dispersion of battleship guns

[Colorado]

.do you know what is the dsipersion of the guns of the battleships for example Iowa=? If the guns fire from 15 miles how close or far from the target get the shells? thanks

[Werner]

A good rule of thumb is dispersion will be 1% to 2% of range for really good guns like British or Japanese cruisers, and much more for bad guns, say, USS Portland in the early 1930s.

Dispersion depended on a number of fixed and variable factors, including the effective distance between barrels, temperature, humidity, range, uniformity of powder, and whether the barrel was hot or cold. US cruisers improved in dispersion in the 1930s by adding delay coils, which made the firing signal travel through a much longer path before igniting the powder. This increased the effective distance between the bores by making one gun in a turret fire a 10th of a second or so after the others. In the 1980s the USN added a doppler Radar over the center gun of each turret on the Iowa class to measure the initial velocity of each round. This simplified the procedure as it stood in for such variables as powder temperature and dampness. These ships frequently had dispersion much better than 1% of range as a result.

[Colorado]

thank you Mr. Werner.
If we take a 1% dispersion for example and the guns fire from 10 000 yards the dispersion is 100 yards. this means the shells can land anywhere inside a circle with a diameter of 100 yards -then to get hits inside that circle you just have to be lucky is that correct explanation?

[Werner]

Generally speaking, that would be correct. It's also important to remember that dispersion also changes over the lifespan of the barrel liner, and over the course of a battle, as the gun tube changes temperature.

Most dispersion is caused by interference of the shock waves of adjacent guns when they are fired at about the same time. In the 1920s and 1930s, naval scientists added long spools of wire to "slow" the ignition of the middle tube, so it fired well after the others. These delay coils could also be found in other navies. Nowadays, you could order computers to fire with several millisecond delays, but in the early 20th Century, they had to rely on the physics of wire to cause such events. Other navies, like the RN, used alternate gun firing to halve the rate of fire, but greatly increase the accuracy. In the USN this was not possible, because in several classes of ships the guns were all on one slide and had to be loaded and fired as a single unit.

The Japanese Navy was probably unique in having a manned instrument which could order a slight change in dispersion aboard the Yamato, Mogami, and Tone classes.

As you can see in the illustration below, it is possible to target the "E" ring within the Pentagon with two salvos at 12 1/2 and 25 miles from the target. Of course, when the battleships were retired, there were active programs for guided projectiles, sabot and "Arrowhead" projectiles with extended range (250+ miles) and GPS accuracy. You can see the benefit when you compare a $1,000 16-inch shell to a $1 million "reduced price" Tactical Tomahawk.

In November 1987, with the doppler Radar and rebagged propellant, USS Iowa demonstrated a pattern size of 219 yards at 34,000 yards (0.64% of range), with a standard deviation of +- 123 yards (0.36% of range), probably the best performance ever demonstrated with a large gun at long range.

[chuck]

thank you Mr. Werner.
If we take a 1% dispersion for example and the guns fire from 10 000 yards the dispersion is 100 yards. this means the shells can land anywhere inside a circle with a diameter of 100 yards -then to get hits inside that circle you just have to be lucky is that correct explanation?

Actually, when you say the dispersion is 100 yards, that means statistically 50% of the shells fired will land in a circle with a 100 yards diameter circle about the point of aim. You may have one bad barrel that makes wild shots, but it will not count towards your dispersion so long as half of all barrels firing are doing much better than that one bad barrel. In a more modern era 1/2 of the dispersion would would become known as "circular probability of error" and used to measure the accuracy of ICBMs.

When firing against a cruiser sized target 200 meters long and 20 meters wide, an accurately aimed 9 gun salvo with a 100 yard dispersion can expect to achieve approximately 2 hit in every 3 salvos at long range.

[chuck]

The Japanese Navy was probably unique in having a manned instrument which could order a slight change in dispersion aboard the Yamato, Mogami, and Tone classes..

The IJN was not unique in this. The Italians at least had similar electrical firing circuits that caused adjacent guns in a turret to fire with a several hundredth of a second stagger between them. But the design of the Italians guns caused wide dispersion nonetheless.

[Werner]

The IJN was not unique in this. The Italians at least had similar electrical firing circuits that caused adjacent guns in a turret to fire with a several hundredth of a second stagger between them. But the design of the Italians guns caused wide dispersion nonetheless.

I think you misunderstood. What you describe are delay coils, and many navies used them. The Japanese, I believe, were unique in having a separate instrument with telescope whose job was to "set" the dispersion or concentration on the ships cited above. It may have been related to the "diving shell".

[Colorado]

Excellent explanation Mr. Warner. thank you very much.
When I look in your illustration I see that at the distance of 25 000 yards the distance between the two shells that landed more away from center target is about 600 yards (+400 , -200 in the axis). That would be about 2.5% dispersion.
then when I look at the distance of 36 000 yards the distance between the most away shells is about 800 yards, that would be about 2.3% dispersion. is corious there is more dispersion in 36 000 yards distance than 25 000 distance.

[Guest]

[Actually, when you say the dispersion is 100 yards, that means statistically 50% of the shells fired will land in a circle with a 100 yards diameter circle about the point of aim. You may have one bad barrel that makes wild shots, but it will not count towards your dispersion so long as half of all barrels firing are doing much better than that one bad barrel. In a more modern era 1/2 of the dispersion would would become known as "circular probability of error" and used to measure the accuracy of ICBMs.

is that true? the dispersion only take into account 50% of the shells fired?
then in the illustration of Werner maybe some shells are from bad barrels? and the dispersion is really 1,5% for 25 000 yards and 1.2% in 36 000 yards?