I spoke with Lewmar's customer service rep, and gained a bit of additional information that was good. He didn't answer my questions though really, but searching around I feel like I have a somewhat ok grasp on what the trade-offs are. The relevant part of his response (where he wasn't telling me that I need to have a professional shop install it) is this:
"The further away from the barycenter the thruster is the more leverage it will have to push the bow (or stern). The tube needs to be far enough below the water line so that the exit water does not interact with the surface or thrust will be lost.
At the same time you have to keep the tube far enough away from the hull bottom so there is enough strength to prevent it from breaking."
I also found this document that details lots of things around tunnel thruster installation:
http://www.marinewarehouse.net/images/sidepower/Tunnel_Installation_Guide.pdfThe question that seems like biggest shortcoming of installing the tube forward of the cad drawing is that the length of the bottom of the tube might be shorter, and might be closer to the surface and/or closer to the keel. The recommended length of the tube is 2x the diameter of the tube, diameter being 5.5". I was reviewing Chuck's install, and just ball-parking from the pictures it doesn't look like his tube is 11" long. I did speak with him about it though and he said the system works great.
So, couple more questions for you Brian. 1. Do you have the shortest length of the tube as designed in the cad drawing? 2. How close can the tube be installed to the keel to maintain structural integrity?