Nautical Slide Rule - More Than Meets the Eye

Weams & Plath Nautical Slide Rule photo by K.C.
At first glance the Nautical Slide Rule is simply a tool used to solve speed-time-distance problems. The real power of the tool though, is hidden, consider this: (from Hutchins (PDF))

"At this end of the technological spectrum at least, the computational power of the system composed of person and technology is not determined primarily by the information-processing capacity internal to the technological device, but by the way the technology exploits the cognitive resources of the task performer."

"Cognitive resources of the task performer" - that's the key. In practical terms, when I arrive on the bridge at 3 a.m. prior to port arrival I check the speed required before I have that first cup of coffee, cognitive resources are sometimes short of what is required to reliably solve the problem. Using the slide rule gives me confidence that I fully understand the situation in just seconds. I can hold the solution in my hand and by moving the dials slightly I can literally get the "feel" of the problem and solution. Using the slide rule has been compared to the practice of rearranging scrabble tiles to search for words.

The full power of the nautical slide rule becomes more apparent when put to use.

-The solution found can be quickly rechecked in a glance as opposed to having to repeat the operation as is the case with a calculator.

-In the case the ship's speed may be within a range, for example when "loading up", (slowly increasing speed) the range of solutions can be quickly seen by moving the dial over that range.

-The solution is in the same form as it will be used, distance on the dial is related to distance on a clock or the distance on the chart itself

- In the case where the solution is changing, for example if speed required is 12 knots but the ship is doing 13 knots, the rate of change of the solution can be sensed by resolving the problem, which can be done in just seconds.

- The precision of the solution is consistent with the precision of the inputs

- Using the slide rule significantly simplifies the problem and reduces the chance of error. In the words of a scientist (From Hutchins):

" The navigation tools do not improve cognitive abilities but simply make them easier and less prone to error. They map the problem into a domain where the solution is more readily apparent.",

The full paper is "The Technology of Team Navigation (PDF FILE) by Edwin Hutchins. Here is a couple samples from the paper:

The knowledge of the equation for distance, rate, and time and the knowledge of the constants required for the unit conversions are specific to the task, but they provide little help in structuring the actions of the task performer. As a result, the procedures for doing the computation are complex. When we write them out at even the shallow level of detail previously given, we find that they contain many steps. If we actually got down to counting each symbol written on the paper or each key press on the calculator as a step (not an unusually detailed level for a cognitive analysis), we would find that they each run to many tens of steps.

and this:

The ..... slide-rule transform the task from one of computation planning—figuring out what to divide by what—to one of simple manipulation of external devices.In the first two conditions, all that stands between the task performer and the nonsensical expressions R= DT and R=^T/D is a knowledge of the syntax of algebraic transformations. When using the nomogram or the slide-rule, the structure of the artifacts themselves obviate or lock out such relations among the terms. The rela- tions D=RT, R=DIT, and T=D/R are built into the structure of the nomogram and slide-rule.7 The task performer still needs to know something, but the knowledge that is invoked to solve the problem with these tools is less complicated, and less general, than the knowledge required with the paper and pencil or calculator versions. A good deal of what needs to be done can be inferred from the structure of the artifacts themselves. They constrain the organization of action of the task performer by completely eliminating the possibility of certain syntactically incorrect relationships among the terms of the computation.

The Nautical Slide Rule is rarely seen aboard merchant ships but I don't like to be without one. I call it my prayer wheel.

K.C.

Aside from the slide rule the paper is an interesting look at a navy bridge team from the point of view of a social scientist. It's The Technology of Team Navigation available here as a pdf - if you google it you can take a quick look at HTML version.