Just did a B60-20 on my Gulfstream. Can't tell you where to mount it, other than probably the best place would be close to the keel, near the transom... and of course, it must be in a location where you have access and clearance on the inside of the hull. For installation, I used black electrical tape across the face of the ducer, and used a sharp blade to slice the excess tape from around the edge like cutting the crust from a pie. I measured VEEERRRY carefully for the location (I have a bunk trailer and didn't want the ducer resting on a bunk). I drilled a pilot hole from the inside, ensuring there was enough clearance for the nut and a wrench, then cut the hole from the outside, ensuring the cutting surface of the hole saw was flat against the hull. Clean bottom paint away from the edge of the hole. Dry fit the ducer in the hole, and tighten the nut down. This cleans any possible burrs off the threads, and will show if the ducer face will lie flat against the outside of the hull. You may need to use a grinder (I used a dremel with barrel sander bit) to even the fiberglass on the inside of the hull so the ducer face rests evenly against the outside hull. While the ducer is still in place, tape around the edge of the ducer so you will have a nice little fillet of 3M 4200. Drop the ducer down and apply 4200 as the directions state, ensuring you have 4200 come up the threads of the ducer barrel just greater than the thickness of the hull and nut. Having some one outside holding the ducer up against the hull, run a bead of 4200 inside of the hull around the ducer. Orient the ducer so the arrow is pointing to the keel and tighten the nut down. Many recommend NOT to use the rubber washer. It will have a tendency to squeeze out from the nut. Then, on the outside, use your finger to remove excess 4200 and create a nice small fillet around the face of the ducer. You can then remove the tapes before the 4200 starts to set up.
