Speed

What is the optimal trolling speed?

does it change under different conditions?

When in doubt, watch your lure over the side of the boat.  If it has good action, then that's a good speed to start!  I usually start around 2 mph and make turns often.  Once I get a bite, I slow down if it was on the inside of the turn or speed up a bit if it's on the outside.  Every day is different and the currents change regularly as well...so stay flexible and paying attention to the details of the day are critical.

Good luck.

Optimal speed is the speed that catches fish! Somewhere between 1.9 and 3.5 mph. 

Speed over ground, and speed at your bait may be significantly different due to current and conditions. The styles and types of baits you use will dictate your speed. Keep in mind that not all bait speed ranges overlap. 

Check your bait manufacturers recommendations for a starting point on bait speeds.