, even if he wasn't in it at the time, he'd go over and chase them out. He'd chase them away from any food dishes. He'd often, even if the other ducks were minding their own business in another place across the yard, just decide to put his head down and start running toward them.
The previous owner had fortunately told me she'd take any of them back if they didn't work out, so I tried by the second morning to get in touch with her to say I needed to bring the drake back. By the time my husband got home, I was crying and so upset that I said (apologies to all duck lovers since I'm one of them), "That drake is leaving tonight, dead or alive, and I'm not sure I care which one at this point. We're going to at least crate him until we hear back from (previous owner)."
I so much hope you get more responses since it sounds like you raised all these yourself and don't have the option of taking the aggressive drake back to a previous owner like I did. It was incredibly upsetting to me to see all our normally friendly ducks huddled at the back of our property and afraid to go near their ponds or food dishes because they were so terrified of that drake.