Before you glue in the GT-15, you need to test the location to be sure it works. You dont want to have to un-glue it!

There are a few different ways. Most common is to:

1) Clean/sand the hull inside where you want to mount the ducer. It needs to be as flat/smooth as possible and no delams, holes, ridges etc that could cause air pockets
2) At least partially fill a 1 gal baggie with water, place it in the sanded bilge area where you want to test.
3) Figure out some way to hold the transducer flat on top of the baggie (usually a really good friend) while you drive the boat at up-to full speed to see if you get good bottom readings.

If it doesnt work very well or at all, try another spot.

Repeat as needed.