I'd put it where the mfr says it will give best results. I have two on the bottom: one nearly the size of my foot hanging from the hull, the other flat as a pancake. The flat one gives consistent readings, the one with the large profile will occasionally get weird, perhaps from trapping some seaweed - but it always comes back.
(Harrumph! This ain't a tip or trick thread. It should, I guess, be in the Electronics dept. Bah Humbug. Oooh, wait: we ain't got no stinkin Electronics section. Maybe we should - as opposed to an electrical department. Electronics would be for discussion of our gadgets such as nav gear, radios, radar, etc - HEY JOE. . . . . .)
Joel Albert, Potomac MD
"Charlie B" - 1988 32' FBS
Twin 318's/FWC/16x15 nibral props
docked Deale, MD