Nothing to do with languages. Sounds like a sailing term to me.

A rudder is the flat piece of material at the back of a boat which allows the captain to steer the boat.

Double means two.

And Dutch means coming from the Netherlands (a country with a strong tradition in seafaring).

I'm just guessing (not being familiar with the context) that it's a rudder with two flat pieces of wood or fiberglass, or whatever they would be made of, and they are made in a Dutch fashion.