I know this is an older thread, but to add my two cents ....
One very important thing when using sprint-and-drift is to alter your course often. Sprint for maybe 5-10 minutes if contacts are close, or 10-15 minutes if they're far. If you have no contacts, you can even sprint for 20-30 minutes.
Then you make a radical course change, and check above and below the layer (if there is one) for a good 5 minutes each.
Rinse and repeat.
As others have said tho, if you have close contact, sprint-and-drift isn't going to be much use ............. BUT you can do shorter sprints, with more radical course changes.
Course and speed changes are going to mess up their firing solution. You can even put on light rudder, say 5 degrees or less, to give yourself a slight turn while sprinting. This will also mess up their firing solution. Also reverse that rudder now and then so you're kinda weaving a shallow "S" thru the water.
I've had good experiences using these methods. YMMV
