Our depth sounder is located in the port seatte, aft end; its mounted in a fiberglass box which is glassed to the hull, the box has a outward flange to which an acrylic lid is bolted. The lid has a hole for the transducer to through bolt and two other plugged holes, one hole is the fill hole, while the other allows air to escape while filling with distilled water. We set up the offset to be keel plus 2" to read zero depth on the West Marine Multi-Data instrument (Standard/Horizon).
We have good results with this setup. The knot log is the same location as most stock boats, located forward and to port of the mast. Repeated removal and cleaning of the paddle wheel has allowed seawater to enter and rest around the mast, bad idea, resulted in minor corrosion to the mast base; would perfer other location with direct vent of water into the bilge, perferably in the shower sump area, where the seawater could more easily be captured and the area cleaned.
I have had the same problem and results as Marty: ingress of seawater every time I swapped the paddlewheel transducer in and out resulted in minor corrosion of the mast base. Very, very annoying. So annoying that I often sailed without the waterspeed transducer in place.
I’ve come up with three solutions: the first is too expensive (unless you start off that way); the second is now SOP on Zygote; and the third is what I would have tried if solution #2 hadn’t worked.
Solution #1 (which may or may not work) is to be equipped with one of the new high-tech non-paddlewheel water speed transducers that can tolerate mild fouling.
Solution #2 (which works for us) is to send the cook over the side at the start and end of each major passage. The cook carries with her a plumber’s helper.
At the start of a voyage, the cook’s job is to make sure that the prop and the water intake for engine cooling are clean. Then she holds the plumber’s helper over the waterspeed transducer thru-hull while I, the cook’s assistant, swap the dummy transducer for the real thing.
At the end of a voyage, the cook’s job is to plaster the prop with water resistant grease (reduces fouling of the prop to almost zero for about one month of stay in a very barnacle-rich tropical marina) and then to hold the plumber’s helper in the magic spot while I swap the transducer for the dummy.
Use of the plumber’s helper reduces the water ingress to about one cupful (250 ml or thereabouts).
Solution #3 (which I have toyed with but not yet employed) is to get a short length (5 cm should be enough) of PVC piping, of a diameter large enough for me to slip my hand (when clenched around a transducer) thru, and epoxy the pvc pipe to the hull concentric to the transducer thru-hull. Then a length of flexible fire hose-type hose, with a diameter to suit the piping and about the same length as my arm, can be clamped - with a hose clamp - to the pvc pipe. The deal being that seawater ingress is constrained within the hosing and can be emptied into the bilge so it does not splash the mast base.