Planer boards have been an open secret for years. More and more pike anglers are using these rudder-like bits of kit for trolling and even so-called walk-trolling along rivers, drains and canals .
Some have dabbled with making their own. This US website has plans and instructions for the DIY-minded.
Hat tip the excellent Trollscot for flagging it up, here's an article on using planers. The rest of the site is well worth a look if you're into fishing big waters.