Next message: Randy Hamblin: "TWL: Fuel Metering"
I find measurement of shaft RPM more convenient, cheaper, and more useful
than engine RPM. On my Willard Horizon I have an upper steering (and s--l
handling station) that contains a suite of engine controls but no instrumentation.
The engine has a perfectly good Stewart Warner mechanical tach in the pilothouse
instrument panel but it is out of sight when at the upper station. Rather
than fit another tach, I bought a CatEye bicycle computer for $25 at my local
bike store. This sophisticated little gem is about 1/4 the size of a pack of
cigarettes. It picks up the rotation of the bicycle front wheel with a magnetic
sensor and converts it into instantaneous speed, average speed, distance, trip
time, and clock time, all available sequentially at the press of a button. The
computer is self powered and runs for two years on a watch battery.
I epoxied a small magnet to the prop shaft behind the coupling, fitted the
magnetic pickup in close proximity, and ran the connecting wire to the computer
on the deck. Current, slightly more expensive models don't even need the
wire. They communicate to the computer via a short range radio link. The
rotational RPM of the prop shaft is displayed. By telling the computer that my "bike"
wheel is 11 cm. in diameter, the reading is converted to an indication of
calculated boat speed. With this correction, the distance indications are
reasonably correct, as is the average speed indication. Naturally time of day needs no
correction.
Knowledge of shaft speed is quite useful. Since I know the gear reduction
rating, engine RPM is immediately available (in gear). I also know when the shaft
has stopped rotating. This is useful when tying up to a pier with the engine
still on since the mechanical gear shift is far from precise.
In two years of using the bike computer, I have had no problems whatever. The
device is fully waterproof and has been exposed to the weather since I got
it. I always marvel at the gap between well designed mass market equipment and
the stuff foisted on the boating community.
Larry Z
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