Today we have a dev diary for you all on torpedo mechanics in RTW3.
Torpedo hit chances is a topic that is a frequently discussed topic among players of RTW3. Torpedoes are a serious threat, and in RTW3, as in real naval combat, a single torpedo hit can mean the difference between victory and defeat. A torpedo hit on a heavy warship will lead to serious damage and speed loss, which might expose the ship to further danger. At the very least, you will face a couple of months repair time. As an example, a single torpedo hit on the Scharnhorst after she sank the carrier Glorious was a career ending event for the commander of the German force, Admiral Marschall.
Players are likely to take more risks than real admirals. After all, it is not their lives and careers that are at stake, only some electrons. But it can still be very annoying to have your victory spoiled by unnecessary torpedo hits. Thus, understanding torpedo combat and torpedo hit chances is important. Torpedo firing and hit chances is actually a much more complex subject that one might think, as we explain below.
First, torpedoes are not all that much faster than a warship. Roughly, a torpedo at fast setting will have about 1.5 times the speed of a fast warship (both ship and torpedo speeds improve over time). And torpedoes in the time frame of the game are not homing, they will continue on a straight course after they leave their launch tube. This means that someone firing a torpedo at a moving ship must lead the target by a considerable distance. It also means that torpedoes fired from abeam or on the stern quarters must overtake the target ship, and that means they have considerably shorter range and will hit at a more disadvantageous angle.
To have a maximum chance of hitting, a torpedo should be fired so that it will intercept the target ship at 90 degrees angle. At anything else than 90 degrees, the apparent size of the target will be smaller. Also, any minor course change by the target will have a larger impact on the hit chances.You can see this in the image below. A torpedo fired from a destroyer at about 60 or 300 degrees bearing from the target ship (Torpedo track 1) will have the best chance of hitting, while a torpedo fired from straight abeam, 90 or 270 (Torpedo track 2), will actually have longer to travel and will intercept the ship at an angle that gives lower hit chances.
To further illustrate this you can see the diagram below, which shows a threat circle for torpedo fire against a moving ship. It illustrates the bearings at which a torpedo firing enemy ship within the possible range circle will have the best hit chances, and thus be the largest threat. The sectors marked shows the level of threat from a destroyer on those bearings. The size of the target ship in the illustration is exaggerated compared to the torpedo range.
Note that the circle that illustrates torpedo range will be offset ahead of the ship, as the ship will be moving forward. This also illustrates why simply showing the torpedo range from a ship in the battle plot will not give a correct idea of the true torpedo threat area, and can actually be misleading.
Thus, if you have enemy destroyers in the high threat sectors, you should change course, and the course change should be at least 45 degrees, or more if the enemy destroyers are closer. Torpedo evasion should be done by turning together, so that the trailing ships in the column will not enter the risk area of torpedoes fired at the leading ship.
It might seem tempting to turn towards the torpedo firing ship, and that can work, especially if it is a single ship, but it does carry higher risks. If several destroyers are firing torpedoes, you cannot be certain which angles the torpedoes are coming from.
Speaking about torpedo avoidance, when the AI decides to do an evasive maneuver, it makes its decision on the presence of enemy destroyers in the risk areas in the illustration above. It will evaluate the position of enemy destroyers depending on range and bearing, and then make a likelihood roll. If it makes the roll, the AI will evade. The AI does not ever take torpedoes in the water into account for these decisions.
You can actually use this to your advantage. If you send in a group of destroyers on the port or starboard bow of an enemy battleship division, even destroyers that have spent their torpedoes, the AI is likely to turn away in a few minutes. You can then recall the destroyers before they are chewed up by defensive fire. By making such feint attacks you can disrupt the enemy formation and upset their gunnery solutions. Sending destroyers at the enemy is also an excellent way of facilitating your own retreat, as the enemy will be compelled to turn away and open the range. In that way, destroyers are the cavalry of a naval force.
When the AI fires torpedoes in RTW3, it uses a digital version of a torpedo firing computer fitted to warships in WW2, with the result made more fuzzy depending on year and tech development. The inputs are speed and course of the target ship, and speed of the torpedo. The output is what angle the torpedo should be fired at. Once fired, torpedoes in RTW3 will continue straight ahead, just like real torpedoes of the time. If they intersect a ship, a hit check is made depending on target size and angle, with a chance for the target to do last minute evasion. When a torpedo fails the hit check, you will see a message that one of your ships is evading torpedoes. Very occasionally, your ships might also report imaginary torpedoes, as happened throughout history.
At the end of the game, homing torpedoes do appear. These were initially only deployed in small numbers and in specific circumstances, usually by or against submarines. This is simulated in RTW3 by giving them better hit chances, which is admittedly a simplification.
If you want to read more about torpedo firing solutions, there are several articles on the internet.
Here is one about the torpedo computers used in US submarines during WW2.