Authentic speed measurement in SH4

[musikerz]

Hello, everyone here, I am a new cadet skipper who has just been introduced to the great Silent Hunter 4 and its mods, and still learning to survive in the 100 percent realism and how to use TDC manually.

So my question is how do u guys measure vessels' speed by listening to its propeller rotation? Because i heard that those real skippers did use this method in the history, isn't it?

[badaboom]

Ahooy Skipper and welcome to Subsim!!!!!!! See this great thread for a wealth of info including links to speed calculations made easy,

http://www.subsim.com/radioroom/showthread.php?t=107783

[Fincuan]

Originally Posted by musikerz

So my question is how do u guys measure vessels' speed by listening to its propeller rotation? Because i heard that those real skippers did use this method in the history, isn't it?

You assign the contact a tracker on the broadband sonar and identify the target. You then find its TPK(turns per knot) from the USNI reference, dial that to the DEMON screen, move the cursor to the first line and voila, you get the speed. Oh wait, that was in Dangerous Waters

In SH4 you basically you need to know the target 's TPK(turns per knot) value. You follow the target on the hydrophone for 60 seconds(or 15 seconds and multiply the result by 4 etc, you get the idea) and count the turns. You then divide the resulting number by the TPK to get the target's speed in knots. I remember that someone did a TPK table for SH3 and this method worked to some extent, but I haven't seen such table for SH4 nor do I know if the above method works at all in the game.

[Rockin Robbins]

Unfortunately, in SH4 the tpm does not correlate to the speed of the target well enough to use. Same with SH3, and anyone who has tried it has abandoned the attempt. As a practical matter, only relative speeds, "target is speeding up" or "target is slowing down" could be determined because turns per knot was not a known quantity like it is today.

A large part of what modern subs do is monitoring known ships and determining turns per knot for that individual ship. Now these are in a database allowing sonar to identify thousands of ships by name and pretty precisely peg their speed from the accumulated database. That wasn't possible in WWII.

[musikerz]

Thx, badaboom, i will check it out, and Fincuan 2. And to RR, so u actually mean it's not possible to do that in SH4? Then i don't get it, some article i read in the internet did mention this "propeller method", even the documentary "silent service" said that.

quote from http://jtmcdaniel.com/periscope.html

Target speed

The final factor needed is the target's speed. There are several methods of obtaining this, though none can really be called 100% accurate.

First, periscope observation. The periscope optics are marked in degrees in both the vertical and horizontal axes. If the distance to the target is known, it is possible to determine speed by timing the elapsed time required for it to travel a given number of degrees. The problem with this method is, of course, that it is dependent upon an accurate range, since you are measuring the length of time the target takes to traverse a known number of degrees, and it also means exposing the periscope while you do it, which is potentially dangerous if the enemy spots it. (Japanese merchantmen not only carried deck guns, frequently with gun crews who actually knew what they were doing, but also depth charges, and didn't hesitate to use them.)

Second, general knowledge. Some types of vessels are known to routinely travel at certain speeds. This will usually be the most economical cruising speed. However, since the target's captain may be in a hurry, or may be moving slower than usual, this will tend to be the least accurate method of determining speed. An experienced captain can often make a fairly accurate guess at a target's speed by the appearance of the bow wave.

Third, counting revolutions. The sonar operator can listen to the sound of the target's propellers and determine the number of revolutions per minute. If the submarine's captain knows the pitch (the distance travelled in one revolutiion) of the target's propellers, he can make a fairly accurate estimate of speed. For instance, a screw with a 24-foot pitch should move the ship forward 24 feet for each revolution. One hundred revolutions per minute should, therefore, move the ship forward 2,400 feet, or 800 yards. This would give a rough distance travelled of 1 nautical mile (2,025 yards) every 2-1/2 minutes, or a speed of about 23.7 knots. This sort of speed would generally indicate a large warship or liner. Freighters were usually slower, with the fastest generally limited to about 16 knots. (Fuel economy was the major factor—fast ships use a lot of fuel, so high speed was mostly found in passenger ships, where the line could charge extra for a fast passage, specialised freighters like banana boats, which had to deliver their cargo before it spoiled, and warships, where cost wasn't a major consideration. In any case, if screw pitch is known, and an accurate count obtained, this can give a fairly accurate speed estimate. Also a ship's screws are far from being 100% efficient—the only vessels that manage this are submarines at a considerable depth, where the great sea pressure suppresses wake and cavitation.)

Well, i know this is a little 2 crazy for a gamer to listen to the hydrophone and count the rotations, and have to look up a TPK table for each ship which are not provided in the game, but it seems to me, without the map update this is the best and easiest way to find out how fast the target is, for the sonar operator at least, especially in the darkness.

[joegrundman]

Well as the war went on, i think they got better at it,

and the british made it a key point from the beginning, but they had the advntage that most of the world's merchant marine engines were built in Britain in the years before the war started.

Anyway, even with what you have, you can do pretty well:

A merchant moving at medium speed according to your hydrophone operator is either travelling at 8 or 9 knots

Now that piece of information helps you a lot, and if you are close, 1 knots difference will not stop your fish hitting.

there are other factors too: if you can see the target isn't too far away, but your hydrophone operator is having trouble hearing him, chances are it's very slow, say 5 knots or less

joe

[tedhealy]

It was done in the war. O'Kane's 'Clear The Bridge!' makes mention of buying the sonar man a metronome to more accurately determine target speed by counting prop turns.