Tony, yes, there are many variables when it comes to trolling.
My point is, it is the ACTION of the lure that determines the speed of the boat. You just can not say, troll at 3 knots or troll at 5 knots. You can be trolling one day at 3 knots and the lure will have no action at all. Troll that same lure the next day at 3 knots and the lure may run perfectly. Troll it another day at 3 knots and the lure can be spinning all over the place. Therefore, it is the action of the lure that is determining the speed, it is not the speed of the boat determining the action of the lure.