Quick question on speed calculation
 

Hi guys,

I've searched high and low for the answer to this but so far nothing. Anyway...

I play SH3 with full manual targeting etc., with OLC's mods and GUI.

For speed calculation I use the 1.94 x ship length / time (seconds) to get the solution, but I remember reading on these forums what the actual figure for 1.94 is - it's something like 1.93##### (about 6 or 7 decimal places).

Does anyone happen to know what the true figure is?

I'm curious to try the exact one as part of my calculations, but I just can't find it!

Cheers all!

Px3000

I've been toying around with that as well, lol

what I did come up with so far is this:

your average destroyer is round about 100 meters long. 195/time it needs to cross your view = speed (roughly)
this comes out reasonably accurate, launch fast eel x degrees ahead or slow eel 2x degrees ahead has scored me several hits so far

i suck at math tho, completely unable to come up with a definitive solution for the value :know:

I confess that I don't know the formula to the 6th decimal place. I use 1.94 and it works pretty well.

I am curious about your interest in the "exact" figure. Is this purely academic interest or do you think you will get more accurate speed calculations if you use a six decimal place factor instead of a two?

I don't think using a six decimal factor would increase your accuracy as I think there are more errors intrinsically in your timing (do you time from when the first pixel or your target firsts hits the first pixel of the cross hair or when the middle of the first pixel crosses the middle pixel of the cross hair?

Have you ever tried to locate the very last pixel that is rendered on the stern of your target? Aint easy!

I would imagine that computing variations in your graphics card, CPU, and monitor would negate any "accuracy" in taking the 1.94 down to six decimal places.

I would also imagine that the mods incorporate the historical inaccuracies of the TDC and torpedo performance.

For warshot situations, when pressed for time, I have actually used 2.0 instead of 1.94 and still poked holes in the floaty things.

So from a practical standpoint, I think that 1.94 is way too much mathematical accuracy as it is, taking the factor to six decimal places I don't think will have any practical use.

Now if this is simply an academic interest, you could easily find this number out with a piece of paper, ruler, and some simple trig. It would be an interesting problem to noodle out :)

Hi guys, thanks for the replies.

@Platapus, yes its more 'academic interest' than anything else. I was reading through something the other evening and got onto speed calculation etc., and I remembered I had seen the exact figure. I've searched the forums and via Google, but no luck so far.

I wonder if it would be more accurate for extremely long range shots though where speed is much more important.... worth a try I guess.

Perhaps trigonometry is the answer here.....:up:

Cheers!

Px3000

I'm not familiar with this method of speed calculation. I use the 3:15 rule- how does this one work?

Hi Predavolk,

This method is very reliable (well, for me anyway).

OK here's what you need to do:

1. Identify the ship first to obtain its length - very important. For this example lets say its 200 metres.

2. Be at full-stop ideally (or no more than 1 kt) and try to be side-on to the vessel as much as possible - it doesn't work from straight ahead, but 90 degrees + or - 50 degrees is usually fine.

3. Turn your periscope so the vertical line is ahead of the bow of the approaching ship. Don't move the periscope any more - very important.

4. Now wait until the ship's bow reaches the vertical line and start a stopwatch.

5. Its safe to lower the periscope whilst you are waiting for the ship to cross the peri's path, but don't turn it left or right.

6. OK, as the ship passes the vertical line stop the stopwatch the moment the stern passes the line. For this example lets say it took 55 seconds.

7. Now the mathematics....

Take the figure of 1.94, multiply it by ships length (200) then divide by the time to cross the line in seconds (55).

So we have 1.94 x 200 / 55

= 7.05 knots

There you go!

Hope that helps.

Cheers,

Px3000

Phoenix3000,

It's a straight conversion factor.

Meters per Second * 1.943844 = knots

or in SH3: Length (in meters) / Time (in seconds) * 1.943844 = knots

Bob

But why are you doing it like this anyway? The U-jagd tools found in the OLC GUI already provide 2 ways of doing this:

1 the U-jagd chrono is plenty accurate enough, but requires a little "rule of thumb" error correction - but the adavntage is that the calculation time is reduced to zero

The aob finder itself can be used for a more precise calculation of speed, if for whatever reason the U-jagd chrono doesn't satisfy. You seek out the ships length on the Basis ring, rotate the middle ring until the time in seconds matches the ships length, and then read off the speed opposite the little "notch" on the inner ring.

It's all there in the documentation, but in fact i hardly ever do that - the u-jagd chrono is plenty good enough


(also for me i use an enhanced U-jagd chrono image with extra scales to provide accurate measurements for more target classes)

Alright you asked for decimals... there you have them:

1,9438444924406047516198704103672

Courtesy of the Windows Calculator.exe. The m/s-to-knot factor is simply the division of 3600 seconds by 1852 meters: 1.94...= 3600/1852

Normally I love to put as many decimals into a calculator as I can, but I have to admit it's more sensible 'in the heat of battle' to just simply double the value. Only beyond a targetspeed of about 17.5 knots is the error going to be bigger than halve a knot. Well beyond what you wil find a convoy or single ship doing. So unless you are firing at extreme ranges where you need to have a accurate AOB (or be exactly across the targets track) and very acurate speed, it's good enough.

Cheers for the responses guys, didn't expect so many :o

@bobchase, that's the figure I was after! Cheers! :up:

@Pisces - as soon as I saw you post it fell into place - cheers too! :up:

To everyone else, I personally prefer the method I use - either with a calculator or with a pen and paper. Its quite easy to work out quickly when you get the hang of it and doesn't take much time at all - believe me (many don't) :D

Thanks again guys,

Px3000

Thanks for the explanation, but wouldn't AoB greatly affect this calculation? Did you mean +\- 5 degrees or 50?

Yes AoB will affect the outcome. The closer you are to 0/180 AoB the less accurate. But for most purposes the 1.94 formula works pretty well. I tend not to shoot outside the 70/110 AoB "cone" if I can at all avoid it so it works well enough :yep:

And the cooleo thing about this formula is that you don't need to know the range to the target. It took me a long time to believe that (and a lot of graph paper) but it's true.

I actually use both the 3:15 and the 1.94 formula depending on the circumstances

No. AOB only makes a difference if it's so small that the beam of the ship starts to interfere with judging the position of the bow and stern (that is 30 degrees or less)

Not necessary, although it helps if you are slow. The easiest if you are making headway is to point your bow at the target while you make the measurement - then the effect of your motion on the measurement is minimalised. And even so, with a little practice it's easy enough to account for your own speed on the measurement.

I am not sure what you mean by this. Is the target supposed to be on your side (90/270) or are you to be on the target's side (in other words AOB=90 or 270)?. If the later your are correct (for the reason Platapus described below), but the former is not to be recommended because your own speed (even if small) distorts the measurement in the worst possible way. Put the target just in front of your bow or stern and you'll minimise the effect of your speed.

Just make yourself one of these:

http://img185.imageshack.us/img185/5185/dsc00013ym5.jpg

Then, you just match the length of the ship in meters against the amount of time it took to travel against your fixed point (ie., periscope/UZO crosshair), and bingo, you read the target speed in knots.

By the way, I've used the fixed wire method while overtaking a ship. When you are on a parallel course, set your periscope/UZO wire at 90 degrees from your course, then see how long it takes the ship to pass. Say you get a result of 6 knots, and you are travelling at 13 knots. Target speed: 13-6=7 knots.