Thursday, March 19 2015
How fast can those big guns shoot?
So far, I have discussed the most important aspects of naval artillery, but there is one more topic to cover: a naval gun’s rate of fire and what difference rate of fire makes.
It takes about a minute to reload the big battleship’s sixteen-inch guns, so they can fire once per minute. On the other end of the spectrum, the destroyer’s five-inch guns can fire one round about every four seconds, or fifteen rounds per minute. Rate of fire can affect the outcome of battles, as I demonstrate in my next novel Vows to the Fallen.
A battleship attempting to bracket an enemy ship at ten nautical miles has to wait thirty seconds for their shells to fall, but that is not a problem since they can only fire once per minute. However, a destroyer firing at a target five miles away has to wait twenty seconds before their shots fall. In that time, they could have fired five more times. So what’s a captain to do? Keep firing even though he might not have the correct range, or wait until his shots fall so he can get the right range and not waste ammunition?
With the smaller five-inch guns, it makes sense to ladder the shots so the ship can keep firing. However, the US Navy ships had the benefit of radar and at the shorter ranges of the five-inch guns bracketing or laddering wasn’t necessary. The Japanese didn’t use radar, and this put them at a distinct disadvantage. Readers will be able to see this play out in Vows to the Fallen in the Battles of Kogeri and Ubella Atoll especially when the Japanese Cruiser captain tries to conserve ammunition. It turned out to be a mistake since his eight inch guns could have fired four salvo's per minute!
Thursday, March 12 2015
The most important aspect of naval artillery
To obtain a targeting solution on an enemy ship there are three variables: target speed, target course, and range to the target. It takes about three minutes to obtain a reasonably accurate calculation of a target’s course and speed mathematically. Sometimes ships used quicker methods (eyeballing the course and estimating speed based on the bow wake of the target) in an emergency, but that sort of accuracy depends on the skill and experience of the targeting crews.
Before radar, obtaining the range to a target was the most difficult of the variables. In this regard, the US Navy had a distinct advantage over the Japanese because the first mention of the Japanese using radar on ships wasn’t until 1944. Still, there are numerous variables relating to the atmospherics that could affect range accuracy, especially at long range. Naval gunners assumed a few salvos would be required to obtain the correct effective range to the target.
Gunners used one of two methods to obtain the proper effective range: laddering and bracketing. Gun crews used laddering at longer ranges. When using laddering, ships would deliberately fire their first salvo short of the target and then increase the range until the shots fell on the far side of the target. Once the shots fell long, gunners could make a final attempt to obtain the correct effective range.
Using the bracketing technique, ships would observe the hits from their first salvo. If the shot landed long, the ship would try to make the next salvo short and vice versa. With one shot long and one shot short, spotters would estimate the distance between the shell impact points and interpolate to obtain the range to target.
One interesting aspect of bracketing is the target ship can almost estimate where the next shot will fall. Readers will see O’Toole using this in Vows to the Fallen. During World War II, US ship captains got into the habit of changing course so their range from the enemy would be the range of the last shot fall. When the enemy adjusted their range and fired again, their shells would fall at the range the US ship was before it had changed course. The common thing to do was to steer for the last shell impact point. As you can guess, this could result in a guessing game between ships.
Thursday, March 05 2015
How long does the enemy ship have to get out of the way?
Hollywood movies sometimes portray incoming projectiles by the sound they make as they scream overhead, but I have yet to see a movie that exposes the problems created by projectile flight times. For example, if a five-inch gun fires at a target ship at 10,000 yards (five nautical miles) away, and the target ship is traveling at thirty knots, the target ship would move more than the length of three football fields before the shell hits because the flight time of the shell would be twenty-two seconds. In Vows to the Fallen, readers will see how flight times can affect a battle.
The navy considered the destroyer’s five-inch guns as dual-purpose weapons usable against surface and air targets. Standard procedure was for destroyers to open fire on incoming aircraft at 9,000 yards. Even assuming flight times are linear with range, which they aren’t, the shell wouldn’t hit the aircraft for thirteen seconds at a range of about 6,000 yards.
Flight times get larger the greater the range and the larger the shell. Here are some example flight times for five, eight, and sixteen-inch projectiles.
Projectile flight times are important because the time a projectile is in flight is also the time the target ship has to evade. In a previous post on accuracy, I discussed how the probability of hitting the target depended on the target’s being cooperative and not changing course or speed while the projectile was in flight.
In the next post, I will cover targeting techniques and pull all of the artillery blogs together with a final post on rate of fire.
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.