Thursday, February 26 2015
Trying to hit the broadside of a ship
With the number of variables involved in naval artillery, it’s a wonder warships can hit anything with their guns. The ship moves in a particular direction at a particular speed, the bow pitches up and down, the ship rolls from right to left, and the target moves. To these variables, add the Coriolis effect (due to the earth’s rotation), wind, temperature, barometric pressure, and humidity.
Fire control computers calculate the firing angle and gun elevation considering the forward motion of the ship and target, Coriolis, target lead angle, and range to the target. Sensors measure the pitch and roll of the ship. Fire control circuits feed this information to large motors that turn the guns and adjust the guns’ elevation. Even in heavy seas, the barrel of a naval gun will appear to remain motionless as the ship rocks beneath it.
The larger the gun, the heavier the gun, and the harder it is for the gun motors to keep the gun steady on a moving ship. Even a fraction of a second’s lag between the ship’s rolling and the motors’ adjusting the gun’s aim can cause the target to be missed.
What does this mean? Hollywood never quite gets it right, and in Vows to the Fallen I portrayed the battles as they really would have happened. The probability of hitting a target with a naval gun is surprising small. Here is a table showing the probability of hitting a cooperative target with sixteen, eight, and five-inch guns.
The table above starts at 0.35 nautical miles because the shells do not arm themselves for the first 690 yards. These values also assume the target ship is broadside to the firing ship. If the target ship is facing the firing ship, it presents a narrower target, and the probability of scoring a hit is less. At longer ranges, a bow-on target cuts the probability of a hit in half. This fact plays a significant role in naval tactics and in the battle scenes from Vows to the Fallen.
The table above also assumes a cooperative target, which means it maintains a constant course and speed. Normally, an enemy ship would not want to be a cooperative target and would zigzag or take other evasive action. Evasive action is a double-edged sword since, the more a ship evades, the harder it is to obtain a firing solution on the enemy. As I will explain in future posts, taking evasive action raises interesting questions about World War II naval doctrines that required arranging fleets in static formations in which evasive maneuvers were difficult at best. This will also be the subject of a future post. Again, in Vows to the Fallen, I attempted to portray the battles in a way that exposes the trade-offs and realities of naval combat.