A foil comes into play when a planing hull is under powered.
 If correctly powered for the weight of the boat, leg correct height  reasonable weigh distribution then it will not be needed.
They are fitted far too often when an understanding of the above is lacking, and usually as a 'patch fix'
yes it works as a temp fix, like a bit of duct tape on a hole in the hull to get home.. but long term..
 
You will find manufactures do not support foils with making after market foils.. will not support warranties when fitted

Also consider the extra drag...yes the economy does sometimes improve from before fitting.. lets not forget the real reason is , leg too low , under powered ... etc.
 Fix the actual problem and results are far better short and long term all around.

Thanks heaps for this - excellent advice which I shall take....