Wind Behavior: 0400 - 0600

Discussions about the history of the ship, technical details, etc.

Moderator: Bill Jurens

Bill Jurens
Moderator
Posts: 743
Joined: Mon Oct 18, 2004 4:21 am
Location: USA

Re: Wind Behavior: 0400 - 0600

Post by Bill Jurens » Wed Apr 08, 2020 4:29 am

Range and deflection corrections for wind and target/firing ship motion can vary drastically depending upon the characteristics of the gun and the magnitude of the wind and speed one is experiencing. These corrections are discussed in detail in any number of textbooks on ballistics and fire control. To give you some idea of the orders of magnitude involved, for a fairly typical big gun at a range of about 20000 yards, range might be expected to change about 7 yards per knot of headwind or tailwind, about 10 yards per knot of ship speed along the plane of fire, and about 17 yards per knot of target motion in the plane of fire. These sorts of corrections are needed to get the gun on target when conditions do not match the zero wind, zero gun, zero target motion usually assumed in range tables. So for a 10 knot headwind, one might expect to see the projectile fall about 70 yards 'short'. This is, of course, corrected for by very slightly increasing the angle of departure. To increase the range by 70 yards, one might increase the angle of departure by about 3.5 minutes of arc, which is -- in reality -- a very small angle indeed. This is about 1 part in 1000, or the apparent width of the front door of a house a thousand yards away.

As range changes, there are associated small changes in the angle of fall with respect to the target and the striking velocity as well, but these are so small -- and, in any case completely beyond the control of the gunner, and so are usually ignored.

Again, a lot of this is covered in detail in most standard textbooks dealing with the delivery of accurate artillery fire.

Bill Jurens

paul.mercer
Senior Member
Posts: 886
Joined: Fri Mar 26, 2010 10:25 pm

Re: Wind Behavior: 0400 - 0600

Post by paul.mercer » Wed Apr 08, 2020 9:42 am

Thanks Bill,
Given all that data I'm surprised that ships managed to hit each other at all. particularly at any great range!
Re striking velocity, if and average 15" shell left the muzzle at around 2700 foot per second, what would it roughly be when it hit at say 20,000 yards and would the speed start to increase again when it started to fall once it had reached its maximum height?
This brings me back to the shell v armour debate,(off topic i know!) although one can say that a ships immune zone is X yards away, surely the whole thing depends entirely on the angle of impact, the shell itself and its construction and many other things including all that you said in your last post,so whether a particular shell penetrates a particular piece of armour or not is sometimes a matter of luck - as being able to hit another ship at long range in adverse conditions?

User avatar
marcelo_malara
Senior Member
Posts: 1242
Joined: Sun Oct 02, 2005 11:14 pm
Location: buenos aires

Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Wed Apr 08, 2020 4:56 pm

Paul, that´s why the hit rate is so low, about 2% in the battle of Jutland for example. You can find many ballistic data in the navweaps site.

Bill, thanks four your posting.

" for a fairly typical big gun at a range of about 20000 yards, range might be expected to change about 7 yards per knot of headwind or tailwind"

This is assuming the wind blows contrary to the shell during all the trajectory? Or just the sea level wind?

Regards

Bill Jurens
Moderator
Posts: 743
Joined: Mon Oct 18, 2004 4:21 am
Location: USA

Re: Wind Behavior: 0400 - 0600

Post by Bill Jurens » Wed Apr 08, 2020 6:02 pm

Mr. Mercer wrote:

"Given all that data I'm surprised that ships managed to hit each other at all. particularly at any great range!"

As many have noted, hits at substantial ranges were quite rare, and in many cases can be attributed as much to good luck as good skill.

"...if an average 15" shell left the muzzle at around 2700 foot per second, what would it roughly be when it hit at say 20,000 yards and would the speed start to increase again when it started to fall once it had reached its maximum height?”

It's difficult to define an 'average' 15" shell with great precision as there were so many variants. Whether these differences were, in practical terms, of any great significance remains a somewhat debatable issue. That being said, taking 2700 f/s as a given initial velocity, and equating to the mean muzzle energy of a variety of 15” guns, we get an ‘average’ projectile weight of about 1816 lb. Using Projectile Type 6 as a model and a reasonable mean air density, for 20000 yards we get a striking velocity of about 1623 f/s, at an angle of fall of 15.5 degrees. Time of Flight is about 29.4 seconds. The maximum ordinate, i.e. the highest point on the trajectory is only around 3500 feet, which is not much, and the projectile never begins to increase velocity during the descending leg of the trajectory – these sorts of striking velocity increases can occur, but only when the range is very long, nearly at the maximum range of the gun.

“… although one can say that a ships immune zone is X yards away, surely the whole thing depends entirely on the angle of impact, the shell itself and its construction and many other things including all that you said in your last post,so whether a particular shell penetrates a particular piece of armour or not is sometimes a matter of luck - as being able to hit another ship at long range in adverse conditions?”

Yes, luck has an awful lot to do with it. I’ve seen target exercises where one ship straddled four times in a row without getting a single hit, while the ship next in line got two hits in a single straddle. There are also large problems with armor penetration, as the zone of ‘mixed results’, i.e. the area where penetration or rejection of the incoming projectile is essentially unpredictable can be quite large. As a ‘rule of thumb’, if my recollection is correct, USN designers during the 1930s didn’t consider the armor thickness really reliable, protection wise, until it was about 11% over the calculated values. There is – and has been – a tendency to over-value the relative importance of purely technical issues, probably because these are most easily amenable to mathematical analysis, and to ignore other issues which are probably just as important but not nearly so easily quantified. When setting up for gunnery exercises, captains of USN ships during the 30’s hardly ever complained that their equipment wasn’t good enough, but they did complain a lot that various and sundry commands were, by subterfuge or other methods, able to raise their scores significantly by assembling a team of highly skilled and experienced individuals to man the gunnery team.

Bill Jurens

Bill Jurens
Moderator
Posts: 743
Joined: Mon Oct 18, 2004 4:21 am
Location: USA

Re: Wind Behavior: 0400 - 0600

Post by Bill Jurens » Wed Apr 08, 2020 6:18 pm

Mr. Malara wrote:


"...' for a fairly typical big gun at a range of about 20000 yards, range might be expected to change about 7 yards per knot of headwind or tailwind"

This is assuming the wind blows contrary to the shell during all the trajectory? Or just the sea level wind?'


Yes. Just the sea-level wind(s).

Technically one should take into account the variations in air density, temperature and humidity with altitude all along the trajectory, as well as variations in wind with altitude. This was rarely practical even in land-based artillery -- I worked on several projects using rocket vehicles to measure these parameters in real time before firing began, but (at least in pre-weather-radar days) one could could rarely if ever cover the entirety of a long range trajectory anyway, and (at least in pre-computer days) one could not perform the necessary calculations in real-time anyway, so one tended to fire one or more ranging rounds, and use those to correct further fall of shot empirically, knowing that if the engagement were a long one, even these corrections would be subject to various degrees of drift as, for example, the sun set and temperatures and winds tended to vary quite rapidly. All of this was more important in land-type engagements where friendly troops could be quite close to the required points of fall and misplaced shots could be disastrous, at least from one's own viewpoint.

At sea, even late in the game, measuring and accounting for small variations in atmospheric conditions along the path of the trajectory was(correctly) considered essentially impossible, so corrections were only done to compensate for observable and measurable effects, which amounted to relative motion of firing and target ship, and the effects of wind so far as they could be measured aboard the firing ship. This amounts to assuming that weather conditions at the target and along the flight line were, in practical terms, identical to those seen aboard the firing ship.

Bill Jurens

User avatar
marcelo_malara
Senior Member
Posts: 1242
Joined: Sun Oct 02, 2005 11:14 pm
Location: buenos aires

Re: Wind Behavior: 0400 - 0600

Post by marcelo_malara » Fri Apr 10, 2020 7:52 pm

Thanks Bill.

Post Reply