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.

[joegrundman]

Originally Posted by 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.

Yes, but does this information make it clear to what degree of accuracy you could make your estimate - the counting part is relatively easy!

Prop count to speed is dependent on many variables such as size and type of engine and propellor, weather conditions and amount of cargo being carried

Also when in the war is a relevant issue

So when they say they did it to gain more accuracy, do we know if the accuracy was to the nearest 0.1 knot, 1 knot or 2 knots, for example?

[Joe Armstrong]

Propellers have been well understood since about 1861 in fact writings of that period mention The Franklin having a 19 foot propeller with a 28 foot pitch and the Mersey have a 20 foot propeller with a 29 foot pitch.

The first actions required of a sonarman was to count the turns of the prop and report it. In theory when a propeller with a 30 foot pitch makes one revolution the propeller has travelled 30 feet. It should make sense that a ship at a stand still could spin its prop very fast and hardly move and at a-head full this effect would be less. A well designed propeller would have about a 10% slip and lesser designed propeller could have as much as 30% slip. (or efficiency) Once a ship is at a certain speed and the TPK registered this speed if properly recorded and reported would eventually be placed in the ONI 41 - ?? Ships Identification Book. It was not uncommon for a ship to have only one identified TPK and corresponding speed at late in the war as 1944.

I have built a sound chart and corresponding TPK table that is about 60% accurate in SH4. I can receive contact information and dive to PSD and with 60% accuracy tell you what type of ship and how fast it is going with enough accuracy to start up my attack. I usually run one game at a time only (don’t return to port re-fit and go back out) so my sonar, radar and corresponding navigator are only about 90% accurate on what they give me because they are a little short on rank and medals. With 30% accuracy on TPK and 90 % accuracy from my reporters I am still able to take out the target by just getting a little further ahead. Sometimes just in time and sometimes a wait around, but I am in position early enough to use sonar to accurately set up my shots buy last minute maneuvering.

I would eventually like to see this part of game improved with TPK tables at cruising speed for all targets. Then as long as I am not spotted by the enemy they would hold the speed they were at allowing me to get ahead and then use my instruments to properly set up my shots.

[musikerz]

Originally Posted by Joe Armstrong

Propellers have been well understood since about 1861 in fact writings of that period mention The Franklin having a 19 foot propeller with a 28 foot pitch and the Mersey have a 20 foot propeller with a 29 foot pitch.

The first actions required of a sonarman was to count the turns of the prop and report it. In theory when a propeller with a 30 foot pitch makes one revolution the propeller has travelled 30 feet. It should make sense that a ship at a stand still could spin its prop very fast and hardly move and at a-head full this effect would be less. A well designed propeller would have about a 10% slip and lesser designed propeller could have as much as 30% slip. (or efficiency) Once a ship is at a certain speed and the TPK registered this speed if properly recorded and reported would eventually be placed in the ONI 41 - ?? Ships Identification Book. It was not uncommon for a ship to have only one identified TPK and corresponding speed at late in the war as 1944.

I have built a sound chart and corresponding TPK table that is about 60% accurate in SH4. I can receive contact information and dive to PSD and with 60% accuracy tell you what type of ship and how fast it is going with enough accuracy to start up my attack. I usually run one game at a time only (don’t return to port re-fit and go back out) so my sonar, radar and corresponding navigator are only about 90% accurate on what they give me because they are a little short on rank and medals. With 30% accuracy on TPK and 90 % accuracy from my reporters I am still able to take out the target by just getting a little further ahead. Sometimes just in time and sometimes a wait around, but I am in position early enough to use sonar to accurately set up my shots buy last minute maneuvering.

I would eventually like to see this part of game improved with TPK tables at cruising speed for all targets. Then as long as I am not spotted by the enemy they would hold the speed they were at allowing me to get ahead and then use my instruments to properly set up my shots.

yeah, i agree with u 200%, and also my proposal is that since TMO has the crew to help u with the ship recognition, can we add a sonar operater to help us listen 2 the revolution and estimate the target speed for us, after all, it's part of their job, right?

[joegrundman]

I think that's very interesting Joe Armstrong, but as you say data was incomplete for this to be a comprehensive method even later in the war

Here's what the 1950 submarine torpedo fire control manual has to say about it:

TURN COUNTThe Approach Officer should have available for ready reference the most complete set of curves that he can obtain of different types and classes of enemy vessels. Their use will furnish a means of approximating the speed of an observed target.
Due consideration must, of course, be given to wind and sea conditions and condition of loading as they increase or decrease the speed of any given R.P.M. Turn count is most valuable in determining a sudden change of target speed.



and a bit further on it says

(b) Although five methods of determining speed have been given above, it still remains that under almost all circumstances Plot and TDC are the two primary methods which the submarine must employ to obtain target speed. In both of these methods any error in underwater log speed will introduce a corresponding error in solution of target speed.

Now this is written in 1950, 5 years after the war finished, and with 5 years more experience and technological improvement and it makes it clear that turn count was not considered the primary speed determination method.

Referencing variations in wind and sea state as well as loading, and emphasising that turn count is most useful for registering changes in target speed, rather than an accurate estimate of target speed - one should ask before assuming that this is how it ought to be done in game, HOW ACCURATE WAS THIS METHOD AT THE TIME?

I think from this manual we can conclude that at the time, turn count was considered a supplementary source of speed estimate by the USN, and not a primary one - and the only reason this could be is that it was less reliable in most circumstances, with the technology of the day.

However, it is also clear that they were striving to improve the estimates furnished by this method.

ref for the manual: http://hnsa.org/doc/attack/index.htm