Hi Steve, Chris,
No Chris yours wasn't the thread I was thinking of but this
https://model-boats.com/search/Hobby-Engine-Richardson-Upgrade
Same model just a different name.
The transducer idea was sparked off I think by a video of 'Noisy Thing' in an RAF launch - by PMDEVLIN I think.
If we'd seen your thread we might have said that fitting the transducer horizontally under the deck is not the best idea. That also directs the sound upwards and the deck is too stiff, due to coamings and superstructure etc, to resonate with the transducer.
Recommendation is to fit to the hull on either side, wired in series so that two 4Ohm transducers give the 8Ohm load impedance needed by most amps.
The idea is to bring the hull sides to vibrate thus acting as a 'speaker cone' and project the sound sideways across the water and not straight up. They should be fitted to the largest flattish area available for maximum effect.
I like the 'sound-palettes' idea 👍 Let us know how it goes, Doug 😎