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--- Lu Abel  wrote:

> Rotate the front disk by the amount of local variation and tape it
> to the back disk.  Draw a course line with any convenient
> straightedge, slap the PIN down so the hollow rivet is over the
> course line, align it properly with the parallels and meridians on a
> chart and you've instantly got your magnetic course.   Way easier to
> use than describe.  Nearest currently available equivalent is the W&P

> Compute-A-Course that's shown below the Portland plotter.

It's a little more elaborate than a garden-variety plotter, but the
ChartPak plotter arm has a similar feature - you can lock the degree
wheel to either the true or magnetic compass rose (or anything else
entirely, I suppose it could also be used for grid navigation if you're
in a polar area). The plotter arm can then either be left free or
locked to a bearing (it works a lot like a drafting tool, only a lot
more versatile and better-suited to navigation work).

> Something I can't figure out is why many different designs of really
> good plotters (like the PIN) appear, but none seems to stick around
> except the traditional 4x15 plotter and parallel rules.

As someone mentioned in another thread, a lot of navigation is steeped
in tradition, and traditions sometimes apparently die hard.

--
GregR


> Greg R. wrote:
> > --- Guy Schwartz  wrote:
> >
> >
> >> Edges are graduated in inches and centimeters
> >>
> >
> > It would be a lot more useful (at least in my opinion) if it also
> had a
> > scale that matched the plotting sheets, though I guess you could
> always
> > use the inch scale and "do the math" mentally when plotting.
> >
> Several of the typically 4 x 15 inch plotters such as the Weems and
> Plath plotters do indeed feature distance scales for the common chart
>
> scales of 1:40,000 and 1:80,000.   But that's useful only if these
> are
> indeed "common chart scales"   In a cruise along the coast of Nova
> Scotia, I was more than a little surprised to discover that their
> coastal charts are not of a constant scale (eg, 1:80,000) the way US
> charts are scaled, but vary somewhat from place to place.
>
> On another note, my favorite plotter (for local coastal navigation,
> at
> least) is a non-longer-available device called the Pocket Instant
> Navigator.   Incredibly simple device -- two clear plastic disks
> joined
> at their centers by a hollow rivet and able to rotate with respect to
>
> one another.   Disks are inscribed 0-360 degrees.  Back disk also has
>
> square grid at aligned N-S and E-W.  Rotate the front disk by the
> amount
> of local variation and tape it to the back disk.   Draw a course line
>
> with any convenient straightedge, slap the PIN down so the hollow
> rivet
> is over the course line, align it properly with the parallels and
> meridians on a chart and you've instantly got your magnetic course.
>
> Way easier to use than describe.  Nearest currently available
> equivalent
> is the W&P Compute-A-Course that's shown below the Portland plotter.
>
> Something I can't figure out is why many different designs of really
> good plotters (like the PIN) appear, but none seems to stick around
> except the traditional 4x15 plotter and parallel rules.
>
> Lu Abel
>
> >
>


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