View Single Post
  #5   Report Post  
Old March 28th 04, 11:57 AM
Posts: n/a
Default Digital navigation? was Good TSD formula?

Gould 0738 wrote in message

"Every three minutes, a boat will travel 100 yards x the speed

in knots."
The rule of thumb would obviously disregard variables like,

wind, current,
tracking error, and so forth..............but as for the

premise itself.....

Always true?
Never true?
True only at certain speeds?........(not likely, would probably

work at a
variety of speeds since the time and distance are in consistent


Want another useful approximation? Try the digital sextant.

My little finger's width viewed at arm's length subtends one
degree. And all its corollaries are useful; my fist subtends 6
degrees, my stretched hand from thumb tip to little finger tip
subtends 15 degrees. Rests on the fact that a degree is
1/57.whatever of a radian.

My hand, outstretched, is a convenient 57 half inches from my
eye; so a half inch at that distance is a degree. Give or take a
few seconds. Calibrate yourself and your crew.

Used as a vertical sextant this is useful (in the absence of
radar!) to discover if you're overtaking the yacht ahead. As a
horizontal sextant it's great for helping other crew members
identify obscure marks. 'See the tall chimney? OK, 18 degrees to
the right and two degrees above the waterline . . .' means 'one
hand span plus three fingers right and two fingers above'. Much
better than just saying 'just right of the shrouds'. Not as good
as putting someone's head in transit with the shrouds and the
mark , though.

Other mental dead reckoning tricks (like the sine clock) are
sadly being made redundant by GPS.


to other crew members if they kn