[Public-List] knot meters

[gordon white via Public-List]

A tip to anyone who has an older Raymarine or similar knot meter

     I have had a knotmeter for a number of years that is run off a 
thru-hull pickup with a little impeller. The thru-hull unit died last 
fall and I have found that the style is no longer made. The 
currently-made pickup will not fit in the old thru-hull housing, thus to 
use a new one requires hauling one's boat and chiseling out the original 
housing.

     (The change was made to facilitate pulling out the impeller to 
clean it of gunk without hauling the boat. The old style came with a 
plug. You pulled out the impeller, getting a bit of water in the boat 
before you could get the plug in place. The new ones have a flapper 
valve that reduces the amount of water that gets in to a minimum, but 
the new unit is not interchangeable with the old one.)

     I have found that when the old impeller units fail, most of the 
time it is in the associated water temperature transducer, built into 
the impeller housing. When the temperature pickup fails the speed meter 
will read only - - - - (dashes).

     But if you can "fool" the instrument into thinking it is still 
getting temperature data, the speed part of the indicator will work.

     There are usually five leads in the wire to the instrument. Red and 
Green are for speed, Brown and White for temperature, plus a grounded 
shielding wire.

     If you attach the brown and white wires from the read-out 
instrument through a small (1/4 watt or smaller) 100 K resistor, the 
temperature reading will not work but the speed portion will. Has worked 
for me with two impeller units.

     Also, it is well to lubricate the O-rings on the impeller unit with 
a little silicone lube.

     And if your local marine instrument shop has any of the old-style 
impeller units, even used, might be a good idea to lay in a spare now.


     _ Gordon White, Brigadoon II