October, 2001
By Tom Snyder
Rules of thumb. What a funny little expression – but what a lifesaver! A rule of thumb is a bit of mental shorthand that can be called into action when time and circumstances don’t allow logical contemplation. Rules of thumb are to be found in every demanding discipline because any situation with more than few variables can quickly become chaotic and counter-intuitive, at which point a wise practitioner will surrender to a highly simplified set of responses that are based on likely averages evolved, in many cases, over hundreds of years.
I offer to you a set of my favorite rules of thumb but remember, please: The charm and power of these rules lies not in their accuracy but in their general predictive power when the human mind is asked to do more than it can.
Navigation: It’s the dark of night – where am I?
Face in a northerly direction. Extend your right arm straight up and over your head while extending your left arm in a forward direction. Close your eyes for 10 seconds to allow them to adjust to the darkness. Tilt your head back, open your eyes and locate the planet Jupiter. This body will stand out because planets transit the heavens relatively rapidly due to their proximity to Earth.
Once you are confident that what you are seeing is Jupiter and not a satellite, a commercial airliner, or a comet, slowly arc your arms, still fully extended, half way to the planet. Your arms will now be describing an angle. Take mental note of that angle. Now locate the tiny moons of Jupiter. Because the reflected light from these moons tends to be overwhelmed by the colossal mass of their mother planet, you will need to pull back on the skin on the sides of your face, narrowing your eyes and exerting back-pressure on the cornea and the optic muscle.
With this greatly augmented vision, identify the moon closest to the planet’s surface. Wait for this moon to begin to traverse the angry red storm on the surface of the planet. Begin counting seconds until the moon moves through the angle formerly described by your outstretched arms. The number of seconds, when multiplied by 10 will represent your longitude (and your latitude if they happen to be the same number.)
Boat Speed: How fast am I sailing?
Remove the cabin or cockpit hatch that covers your propeller shaft. Loosen the stuffing box so that water is freely flowing into your bilge. Loosely grasp the shaft with both hands. When ready, inhale, attempt to stop the shaft with your hands by tightening your grip, exhale, and hold this position for 50 revolutions of the shaft. When done, refer mentally to the Cheney Scale of Resistance, against which you will compare your effort exerted. Much like the Beaufort Wind Scale, the Cheney Scale uses simple and accessible measures that can be easily pictured or imagined. There are two levels in the Cheney Scale:
Resistance 1: Easy to stop the shaft with almost no pressure (speed = 0 knots).
Resistance 2: Cannot stop the shaft no matter how hard you try (speed = greater than 0 knots).
Food: How long will this mayonnaise last?
Remove from storage the mayonnaise jar in question.
Step 1: Open the jar and inspect the surface of the product. If the product is whitish in color and free of blemishes or colonial development, proceed to step 2. If not, return the mayonnaise to storage.
Step 2: If the jar you are holding has been opened or tampered with, it will not last much longer and you would be wise to dispose of it. However, if the jar has never been opened, it will last for a good long time.
These three rules of thumb are, of course, not the only two, but they set a standard against which all rules of thumb might be measured. If you have come across other rules of thumb or have invented your own, please share them with this magazine. Remember – an adequate rule of thumb is better than careful planning and cautious reasoning, which usually take too long.
Note: Don’t forget to retighten your stuffing box.
Tom Snyder sails his Island Packet 350, Blue Moon, out of Peaks Island, Maine.
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