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Larry:
Excellent suggestion. The only modification I would suggest is to epoxy
the magnet to the hub or coupling at the front of the driveshaft instead
of the shaft itself. It will then not interfere with removal of the
shaft or stuffing box when that becomes necessary.
Wesley
wrote:
>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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