How did real Uboot sailors determine target range?

[Kpt. Weyprecht]

Originally Posted by Hitman

When submerged, using the periscope split prism stadimeters (early models) or reticles (late war models)

They switched from split-prisms to reticles? Something like moving from what we have in SH4 to a kind of OLC/U-Jagt tools?

[warispeace]

I'm assuming they would have used the mil dots in the targeting reticles to measure the size of the ship, then looked them up on a chart to estimate range. Just a guess though.

[Paul Riley]

I dont like the stadimeter much,much prefer eye estimations,as Hitman said,by judging how much of the target fills the scope you have a fairly good range estimation.You can also draw level with the ship,and track it on a parallel course,match speed,quickly plot your UBoat's position and your target,get the ruler out and get your range that way.
But as has been said already,range is not that important from a perpendicular position,only when using a gyroangle,like when firing from parallel positions,which would be a full 90deg turn by the torpedo (I am actually very successful with these attacks)

[irish1958]

Originally Posted by Paul Riley

But as has been said already,range is not that important from a perpendicular position,only when using a gyroangle,like when firing from parallel positions,which would be a full 90deg turn by the torpedo (I am actually very successful with these attacks)

Actually range is always important, as it tell you (or the torpedo computer) how far ahead to "lead" the target.
However, the closer you are to the target, the less important this is to be correct.

[Kpt. Weyprecht]

Thanks, Pisces. But I still don't get what actually changed between early-war and late-war periscopes that Hitman mentioned?

[Pisces]

Hmm, yes, now I'm curious too what he means. Looks to me it would be the other way around. Split prism mechanism being developed late war.

[joegrundman]

no the split-prism was dropped at some point in favour of the much more advanced binocular periscope which gave a 3d image and depth perception. But the increased size of the mechanism meant dropping the split prism and aob finder and so relying on telemetry scales only.

it was considered an advantageous tradeoff

however, until our pc screens are similarly able to render in very high resolution 3d, to simulate this technological progress would be a step backwards, so we should stick with modelling the former.

[joegrundman]

Originally Posted by irish1958

Actually range is always important, as it tell you (or the torpedo computer) how far ahead to "lead" the target.
However, the closer you are to the target, the less important this is to be correct.

this not correct mathematics

[Sledgehammer427]

IIRC (as in, reciting from memory) the range to the target was measured with a trigonometric equation involving a right-angle triangle. starting with the periscope head at the bottom end, the known height of the ship as the vertical line, one must find the length of the hypotenuse to determine range (the horozontal line in a right-angle triangle)only knowing how tall the ship is.
I do not know the math involved, but thats the gist of it.

[joegrundman]

sine alpha = opposite/hypotenuse

alpha being angle subtended on periscope

opposite = mast height
hypotenuse = range

so range = mastheight / sine angle subtended

but the earlier scopes with the split prism had this calculation automated, as the american scopes do in sh4

Hitman fairly recently demonstrated that the telemetry markings on the periscope reticule were in radians rather than degrees, which apparently makes the range calculation easier

[irish1958]

Originally Posted by joegrundman

this not correct mathematics

Maybe so.
However, it is correct physics.
A torpedo travels about 1000m/min
A 300 meter ship at ten knots travels about 300m/min
If a sub at 1000m, perpendicular to the target fires a torpedo at the middle of the ship, the middle of the ship will have traveled 300 meters by the time the torpedo arrives at the spot at which it was aimed, missing the ship by 150 meters.
If the sub is at 2000 meters, and it fires a torpedo at the center of the ship, it will miss the ship by 450 meters.
Etc.
It appears prudent to lead the ship.

The AOB calculation corrects the equation for the sub not being perpendicular, by adjusting the amount that you lead the ship with your torpedo.

This is done by the torpedo computer when you feed the data into it.
If the torpedo doors are not open when the fire order is given, there will be a delay in the firing of the weapon by several seconds, making the torpedo arrive late and perhaps missing the ship.

The only time range is not important is if both the sub and the target are stopped, and the ship is at least 350 meters away, and no further than the range of the torpedo (?5Km).

[joegrundman]

Maybe so.
However, it is correct physics.

this still not correct physics, either

Anyway, for such a small angle the difference between the hypotenuse and longer cathethus is minimal.

innit, and the difference is in fact vanishingly small except at very close ranges. with sine you are measuring range to top of mast, with tangent you measure range to waterline.

e.g 25m mast subtends 1 degree
range to mast top (using sin) = 1432.47m
range to waterline (using tan) = 1432.25 m

a difference of 22 cm

e.g. 25 m mast subtends 10 degrees
range to masttop (using sin) = 143.97m
range to waterline (using tan) = 141.78m

a difference of just over 2 m, and in either case within minimum torpedo range.

Using sine is more practical for the simple reason that the same circular sliderule can be used to solve this calculation as for all the other sine-based calculations you may need to solve!

[Pisces]

Originally Posted by Kpt. Weyprecht

They switched from split-prisms to reticles? Something like moving from what we have in SH4 to a kind of OLC/U-Jagt tools?

Actually it's the two combined. OLC/Joegrundman had to resort to a simple reticule style because the SH3 engine doesn't support dual image rendering. Sh4 does. But in the real german attack scopes those prism things could be rotated on it's side (90 degrees) to measure the appearant width, just like you do the height. Sh4 can't rotate them, or atleast the Americans didn't use it like that. To mimmic the AOB measurement OLC/Joegrundman also included a horizontal reticule scale to make it as realistically functional as possible. And by god they succeeded!

Also, because you see two of the same images moving up and down together in the attackscope it is easier for your eyes to track the difference. You don't need to worry about keeping the waterline in the same spot. So stabilised view option isn't such a bad idea in my opinion.