One of the perennial problems with through hull speed transducers is that the little paddle wheel seems to work well for a day or two after launch at the beginning of the season when it is all fresh and clean and then it stops. The problem is that those little paddle wheels are very susceptible to clogging up with any small bit of weed or other floating debris. Most of the transducers made for the likes of Garmin, Raymarine and so forth are actually produced by Airmar who produce good products and I was very pleased to see that they came out with a new speed transducer a couple of years ago called the CS4500 which has no moving parts and measures the molecules of water moving across the face of the transducer, all clever stuff and of course the best part is that as the transducer looks similar from the outside to the well known depth transducer there is nothing to catch weed or critters. The only problem with this transducer is the cost which runs to about $650 which puts it outside the pocket of many boaters. The only answer is either to get stuck into the cycle of removing and cleaning the paddle wheel every few days and all the problems that that entails or coughing up the lolly for one of the CS4500 transducers.
One of the other possibilities is to install The CruzPro SOG1 which converts a GPS signal into an
electronic impulse that can be read by a log speed display which in my case is the Raymarine ST40 unit. The CruzPro works on the NMEA 0183 protocol so it should work with most any display unit but may wish to check with the manufacturers before going ahead. As you probably know every GPS set calculates speed over the ground (SOG) and there is usually a page on the plotter or at least a small box that can be bought up to display this information. Basically the Cuzpro unit takes this information and feeds it to the speed or log display which is great. The good news of course is that the small CruzPro black box can be installed almost anywhere aboard and being a solid state unit with no moving parts should perform with no attention unlike the paddle wheel that gets clogged at a moments provocation and as we know needs cleaning almost every time we go out boating. The down side is that the speed is SOG not through the water. You could argue that speed over the ground is more helpful and it is in most cases because we want to know how long it will be before we get somewhere but there are times when is most helpful to know how strong the tidal stream is; calculating the best course to steer, how much tide we have moving under us when we are anchored and such like all things that the CruzPro will not give us. We can of course often calculate the tidal stream from the tidal atlas or chart but that is not as convenient as simply reading it off a display.
Think of it this way, if the boat is in the slip at the marina and is therefor not moving we can often turn on the speed display and assuming that the paddle wheel is not contaminated and is functioning properly we can read off the speed of the current which could be an important consideration to take into effect if we are just about to leave the dock and want to know how the current might affect the boat. Assuming that the paddle wheel is operational we only have to
turn it on and with the boat stationary and facing into the current we can read off how fast the current is running.
For all that I still think that the CruzPro unit is a good one and I have one on order, it costs just $100 and will let you know how the installation and sea trials go.