The "Rule of Fives" is a strategy anglers use to set trolling depth[link to "What is trolling depth?" - new tab]. It states:
When trolling at 5 MPH with 50 pound test line and 5 oz. of weight, set lure to 50 feet behind the boat, and the lure will run about 5 feet under the surface.
An alternate method of setting trolling depth is to use a device, such as a downrigger or fishing lure depth planer.