Quick question on speed calculation

[Phoenix3000]

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

[Bosje]

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:

[Platapus]

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 :)

[Phoenix3000]

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

[predavolk]

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

[Phoenix3000]

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

[bobchase]

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

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

[joegrundman]

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)

[Pisces]

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.

[Phoenix3000]

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

[predavolk]

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

[Platapus]

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

[joegrundman]

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

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)

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.

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.

[Pisces]

Hi Predavolk,

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

OK here's what you need to do:

...(snipped)

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.

.... (snipped)
Cheers,

Px3000I 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.

[Puster Bill]

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:

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.

[Sag75]

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


Thanks,

Is known the whole trigonometric formula? I mean, it should be some trigonometric factor to correct the smaller AoB..

[BillCar]

Fixed line is easier than it is made out to be – as joegrundman correctly said, the slower one moves, the more accurate one's reading, but within firing range, I have taken fixed-line readings at flank speed when necessary, and one can easily get away with just doubling the value.

To cop out to the argument of what happened in reality, U-boat commanders would generally use the fixed-line method while having only rough values for the ship's length. These cause a broader deviation in the results than does the doubling of the ship's value, but even when taken together, these two inaccurate estimates aren't particularly bad, and generally proved to be quite adequate when within torpedo range.

To use an example of a tramp steamer crossing your line in 24 seconds:

1.94 x 78m / 24s = 6.305 knots
2 x 78m / 24s = 6.5 knots
1.94 x 75 (as a common estimated length) / 24s = 6.06 knots
2 x 75 (as a common estimated length) / 24s = 6.25 knots

In the example above, all of these values fall within .299 knots of the actual speed. Simply doubling the value by 2 as opposed to 1.94 falls within .195 knots of the actual speed, on the outside.

In my opinion, if one is far enough away for one's firing solution to fall apart based on these sorts of small deviations, one needs to be closer anyway.

[Snestorm]

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.

Hmmm. Uh oh?

I've just been multiplying the vessels length by 1,852.
Although my way works, it's not always "perfect", so usualy I'll just use 1,9.

Have I been doing this "wrong, but close enough for government work"?
To rephrase: Have my "more precise" calculations (x 1,852), actualy been LESS precise than my rounded calculations (x 1,9)?

[BillCar]

Hmmm. Uh oh?

I've just been multiplying the vessels length by 1,852.
Although my way works, it's not always "perfect", so usualy I'll just use 1,9.

Have I been doing this "wrong, but close enough for government work"?
To rephrase: Have my "more precise" calculations (x 1,852), actualy been LESS precise than my rounded calculations (x 1,9)?

Yep. But as I point out above, those little deviations don't add up to too much within firing range, as a general rule.

[Snestorm]

Yep. But as I point out above, those little deviations don't add up to too much within firing range, as a general rule.

Thanks for the confirmation, Mr Bill.

More than once my torpedo strikes had been defined as, "good thing I got so close".

Think I'll now stick to a simple 1,9 . . .
OR an even simpler, and faster, 2.

[Sag75]

thanks for answers, I was looking for the trigonometric formula underneth this method,

for instance this method cannot be applied if I have a 20°-30° AoB ship.. probably because there is a trigonometric factor that we can ignore at AoB 90° (for instance Sen(90)=1)

[ryanglavin]

thanks for answers, I was looking for the trigonometric formula underneth this method,

for instance this method cannot be applied if I have a 20°-30° AoB ship.. probably because there is a trigonometric factor that we can ignore at AoB 90° (for instance Sen(90)=1)

It might be strange, and I don't think this is 100% possible, but couldn't you use the fixed line method at any AoB? lets say the ship thats 24 meters long crosses your sights in 8 seconds while at 30 AoB. Couldn't you, in theory, multiply the answer you get from that by a specific number so it turns into it as if it was a 90 degree AoB?

[BillCar]

It might be strange, and I don't think this is 100% possible, but couldn't you use the fixed line method at any AoB? lets say the ship thats 24 meters long crosses your sights in 8 seconds while at 30 AoB. Couldn't you, in theory, multiply the answer you get from that by a specific number so it turns into it as if it was a 90 degree AoB?

I've done it at very shallow angles before - 20 degrees, at least once or twice, and it did seem to work as normal.

[Paul Riley]

Best and easiest way to get a good speed reading (for me) is to draw level with the target and match speed until it is steady at 90 or 270 degrees in the scope,less time messing around with complex mathematics in a combat situation :yep:
Ideally you want to be observing the target for 15 - 30 mins to get a good steady result.
I personally find making adjustments 'on the fly' and using my own judgement and observation to be more satisfying than performing ridiculous mathematical equations.The MK I eyeball will take some beating yet.

[joegrundman]

@Paul Riley

if the fixed line method is complex, then you have other problems

matching course and speed is a perfectly valid, useful and important approach, but calling your currently preferred method the best way is to simply say you don't understand other methods, nor the limitations of the one you favour.

@Sag75 and the others

To be clear about the AOB with the fixed line method - AOB is NOT a factor in this method.

This is because you are measuring the time it takes for the ship to pass it's own length and the ships own length doesn't change whatever the aob. (Only your torps can change that !) the only requirement is that you be able to see the bow and stern.

But because a ship has a width as well as a length, at small AOBs (ie when the target is coming sharply towards you - or away from you for that matter) the width of the target ship can obscure where exactly the bow and stern are - making it difficult to measure when the target has passed it's own length.

@BillCar - you are quite right about the ability to estimate -personally i never use the recog manual (much too much work with the community units mod) and still get very good results using the kind of assumptions you describe here.

[Paul Riley]

joegrundman,

I perfectly understand the other methods,I just dont like to use them...much :smug:
My own judgement has proven many times to be just as reliable.

[Fader_Berg]

1.943844492440604751619870410367170626349892008639 3088552915766738660907127429
80561555075593952483801295896328293736501079913606 911447084233261339092872570

From that point, it just repeats it self in eternity.
This information is of course totally useless... I just had to find it out.

[Paul Riley]

:06: LOL

[Fader_Berg]

To clearify the AOB problem a little more...

If we would look at a moving target in an AOB of 90 degrees. It would cross our field of view as fast as possible in any given speed. While if we watch it in a AOB of 45 degrees it would appear to cross it slower, or actually (sin(45) =) 0,71 as fast over the same view. But, at a fixed aim point, and as the AOB of 45 degrees also makes the ships footprint only (sin(45) =) 0,71 of its true (90 degree) size. The two factors evens out.

So a ship with an AOB of 45 degrees will appear to move 0,71 slower across our point of aim than a ship with a AOB of 90 degree. But the 0,71 smaller footprint of the 45 degree target compensates for that, and the time for both to travel their full body will be the same.

[BillCar]

joegrundman,

I perfectly understand the other methods,I just dont like to use them...much :smug:
My own judgement has proven many times to be just as reliable.

I use the fixed bearing method you prefer when shadowing, Paul Riley, but I don't use it in combat - takes way too long. I always wind up verifying it again with the fixed line method. The fixed line method takes seconds, is exceptionally accurate, and can be done while closing distance to the target. There is no complex mathematical equation involved (multiply ship length by two, divide by number of seconds).

Furthermore, I've been beta testing Hitman's new GUI for GWX 3.0, and it contains a chart for fixed line, giving speed based on seconds and length of ship, thereby eliminating all mathematics. All you need to do is click the stopwatch for a few seconds and look at the chart, so maybe if you didn't enjoy fixed line before, you will once you're able to try out this GUI! It is pretty great. The finished version should be out within the next short while, from what I hear.

[Sag75]

To clearify the AOB problem a little more...

If we would look at a moving target in an AOB of 90 degrees. It would cross our field of view as fast as possible in any given speed. While if we watch it in a AOB of 45 degrees it would appear to cross it slower, or actually (sin(45) =) 0,71 as fast over the same view. But, at a fixed aim point, and as the AOB of 45 degrees also makes the ships footprint only (sin(45) =) 0,71 of its true (90 degree) size. The two factors evens out.

So a ship with an AOB of 45 degrees will appear to move 0,71 slower across our point of aim than a ship with a AOB of 90 degree. But the 0,71 smaller footprint of the 45 degree target compensates for that, and the time for both to travel their full body will be the same.


Thanks! this is the confirmation I was looking for!

[Snestorm]

@Fader_Berg

Good info. Thanks.

[Pisces]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

[Pisces]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

p.s. If you used 1.852 as conversion factor instead of 1.94...yada yada yada... then your lead will be underestimated by 5%, or about one in twenty degrees too less. If you used simply 2 you'd have over-estimated the speed by 3%. That's better and simpler.

If this is a hit or miss really depends on the size of the target and the range. 1 degree is 17.5 meters wide at 1km. So most small vessels (78.5 meters is common) are between 4 and 5 degrees wide at 1 km distance. When closer disproportionally bigger in degrees. You'll no doubt hit it, but maybe not where you intended.

[Fader_Berg]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.
Was this intended as an answer to my contribution?

[Snestorm]

The only bow or angle you should be worried about when doing this trick is that of your own! Have the periscope look at 0 or 180 degrees, turn until right in front of his bow and then all is good.

p.s. If you used 1.852 as conversion factor instead of 1.94...yada yada yada... then your lead will be underestimated by 5%, or about one in twenty degrees too less. If you used simply 2 you'd have over-estimated the speed by 3%. That's better and simpler.

If this is a hit or miss really depends on the size of the target and the range. 1 degree is 17.5 meters wide at 1km. So most small vessels (78.5 meters is common) are between 4 and 5 degrees wide at 1 km distance. When closer disproportionally bigger in degrees. You'll no doubt hit it, but maybe not where you intended.

You are 100% right.
That's why torpedoes were hitting towards the stern on a regular basis.
Only getting so close allowed me to get away with it so often.

I love this place. Thanks for the assist. (I need it now and then).

[Paul Riley]

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

Easy,effective,jobs a good un :up:

[Pisces]

Was this intended as an answer to my contribution?Not in particular. Just to emphasize in general that an 'angle on own-bow' can easier wreck your measurement. Snestorm asked about how accurate is accurate. Therefore I gave some ballpark figures about how much it is going to be off. No worries mate.

[Snestorm]

Thanks, everybody.
100% improvement in accuracey.
Results, and confidense, are way up following this last patrol.