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if built into (some) void-free/void-consistent substrates, like high density foams or fiberglass, many sonar transducers can be tuned to ignore the hull and provide feedback of the underwater surface without a 'thru-hull' opening on the underside of the craft.

This is useful for boats, but maybe useful as an idea for a project like this as well?

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Yes! I've been considering putting this into one of my hard boards, in which case I'd want to fiberglass over it. I've been wondering how well it would work.

Look up acoustic impedance matching. By analogy, your ultrasound technician needs to smear ky jelly (or similar) on your skin to ensure a good connection between the transducer and your flesh. You need an analogous material to make the sonar transmit efficiently into water. You can get special epoxy that works, but off the shelf fiberglass will likely not be very good.

Are you thinking you could just duct tape one to the top of the board? That certainly seems like a lot less work and a lot less worry about damaging the seaworthiness of the board.

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