Page 219 - NAVAL SCIENCE 3 TEXTBOOK

RULES Or TilE lWAD AN"n ,\IANEUVERING BOARD 225

Alternatively, if you have an electronic calculator, you could own ship's true course and speed vector, and then determine the
find the relative speed by solving the formula specd = distance -;- resultant other vessel's true course and speed vector. 1b do this, lay
time, making sure you use distances in miles and time in fractions one side of the parallel ruler along the RML, and position the other
of an hour. Or you could use either the three- or six-mi1lute rules. side at the tip r of our own ship's vector. Now draw a line from r in
The three-minute rule is the distance traveled in yards in three min- the same direction as the RML. Layoff the relative speed along this
utes -;-100 equals speed in knots. The six-minute rule is the distance line with the dividcrs, being sure to use the same speed scale as was
traveled in miles in six minutes x 10 equals speed in knots. used earlier to plot your own ship's vector, the 3: I scale in this case
In practice, bearings and distances to maneuvering ships used (if dillcrent scales were used, the resulting vector diagram would
to plot points .l.VI , All' AI , and so on are generally taken three or six not be usable). Place an arrowhead at the end of the relative speed
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minutes apart, so the two rules above can be quickly applied to find vector, and label it with a lowercase 111. Vector f11lnOW represents the
the relative speeds between .1.\11 and 1\1] or .l.VI2 and AI , and so on. relative motion speed vector (see the illustration on page 224).
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Having determined the magnitude of the relative speed, we can To complete the problem, the em vector representing the
now plot the relative course and speed vector from the head of our maneuvering (other) vessel's true course and speed is formed

SC.l,LES MANEUVERING BOARD
2,1 3,1
o
11

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lO---j'1-i

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N- llJ ~.~
1
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-----------------"~--'-'-'-~ ~~.!:' .... ,
~".,.o,'" ,,,",,,"hll""_'~' .. ' ..
~. ~". ~, ~"I ,'" ~'''Ih ,.,~, .. "'!'"
"'~, .. , .. ,' ,"'n."" .. "'.j ",~
Completed maneuvering board plot to find the CPA and the maneuvering ship's course and speed.