Alternative to 3.15
 

Since 195 seconds (3 minutes 15 seconds) is divisible by 15, can you use the 3.15 method and then multiply the distance by 13 to get the speed in knots instead? Or is it to innacurate?

The easiest method I found was posted over in the 'hitting fast targets' thread.

Put your periscope just ahead of the ship crossing your FOV. As soon as the bow touches a vertical line, start the stopwatch. Stop it when the stern crosses the line. You already know how long the ship is in meters from your recognition manual, and you know have the time he took to cross that distance.

The EXACT speed is thus:

V (knots) = 1.9438444908 * Length (meters) / T (seconds)

...or you could replace the constant with 1.95, or 2x for a very quick estimate (and one you can likely do in your head. Double the ship's length, and divide by the time it took to cross bow-to-stern through your 'scopes line)

Pardon my asking a dumb nub question but I guess I am what I ask...

This technique does not require any knowledge of the range to the target?

Thank you for sharing this cooleo technique

No, range won't effect how long it takes for a ship to travel through your line of reference.

This is my formula (and I seem to be posting it alot these days):
D = Distance in Kilometers
S = Time in Minutes

(D/S)x30.866 = Speed in Knots

This works because 1 knots equals 30.866 meters per minute. So this way you can get speed from any time period.

Indeed, if the target is closer, the speed will make it seem to move 'faster' through your site. On the other hand, it's closer, so will appear larger, which will always balance out.

This is true for off angles, too. At 90 degrees, the ship will appear the largest, but it's also moving directly across your FOV, so covers the distance fastest. At 45 degrees, it's spending 'half' its movement towards you and half across your FOV...but, then, it also appears half as wide.

etc.

It's really great, as you don't need to know the AOB *or* range to get the speed. Just how long it is!

*grins* Actually Herr Kaleun, that is the formula for the average velocity of an object travelling with a non-constant speed (speed is the rate at which position changes, velocity is the rate at which speed changes, and acceleration is the rate at which velocity changes). In reality, the formula is this:

V=D/T

when V=velocity, D=displacement (the change in position), and T=time.

Sorry, shouldn't be too picky...this Kaleun paid too much attention in physics class. LOL

i have one question, when you say V (knots) = 1.9438444908 * Length (meters) / T (seconds)

do you meen V = (k * L)/T or V = k(L/T)

or put another way..

k=1.9438444908

Umm...Neon...it's the same either way. Order of operations and all - division and multiplication can be done in any order (although, traditionally, they are done left to right).

If there was addition or other functions in there, certainly, the order would matter - but I think you'll find that whatever numbers you plug into your two examples, they come out the same.
That formula...IS V=D/T? You are measuring distance (in meters - ship's length), and time in seconds. The multiplication for 1.94... is just to convert the resulting velocity in 'meters/sec' to 'nautical miles/hour' (or "knots"), which is V.

It makes no mathematical difference either way.

No. Just as distance and displacement have distinctly different meanings (despite their similarities), so do speed and velocity.

Speed is a scalar quantity which refers to "how fast an object is moving."
Velocity is a vector quantity which refers to "the rate at which an object changes its position."

Hence the second does not mean the rate of change of the first one. The main difference between the two is that velocity, being a vector, also gives you direction.

I use this formula: V=1.944xL/T for almost 2 years, it's very accurate. The real speed is when AoB is 90° but as targets move slowly, the error on the speed is very small and you can get a good estimation with AoB 40° or more(if AoB is 0 you can't find a value for speed!). For fast targets it works but you have to be careful and you don't have much time to do this!! :know: