I agree with Christian.
Gluing it inside works fine. I just had a transducer added so we now have depth displayed on our Lowrance plotter. The transducer was glued to the bilge, under the forward end of the cockpit. Great signal and the cable even was connected to the proprietary bus, under the cockpit.
I asked a long time racing skipper/naval architect type about this very subject last weekend. I'm getting tired of the fountain every time I put in and take out the DST. He didn't think the shoot through the hull speed sensors were accurate enough for racing. I have absolutely no idea. Any thoughts about that?
I know there are new ultrasonic speed sensors but I hadn’t heard of them being “through the hull.” I got as far as looking up the price, which is a significant fraction of the value of my boat, and put them out of mind.
I did use the depth sensor in “through the hull” mode for a year before drilling a hole for it at the first opportunity. The problem was that it would blank out in very shallow water - exactly when it was needed most. At the time I was docked up in the back end of a shallow swamp and needed to feel my way very carefully. A foot of water under the keel was a good day. I suspect there were too many reflections at close range. Anyway, the blanking problem never occurred again after installing in a hole.
I thought I had saved the core sample, but can’t find it - maybe I have a picture of it somewhere? I think it was well over an inch thick - forward and outboard of the keel a bit.
With my new Raymarine depthsounder, I could not get a signal theough the hull forward of the keel. The in hull transducer had to be mounted near the prop to get a reading to approx 120 to 125 feet. At the next haul out, I installed a thru hull transducer and I get readings to 1200 feet. That might not be useful to you guys back east but out here we can get readings of 1200 feet 50 feet from shore. That depth reading is a helpful check on your other instrument readings when meandering around in heavy fog.
Turns out that the hull forward of the keel is 1 1/8" making it difficult to send a signal through.
When I was shooting “through the hull” it was not in the same location that I drilled, but in a thinner area under the settee. However, I think that any relatively flat part of the hull (i.e. pointed down) starts to get relatively thick. Though one of the advantages of the various TTH methods is that you can arrange the transducer to point straight down (discounting angle of heel) over curved hull by making a wedge of the mounting material. I experimented with that but don’t recall that it had much effect. One of the problems was that my slip itself was extremely shallow and the bottom was an ill-defined layer of soft muck, so I had to go somewhere to test each iteration.
(Currently my transducers are all in a little bilge below the hanging locker and the fluxgate compass is just above them - makes wiring simpler.)