I don't know if the stingray slows the boat down. By design, it helps the boat to get on plane faster. I've heard arguments that it will reduce top end but I don't see the relevance of 1 or 2 mph. I think the foil can be a detriment to steering, especially if making hard turns - it may grab too much. I also think it also puts a lot of drag on the leading side when trying turn into a quartering sea.
I'll take the foil off, run fresh gas and try to note the difference next weekend. Definitely need to go to 15 pitch to get the rpm up. I didn't calculate efficiency cause my fuel transducer wasn't being read by the meter (always something) . And I was hung up on that instead of just doing the math.
The only reason to have a foil is to assist you in getting on plane if you have stern squat issues. It will also help you stay on plane at slower speeds and therein lies the argument that it will help fuel efficiency. I think a four blade prop or a stern-lifting three blade prop like the Black Max and its equivalents is a better choice. And trim tabs would help a lot more in controlling planing and balance. Also, out here we fish in and around the kelp and the wing is one more thing to get you hung up on kelp. I have another foil, which I may try also. It has better speed ratings in some tests I've read.
http://www.sesport.com/5_29.asp
Anyway, looks like its back to the drawing board,more testing to follow.