My boat came with a depth sounder and a knot log. I don't really have any problems with the older Standard Horizon instruments at all, so if the knot log worked, I wouldn't be touching them. Unfortunately, the speed log is dead. Call me a stickler, but I really don't like the look of mis-matched instruments, so off I go on another project.
The original transducers are installed with wood fairing blocks that I find to be less than elegant. The first problem is that the wood blocks weren't very well protected, and are in rough shape. The second thing I don't like is that there's no reason to have a paddlewheel perpendicular to the waterline. It can hug the hull just fine, and that eliminates the need for a protrusion from the hull.
My research has suggested a CS27 "ideal" practice of mounting the knot log paddle wheel under the floor in the V-birth area close to the bulkhead. The problem with doing so on my boat is that the teak and holley sole is screwed to the floor and the countersunk holes are bunged. There's no way to get in there for maintenance. I'm also not sure of the clearance available since the sole under that teak and holly is solid glass. For now, that location is out.
In the interest of getting the project done, my current preference is to keep the existing through-hull locations. The depth sounder will get a new fairing block cast in thickened epoxy. The speed log will simply parallel the hull. I will install the speed and depth instruments in exactly the same locations they originally occupied.
The choice of devices will be Raymarine ST60+. They seem to be the best value for the money. At this point I have removed the old fairing blocks and wiring and obtained the instruments. I'm waiting for the barrier coat before installing the fairing blocks and through-hulls.