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Originally Posted by greyrider
it seems that rpm's and speed are closely related in sea going vessels,
there are terms used such as," makings turns for 6 knots",
theres a propcalc in the JP sonar, some members have reported some skippers using tachnometers
to count rpm's, from novels or movies that they have read or seen.
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I can't really argue for or against using turns, but I have a couple of possible problems, at least for me to understand.
1) "Making turns for six knots" is fine for your own ship, but different ships have different sized props, so different revs for each class of ship to make the same speed. Do they know all the different possibilities for each enemy ship? If so, how do you tell which ship it is without having looked at it?
2) Even if you can tell the speed from the revs, what about the Doppler effect: if he's heading toward you the pitch changes, but so does the apparent speed. Likewise if he's heading away, and without looking you can't know the direction he's heading.
That said, I've heard claims that some could do it; I just don't understand how.