Why is a straight shot assumed to be the perfect firing angle?

[fireship4]

Why is it that a straight shot is assumed to be the perfect firing angle?

I used to believe that this was because if a torpedo has to turn this will add an error but im not 100 percent sure now - isnt the "torpedo reach" or "torpedo advance" as it's called taken into account? I'm sure you can see it on the attack map.

Would it not be a better solution to have yourself on the target's course and fire when the range is smallest and the track is close to 90 degrees off his bow? This way you can use bow and stern tubes almost simultaneously - maybe bow shots wait a few more seconds until track reaches 90 degrees on the map, i don't know - better than turning around.

Maybe there is some mixup between AOB and gyroangle. I was about to call this thread "why is 90 degrees assumed to be the best shot" but i guess thats because the ship presents his full side to you and you have more chance of hitting. But this doesn't mean you have to shoot straight aswell...

Or maybe im wrong and having a gyroangle causes error.

UPDATE I tried it from 1000 yards at a stationary target with 8 tubes simultaneously. All hit. Text in below picture that is unclear says "20 deg off perfect angle".

Next I will try a moving target...

http://i11.photobucket.com/albums/a155/fireship4/TorpedoFiring1a-1.jpg

http://i11.photobucket.com/albums/a155/fireship4/TorpedoFiring2a-1.jpg

[tedhealy]

The best firing angle is the one that hits.

Also don't forget you are more likely to get duds early in the war when the torpedo hits at 90 degrees.

[tedhealy]

Here's a bit of advice from an actual sub manual

http://www.hnsa.org/doc/attack/index.htm

(d) The optimum torpedo track angle for a 16 knot target for a 46 knot torpedo is about 110 degrees and for a 29 knot torpedo about 125 degrees.
(e) The greatest advantage of straight fire (small gyro angles) is that errors in torpedo run have no appreciable effect on the solution. Therefore, when the range is inaccurate, as in stadimeter and telemeter scale approaches, the submarine must maneuver for a small gyro angle shot.




If I have the time, I set up for a near perpendicular approach, but offset it by about 10 to 20 degrees to avoid the higher chance of duds.


If I don't have time for a long setup or the ships are escorted, I take what I can get.

[fireship4]

I was reading through the manual (infact i had posted a link to those sections of it onto the board just before) to confirm this when i realised that you will still get the shot you want with this method. I'll post a couple of annotated screenshots as I test it.

UPDATE Here is a snippit of the relevent part of it:

810. ANALYSIS OF TORPEDO FIRING:
(a) Straight Fire
Torpedo firing in which small gyro angles (less than 30 degrees) are used is considered to be "Straight Fire". The curves plotted on plates XVII (http://www.hnsa.org/doc/attack/index.htm#platexvii) and XVIII (http://www.hnsa.org/doc/attack/index.htm#platexviii) for 46 and 29 knot torpedoes were developed by plotting the deflection angle against the torpedo track angle for different target speeds. It should be noted that in all cases the gyro angle was zero.
(b) The slope of these curves at any point is the instantaneous rate of change of deflection angle with torpedo track angle. The optimum torpedo track angle for any given target speed is the torpedo track angle for which the rate of change of deflection angle is the least. This is indicated on the curves by the shaded areas.
8-8

[fireship4]

Yes it seems to work with a moving target no problem. And seeing as the gyro angles im using are considered "straight fire" and the track angle is within the guidelines (i think) then im going to try it out more on patrol (when i get out of my lovely s-boat) - I advise other captains to give it a go aswell. The red text above is wrong

Heres some more from the manual:

(c) It is within this range of torpedo track angles that the greatest amount of course error can be absorbed. From a study of the curves it is evident that the maximum deflection angle is obtained when firing on the optimum torpedo track angle and that the optimum torpedo track angle has a value equal to 90 degrees plus the maximum deflection angle. It is also evident that as the target speed increases for any given torpedo speed the slope of the curves becomes sharper. This means that the higher the target speed the greater the rate of change of deflection angle with torpedo track angle. It is therefore true that the optimum torpedo track angle is more effective for absorbing errors in course when the ratio of torpedo speed to target speed is large. It therefore may be stated that the optimum torpedo track angle is a good mean torpedo track angle for firing a salvo of torpedoes if the target speed is less than one-half of the torpedo speed. (d) The optimum torpedo track angle for a 16 knot target for a 46 knot torpedo is about 110 degrees and for a 29 knot torpedo about 125 degrees.
(e) The greatest advantage of straight fire (small gyro angles) is that errors in torpedo run have no appreciable effect on the solution. Therefore, when the range is inaccurate, as in stadimeter and telemeter scale approaches, the submarine must maneuver for a small gyro angle shot.




Actually, from this (correct me if im wrong) 90 is not the perfect track angle (degrees from bow torpedo hits on i think) for our torps. Its more like 115 degrees (with mark 10s)?

[joegrundman]

The specific reason why straight shots are more accurate than curved fire is torpedo tube parallax. it's all there in the manual that you are discussing.

the manual also says that if you have radar range finding, there is no appreciable difference in accuracy with curved fire, but regrettable, radar rangfinding is incompletely modelled in SH4

[fireship4]

Yes it does - here is the section on curved fire:

Torpedo firing in which large gyro angles (over 30) are used is considered to be "Curved Fire".
(a) When using curved fire an additional angular correction must be applied to the deflection angle to correct for reach and turning circle of the torpedo. This correction is automatically computed in the angle solver section of the TDC. This correction varies with torpedo run. The following table was made up by setting up the TDC for target speeds of 10, 15, and 20 knots and adjusted for a starboard 90 degrees torpedo track and 1000 yard torpedo run with gyro angles of 20, 40, 60 and 90 left in each case. The torpedo run was then increased to 1200 yards and the gyro angle difference recorded.

http://i11.photobucket.com/albums/a155/fireship4/table.jpg

From examination of this table it may be readily seen that for a torpedo run error of 200 yards as the gyro angle increases the angular error becomes larger and larger.
(b) In order to have a correct solution of torpedo run it is mandatory that an accurate range be available.

8-10


"Curved Fire" should not be used ten an accurate range is not available.

(c) When radar ranges are available it is not necessary to maneuver to obtain small gyro angles. It has been found in many firings at the Submarine School that when using radar ranges the percentage of hits obtained is the same with "Curved Fire" as with "Straight Fire".

So did stadimeter shots have such a high margin for error? Sounds like it. We dont have this.

[Quillan]

One of the things I've come to love about Trigger Maru is the effect that some of the changes Ducimus made have in the game. He changed all the map icons, so the contact boxes have no tail to give exact course and all ship silhouettes are now just a dot on the map so you can't tell ship type or course from it. The result is that leaving map update on doesn't give away a bunch of information you wouldn't get. As long as you have SJ radar, even the rather precise range you can measure off the nav map isn't something you would not have.

[joegrundman]

So did stadimeter shots have such a high margin for error? Sounds like it. We dont have this

What do you mean we don't have this? Of course there is a margin of error!

I may be wrong, but I think you are confusing something here.

The anglesolver will correctly solve for the input you give, no matter what the angle is, and without regard to the means of gathering data, be it stadimeter or radar rangefinding.

The error arises because of torpedo tube parallax. Somewhere in that document there is a clear description and diagram of this phenomenon. In straight fire the mathematics of triangles means that in most cases, even if your range is wrong by a big margin, you'll still hit.

If you have a perfect right-angled triangle set up with a 000 gyro angle on a tta of 90, then you do not need to input range at all and it will always hit the target so long as your speed solution was correct.

However, because you are making a curved shot, the torpedo correction (for torpedo reach and turning circle as well as the difference in position between the torpedo tube and the scope) is highly dependent on an accurate range.

If your range is inaccurate, then your torpedo shot will not hit the target as you'd hoped. The greater the bearing to target, the more this is the case.

The reason the manual states that curved shots with radar range are more accurate than with stadimeter is simply because radar raneg finding is much more accruate that stadimeter rangefinding.

Hitman had a nice diagram of the effect of torpedo tube parallax here:

http://www.subsim.com/radioroom/showpost.php?p=639851&postcount=1

[fireship4]

I was talking about incorrect range estimation - the table references to a target measured with the stadimeter, incorrect by 200 yards. Therefore I was wondering whether this was a common occurence.

[joegrundman]

OK I see (or at least i think i see :) )

A 200 yard error is certainly possible in rough seas or poor visibility or inaccurate target identification if using the stadimeter.

But the reason for showing variation with a 200 yard error over a 1000yard range, is not that that error is particularly representative, but simply to show how with a constant margin of error, the difference in gyro angle is more critical the greater the curve, with it up to 5 degrees.

Here's an example.

To quick calculate the angular lenght of a target we can use

length of target in feet/100 x 2000/range in yards x sin tta

because we are always talking about tta 90, sin tta=1

Let's say there is a small merchant, length approx 250feet and 1000 yard range

250/100 x 2000/1000 = 2.5 x 2 = 5 degrees

That's what the target subtends.

If it is moving at 10 knots then the table shows the gyro angle error with a 200 yard range error is

20 degree curve = 0.5 degrees
40 degree curve = 1.5 degrees
60 degree curve = 2 degrees
90 degree curve = 5 degrees

If only the range solution is incorrect, then you will still hit the target at curves of 20-60, but a 90 degree curve may well miss entirely, or just clip the bow or stern.

You can take care of this by using a spread.

Hope this clarifies things:D

joe

[fireship4]

I understood that this was what the manual was saying, I was just wondering if we get these sorts of errors in game - they may have been due to innacurate recognition manuals too (i think ours may be perfect). So therefore this may be an aspect we are missing.

Thanks for trying to explain it though.

[swdw]

We hada discussion on range finding among the RFB guys. Here's one of the posts

WARNING_ LONG- but some really good info on this subject

From the fleet boat handbook


The value of the sonar operator to the TDC

By this time it should be apparent why the sonar operator is so important to the TDC. The essential information for TDC comes from various sources:

1. Own ship's course is registered AUTOMATICALLY from the master gyrocompass in the control room.

2. Own ship's speed is also fed AUTOMATICALLY, from the submarine's "log" (actually a speedometer at the keel).

3. Target's course is either estimated by PERISCOPE observation of the angle at which the target ship is traveling, or by plotting from other data. Since this information is only approximate, it is modified as the problem progresses.

4. Target's speed is estimated principally from the turn count obtained by the SONAR operator, or from the periscope identification of the target. This information also is approximate. Therefore, it too is corrected as the problem demands.

5. Target's range can be accurately determined by SONAR single-ping echo-ranging. However, this is used only to supplement estimates from periscope observation,

6. Target's relative bearings can be determined spottily by brief periscope observation. But for a continuous flow of relative bearings, the TDC operator depends on SONAR. (my note, this would be passive sonar which was accurate for bearing in the hands of a good operator)
This is why the sonar operator must give bearings continuously after contact. If he fails to do so, TDC operations are seriously handicapped.

A continuous flow of accurate sonar bearings is important for TDC operation and for a successful torpedo attack.

As you can see, there was actually a very heavy reliance on the sonar (hence the sonar reporting buttons at the attack map and scope positions with the new order bars)

The references I'm seeing with regards to the inaccuracies not only mention the rec manual,but also the stadimeter itself. Remember the lining up of the ships in the stadimeter is a judgement call based on experience of the person manning the scope, the distance to the target, and the visibility. At a longer range accurately lining up the images is more difficult.

Here's some info from another site that talks about an attack approach through the scope:

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

This image shows the view through the periscope with the stadimeter in use. A split prism is used to superimpose a second image of the target over the actual image. The captain adjusts the prism so that the waterline of the second image is set on the masthead of the actual target image. The height of the masthead from the water is entered on the dial, and the reading obtained. The stadimeter actually measures angles, not distance. If the masthead height is entered accurately, the range will be correct. Getting the masthead height wrong gives an incorrect range. (The same principle is used by surveyors, though they have the obvious advantage of basing their ranges on a graduated pole of known length held by an assistant.) In practice, the most accurate ranges were always obtained during exercises, since the subs were operating against units of their own fleet, and masthead heights were always known. Enemy warships and freighters often involved a certain amount of guessing, though recognition books were careful to list masthead heights whenever they were known.

Approach Procedure

Once a submarine finds a target, the approach and attack is essentially an exercise in geometry. The captain needs to determine the precise angle at which to fire his torpedo so that it will hit the target.

In stationary objects, this is easy. You simply point the torpedo directly at the target and, so long as it travels in a straight line, it will hit it. The problem with this, obviously, is that neither the submarine nor the target is likely to actually be stationary. With the rare exception of attacks on anchored vessels—Prien's attack on HMS Royal Oak in Scapa Flow being, perhaps, the best known example—submarines normally encounter their targets at sea, where both the submarine and the target are almost certainly going to be moving.

In this situation, you can't shoot at where the target is. Instead, you have to shoot at where the target will be when the torpedo reaches it.
Bearing
approach01

In this graphic, the approach has begun. The submarine is moving due north at 2 knots. The target is moving due west at 6 knots, and is currently located to the east of the submarine's track, at a range of four nautical miles. (In order to fit everything into the graphic the distances and sizes of the vessels are not to scale. Also, the submarine is shown surfaced for clarity—it would be submerged if this was actually happening.

First, the captain centres the periscope's crosshairs on the middle of the target, or on the point on its hull where he wants the torpedo to hit, calling out, "Bearing." At the moment he has the target exactly centred he then calls out, "Mark!"

The Approach Officer reads the bearing off the bearing ring located on the periscope shaft. This bearing gives the relative angle from the submarine to the target. In this case, 45°. For clarity, the Approach Officer announces the bearing as, "Bearing—zero-four-five." (Target bearings are always given as three numbers, and the digits are always given separately. "Zero-four-five" is less likely to be misunderstood than "forty-five degrees." This is particular so since lookouts call out bearings as "starboard four-five," using two digits and always referring to the side of the ship the sighting is on. Some navies use "red" for port and "green" for starboard in making sighting reports, these being the colours of the navigation lights on those sides.)

Once the target bearing has been determined, it is entered into the Torpedo Data Computer (TDC). This is a highly sophisticated electro-mechanical analogue computer. Two basic types were used during World War II. In most navies, the TDC was an angle solver only, which would give the correct gyro setting for the torpedo based on the data entered at the time of the reading, or at a given time in the future, based on the captain's best guess of where the target would be. The American version added a position keeper, which was capable of keeping track of the target's course in real time. This was a significant advance on the older systems, and made for much more accurate target solutions by eliminating most of the guesswork.

The TDC will always know the submarine's course and speed, as these are constantly updated from the master gyro compass and log. (This log is the submarine's speedometer, by the way, and not the book the captain uses to keep track of daily events.) The TDC now also has the target bearing, but still doesn't have enough information to work out a target solution.
Range to Target

Now the captain needs to determine the range to the target. To do this, he first needs to know just what the target is. Looking through the periscope he can see that it is a large freighter. Submarines carry recognition books which list every enemy warship and merchantman on which information is available. Looking through this book the captain finds the Oyama Maru, a 4,750 ton Japanese freighter, which seems to be the ship he has in his periscope. Since it is mid-1944, and World War II is raging, he decides this is a legitimate target, so he continues with the approach.

Now that he knows—or, at least, believes he knows—the identity of the target, he looks up the masthead height. This is the distance from the waterline to the heighest point on the ship. According to the recognition book, this is 100 feet. This figure is entered into the stadimeter in the periscope.

Range may also be determined by using the active sonar on the single-ping setting. This is one of the two most accurate methods, as it doesn't depend on knowing the target's masthead height. Late war American submarines also incorporated a tiny radar antenna in the search periscope, which would also give an exact range, at the risk of throwing up more spray than the thinner attack periscope.
view through periscope

This graphic shows what the captain sees through the periscope's stadimeter. A split prism is used to place a ghost image of the target so that its waterline is sitting right at the top of the masthead of the "true" image. The stadimeter actually registers the angle above the horizontal to the masthead, not distance. Some basic math is then performed which translates that angle into a distance figure.

The way this works is that, viewed from any particular distance, an object of a given height will be at a particular angle. If you know that the angle of view is 1°, for instance, and the object is 100 feet high, you can calculate that the angle of view and the top of the object will touch only at a distance of one nautical mile. The stadimeter simply does the math for you.

One disadvantage of this, of course, is that accuracy is completely dependent upon knowing the correct height of the object. (In this case, the masthead height of the target.) In our example—but not in the graphic, where the ship is considerably closer than it would be in a true view—the masthead height turns out to be 1/4° above the horizontal. Using the formula R=h/tan(q) this means that the target is four nautical miles from the submarine. The stadimeter does this internally, without the need for the captain or approach officer to calculate it, and indicates that the target is 8,100 yards away.

This figure is read off a dial at the base of the periscope and entered into the TDC, providing another part of the solution.


Also from the fleet boat manual


The second method of obtaining ranges is by means of the stadimeter installed in the Type II periscope. The stadimeter relies for its operation upon the formation of two identical images which can, by means of a handwheel on the periscope, be vertically displaced with relation to each other. Normally the handwheel is at the limit of its counter-clockwise travel. To obtain a range, the handwheel is turned clockwise until the target masthead in one image coincides with the target waterline in the other image. The range is then read on the stadimeter scale opposite the appropriate masthead height. In Plate III, a picture of a stadimeter scale, a masthead height of 60 feet gives a range of 2300 yards. Note that the scale is constructed for high power observation. When ranges are measured in low power the computed value must be divided by four.

(d) The third method of obtaining ranges is by use of the radar installed in the Type IV periscope. In this method the range of the selected target is measured directly by the ST Radar Operator when the periscope is raised and trained on the target.

(e) Of the three methods the radar ranges are the most accurate and depend primarily upon the adjustment of the radar which is usually plus or minus 35 yards. The accuracy of telemeter or stadimeter ranges depend first, upon the skill of the observer and second, upon the accuracy of the estimate of target masthead height.

(f) The value of the masthead height of the target may be obtained by intelligence, estimate, or by a method referred to as "radar stadimeter" (telemeter) estimate. The latter of course is the most accurate and is accomplished as follows; assuming that the target has been tracked using the ST periscope, the Type II periscope is raised immediately following an ST periscope observation, a stadimeter range observation is made as described above, but instead of reading range on the scale, the masthead height is read opposite the value of the TDC generated range.

(g) When radar ranges cannot be obtained the Approach Officer must rely upon his ability to correctly estimate the height of the funnel or masthead, or other prominent mark on the ship's structure above the water line. If the target ship can be properly identified an accurate value may be obtained from intelligence information supplied the ship. If this is not available the following procedure will he of assistance:
(1) Count or estimate the number of decks that are seen above the main deck.

(2) Add to this figure the approximate number of deck heights equal to the observed freeboard.

5-5

(3) Multiply the total by eight to determine the height of the top of the bridge structure above the visible waterline.

(4) Using height of bridge structure above the visible waterline as a yardstick, approximate the masthead height. The masthead heights of merchant ships are on the average about 2.1 times the bridge height (above waterline). A masthead height which appears to be shorter than normal will be about 1.7 to 1.8 times the bridge height, while one which appears to be higher than normal is approximately 2.2 to 23 times the bridge height.

(5) Funnel heights may be estimated by approximating the number of deck heights of the funnel which is seen above the top of the bridge structure and adding this height to the bridge structure height.

(6) At extreme ranges it must be remembered that the waterline is below the horizon. This necessitates estimating the position of the waterline.

5-6

(h) The following points should be kept in mind in height determination:

(1) Masthead heights may be purposely altered by the enemy to cause inaccuracies in periscope ranges.

(2) Tops of masts may be camouflaged in such a manner as to be invisible under average visibility conditions at any except short ranges.

(3) Funnel height is normally sufficient to insure that the smoke which is blown in the direction of the bridge by a tail wind will pass well over the bridge.

(4) Coal burners require taller funnels to insure adequate draft.

(5) Funnels of modern vessels having forced draft do not require as tall a funnel as older vessels without forced draft.

(6) Diesel propelled ships require no draft. Funnels are normally short, are not required, and generally have such dimensions as to provide a good appearance on the ship.

Regardless of the methods employed by the individual Approach Officer, skill in estimating masthead heights, and ability to obtain accurate ranges can be acquired and maintain only by constant practice. Even when radar ranges are available daring an approach the Approach Officer should also obtain telemeter ranges as a means of improving and maintaining his skill.

[swdw]

I understood that this was what the manual was saying, I was just wondering if we get these sorts of errors in game - they may have been due to innacurate recognition manuals too (i think ours may be perfect). So therefore this may be an aspect we are missing.

Thanks for trying to explain it though.
This has already been added to the list of things to change after the next RFB release ;)