Hi Guys!
The Airmar sensor requires a hole almost 3" in diameter! In models this is huge in comparison to a 1/8" pitot tube or just a single pressure tube used with a complete pressure sensor circuit that only takes up .625" (3/8") square! Ultrasonics is great and again the transmitter and receiver are about 1". Mike asked for something as small as possible. 3/8" is about as small as I can think of.

Best, Ed