Quick question on speed calculation

[Sag75]

Originally Posted by bobchase

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.

Originally Posted by 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.

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]

Originally Posted by Snestorm

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.

Originally Posted by BillCar

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]

Originally Posted by 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)

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]

Originally Posted by ryanglavin

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
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
My own judgement has proven many times to be just as reliable.

[Fader_Berg]

1.9438444924406047516198704103671706263498920086393088552915766738660907127429
80561555075593952483801295896328293736501079913606911447084233261339092872570

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]

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]

Originally Posted by Paul Riley

joegrundman,

I perfectly understand the other methods,I just dont like to use them...much
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.