Sorry for not explaining Araldite! Basically by using this 'epoxy resin', you are bonding the transducer to the hull, so that it all becomes Ďoneí. You donít need to drill any holes but it does need to be down to the inside of the solid bottom of the hull, (hence in the sump). As has been said, provided there is no air, bubbles or otherwise, then the ultra sound signal from the transducers will penetrate the 'glue' and hull without hindrance and return with its echo. I had so many air bubbles form at the transom when on the plane that when I had it mounted there, it lost its signal all together. Now, with the thru the hull placing, I do sometimes still loose the pretty little picture of the sea-bed and the little fish shapes, but it does continue to return a depth. If it's bonded like that, then I don't see why any water is needed in the sump as it wonít even get to go between transducer and hull. Other than perhaps to keep the transducer cool?
Interesting that you used silicon sealant, Dave. As you say, the instructions say itís a no-no. Any 'glue' you use should be solid and rigid to allow the signal to pass. The wobbliness of silicon sealant should attenuate the signal. Having said that, if your only using a thin sliver between transducer and hull, (i.e. push hard) and then pack around the side and top for the 'glue it not to move' factor, then that may well work fine. (Well obviously it does cos you've done it!) I wish now I hadn't read the instructions and done that myself. Much easier to remove!
Anyway, if 'jelly-like' substances don't let ultrasound through, how come they stick it on pregnant womenís stomachs in the maternity ward? Perhaps our resident baby expert, John Kennet would like to comment on that one?
(PS. Dave, Thanks for all the private info on your boats)