Cleaning the Knotmeter and Fathometer.

If you’ve been on a sailboat, and paid attention to some of the instrumentation and displays, you will have noticed that there is a likely a device of sorts to provide a readout of the boat speed and distance traveled through the water and another to provide an estimate of the depth of the water. Both of these devices come in various forms and shapes. Modern ones are combined into one unit. Skiron has such a unit. Further, my vessel has GPS and I can obtain an estimate of the speed of Skiron “over ground” compare to the boat speed and compass course, and gain an understanding of the effects of tidal current flows and leeway as a result of wind on progress.

While sailing just a few days ago I noted that the boat speed, as determined by the knotmeter, was reading much less than the GPS-determined speed over ground. This observation suggested to me that I probably should have cleaned the knotmeter before we set sail. I suspected that it was nearly clogged with biological growth. Fortunately, the fathometer seemed to be reading depth that was as expected based on my familiarity with the San Francisco Bay.

Today, I decided to inspect and clean the knotmeter. It is located in the forward part of the boat, under the floor in the V berth or forward cabin.

The floor of the V-berth of Skiron.

I lifted the floor to expose the combined knotmeter and fathometer

Forward through hull is the combined knotmeter and fathometer. The through hull in the rear once held similar instruments and has been sealed with epoxy

Next, I twist the collar of the through hull to loosen it and apply upward pressure to pull the instrument from its seat. This basically creates a situation where there is an influx of water. It is possible to control the amount of water that ingresses. For demonstration purposes, I allowed more water to enter the boat than I usually would.

Twisting the collar of the instrument to loosen it in preparation for examination of the device.

Ingress of water after the device is pulled.

Ready to insert the accompanying plug to stop the ingress of water.

Inspection of the instrument revealed that there was indeed biologic growth affecting the paddle wheel that is turned in the “stream”of water under the boat indicating not only speed but also distance traveled.

Filamentous biological growth along side the paddle wheel of the knotmeter.

I usually employ an old toothbrush to scrub the wheel and the surrounding parts of the device as clean as is possible. It is not, however, possible to remove all of the material. I should try to figure out someway to do that in the future.

Paddle wheel is now free to rotate without obstruction. The flat surface half disk end is the underside of the depth transducer or fathometer.

After cleaning the instrument, I pulled the plug, reseated the instrument, and sponged out the seawater. Then. I wiped out all of the remaining water that entered the vessel with a paper towel. I’m pretty fastidious about keeping the bilges perfectly dry.

The device is now ready to provide accurate and useful information for the next sail.

To be truthful, I’ve only been required to clean this instrument four times in a year and a half of ownership of Skiron. I probably should make it a habit of inspecting the instrument at least every 6 to 8 weeks moving forward.

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