Once you have picked out a spot...
...for the transducer, here's how to check it. Put the 'ducer in a plastic bag with some water. Place it on the deck where you intend to mount it. Make sure to wet the deck just a little to ensure good sound transmission. If you get a reliable depth (you can cross check by holding the 'ducer in the water over the side). If the hull is solid in that spot, you'll get a good reading. If not, you won't. On my O'day 22 I mounted the transducer inside the starboard quarter berth locker, all the way forward and inboard. I am planningf to move it aft because in rough seas even that location tends to come out of the water. Where you are proposing will do liikewise, I think, you may want to go aft a little. You cannot mount the transducer over the keel, as that will block it. I'm going to try the same storage locker locaion, but at the aft end. I don't think the keel/ centerboard will be an issue there. Some info I have seen warns that the keel will block the transducer when you heel, I think that is nonsense, becasue the transducer and keel are fixed in relation to one another - when the boat heels, the keel heels, and the transducer heels too. One more thing - I mounted my transducer in silicne, not epoxy. I sure am glad I did that, now that I want to move it.