Guides and Training

[Frederf]

RR suggested I transplant this explanation of how the nomograph works from another thread:

I think it's too easy to overestimate the complexity of the nomometer and treat it like it's some black voodoo magic. All it does is calculate time, speed, or distance if you know the other two things.

"How much time does it take to go a distance at this speed?"
Time = Distance / Speed

"How much distance is traveled by going speed for time?"
Distance = Speed x Time

"How fast does something have to go some distance within this much time?"
Speed = Distance / Time

These are the only three questions that the nomometer knows how to answer. If you want to know the course or angle or when the moon will rise, you need to look somewhere else. You ask by drawing a straight line through the two values you do know and the answer is discovered by finding where the line crosses the scale of the value you want to find out.

A practical example is (for me) commonly that I have plotted the target's location and I know its course and speed through previous calculations. I've decided that I want to shoot at the target when it gets to some specific future position since that makes for a good shot with short range and good angles. So I measure the distance from the target's current position to the desired future position and maybe it's 4600yd. Also say for example that the target is going at 8 kts. What I want to know is "How much time is it going to take that 8 kt ship to travel 4600yd?" This is important to me because I want to know how long I have to get into shooting position.

I pull out the ruler tool on the navigation map and I start a line from the 8 kt mark on the speed scale. I pull this line through the 4600yd mark on the distance scale until I cross the last scale, time. I notice that this line that passes through "8kt" and "4600yd" also passes through about 17 minutes. This tells me that something takes 17 minutes to go 4600yd at 8 kt.

Just for fun the problem can be extended. Using the previous result of 17 minutes until the target gets where I want him to be for shooting I discover that my submarine is too far away to make a shot. Oh no! Now I pick where I want to be to shoot which I measure to be 1200yd away from my current position. OK, I have 17 minutes to go 1200yd. How fast do I have to go? Leaving the right end of the line I made in the paragraph above at 17minutes, I drag the left end of the line around until the line crosses through 1200yd mark on the range scale. Then I look to the speed scale to figure out my unknown. The line crosses the mark at just over 2 kts. "I must go 2 kt to travel 1200yd in 17 minutes."

17 minutes later the 2 kt submarine has traveled its 1200yd and the 8 kt target has traveled its 4600yd. The nomometer has allowed me to arrive just in time.

[Rockin Robbins]

And I've been playing mods without a nomograph lately! When I loaded up TMO 1.8 beta, there was my old pal the nomograph again. It's great to have a refresher course on just how useful it is. Thanks Frederf!

[Distortion]

I have a question for the excelent vector analysis attack: If for instance the speed of the ship is 5 knots, I draw a line of 5 miles, from where I start the attack. Is it possible to attack let say from a distance of 2 or 3 miles?

If I would plot the attack from 2.5 miles, doe I need to cut any other lines? like torp speed, and the 500 yards line for the lead angle?

cheers

[magic452]

Distortion The values for the lines are 100 yards for each knot of torpedo and target speed. Ex. Mk 14 high speed is 46 knots, your course line would therefore be 4,600 yards long. Target speed of 5 kn. = 500 yard speed line.

You can shoot anywhere along the course line more than 500 yards (arming distance) but not beyond it, the torp won't travel more than 4,600 yards on high speed.

Spectator explains it much better than I can in post #55 in this thread.
He uses an approach angle of 90° but it works at any angle, I always use something like 60°.
Vector analysis is a firing solution method so you must be in range of the torpedos for it to work,


Magic

[Hitman]

I haven't found any actual wartime references to taking prop counts and I have a fair collection of WWII related submarine books

That's interesting, I though I had readed something about it in one of O'Kane's book, must be wrong. What I can confirm is that Erich Topp's sonarman certainly did estimates of speed based on propeller counts. NOt only have I readed it in a book, but there is also video footage in which he appears doing that during Operation Drumbeat.

[Distortion]

Originally Posted by magic452

Distortion The values for the lines are 100 yards for each knot of torpedo and target speed. Ex. Mk 14 high speed is 46 knots, your course line would therefore be 4,600 yards long. Target speed of 5 kn. = 500 yard speed line.

You can shoot anywhere along the course line more than 500 yards (arming distance) but not beyond it, the torp won't travel more than 4,600 yards on high speed.

Spectator explains it much better than I can in post #55 in this thread.
He uses an approach angle of 90° but it works at any angle, I always use something like 60°.
Vector analysis is a firing solution method so you must be in range of the torpedos for it to work,


Magic

Cheers magic, I feel kinda stupid But now it all makes sense.
What I did was a run to the end of the 1hour speed line/course of the target, but sometimes I didnt have the time to do plotting.
Thanks m8

[Rockin Robbins]

Yes, it's really easy to forget that all we're doing is drawing a scale model of the firing solution and start to confuse 4400 yards on the graphic solution with an actual 4600 yards distance required to make the shot.

Keep practicing and it all becomes automatic. But when you're first learning it's easy to become confused about all the details. Keep up the good work!

Hitman, the fact that the Germans did it could have lent a halo effect to the assumption in American sub movies that Americans did it too. The Germans had some important advantages. First the Atlantic was crawling with hundreds of pretty identical cookie-cutter Liberty boats. Once you learned the RPM/speed curve for one you had the keys to the city. Also the open nature of American and British societies, where information valuable to enemy combatants easily enters public knowledge, made information on our merchant shipping much more accessible than the relatively closed society of Japan.

[Hitman]

First the Atlantic was crawling with hundreds of pretty identical cookie-cutter Liberty boats. Once you learned the RPM/speed curve for one you had the keys to the city. Also the open nature of American and British societies, where information valuable to enemy combatants easily enters public knowledge, made information on our merchant shipping much more accessible than the relatively closed society of Japan.

Cheers, good observation

As far as I have seen, boy have you guys been able to capitalize on any new idea that someone has pointed out! The cold war american sonars were simply the best, despite germans having lead the way to that rpm count, and I always like to remember the memorable sentence from Jules Verne in his novel "A Journey to the Moon", where he said "What an american can imagine, another one can do it"

In any case, I wanted also to pay another tribute to the John P. Cromwell attack method (45º AOB shots) with something I recently readed:

The very same U-Boot Commander's Handbook says:

95.) When attacking ships with low and medium speeds, at close range, it ballistically advantageous to fire at an angle of the target of 90, as errors estimating the position will in this case have the least effect, besides which the speed of the enemy can be most accurately gauged in this position. If the range is longer (over 1,000 m), and the target is traveling at a high rate of speed, an attempt should be made to launch the torpedo at a smaller angle, say, 60.

Since the angle the book is referring to is the opposite to the track angle, it is in fact recommending a 40º attack , i.e. the John P. Cromwell one!

Amazing