American militarism pre-WW2 and the Ultimate Battleship

[TheMarcksPlan]

6 - 1 is not a high B:L ratio, it is roughly that of a WWI dreadnought and nothing like the roughly 10:1 the Japanese needed for their cruisers to

You're conflating "B:L" and "L:B".
1:6 is a high B:L ratio; 6:1 is not a high L:B ratio.
I stated "high B:L ratio."

Have you ever tried to use any of the ship design programs out there to create what you are proposing?

No. The primary fun in this exercise is learning what dictates ship HP/speed, understanding how the tradeoffs went historically, and how they might have gone at different scales. An accurate program would steal that fun from me; an inaccurate program based on simplistic rules of thumb would perpetuate bad common knowledge.

I want to understand, for example, why Emma Maersk needs less horsepower than SoDak BB's for near equal speed, despite Emma being ~5x larger.

With that length to beam ratio a high speed is very difficult to obtain, which is why the Iowa's had such a long thin bow

The biggest thing missing from your analysis is the relationship between hull length and wavemaking resistance; longer hulls have less wave resistance (all else being equal). Again, this depends on speed-length ratio: (S in knots) / Sqrt(waterline length in feet).

As megaship is ~4x longer than Iowa, at 33 knots its proportional wavemaking resistance would be similar to Iowa's at 16.5kn (33 / Sqrt(2)). For illustration of the issues, here's Basic Ship Theory:



[To convert S/L ratio to Froude number, multiply by .298]

As you can see, Iowa's S/L ratio of 1.12 (Fn=.33) puts it at an extremely unfavorable point on the wave drag curve, just right of the resistance "hump" at N=3 (Fn=.31). The Iowa's designers therefore had very good reasons to concentrate on ameliorating wave drag.

By contrast, the 40kn megaship (S/L=.70, Fn = .21) is well left of N=4 on Figure 10.5 above.

Also this diagram from Wikipedia illustrates the basic issue fairly well:



So megaship's main resistance issue is friction resistance, Iowa's is avoiding wavemaking resistance.

Iowa's hull form follows the basic precepts for ameliorating wavemaking resistance.

KEY CONCEPT:
Friction resistance follows different optimization parameters from wavemaking resistance.

Friction resistance is largely a matter of wetted area, pressure coefficients, and speed.

The pressure coefficient is applied to wetted area to give total friction resistance of a given hull:



[note- I've been using "friction" interchangeably with what the text describes as "skin friction" plus "viscous pressure" resistance. Shorthand terms vary here and in aerodynamics in my experience]

If you "plug and play" with realistic numbers, you'll see there's a tradeoff between total wetted area and friction coefficient. The minimal wetted area enclosing a given volume (displacement) is, of course, a sphere. But a sphere (L:B ratio of one) has a friction coefficient so high that's it's better to have more wetted area (therefore more skin friction drag but less viscous pressure drag).

If wave drag were not an issue in ship design, the optimal L:B ratio (point at which friction drag is minimized) is usually around 5-6, depending on draft and other parameters (especially block coefficient).

Now let's circle back to the point about Speed-Length ratios and wave drag: It should be apparent that the lower the S:L, the closer to the friction-minimizing hull form we should be. Thus we can't look simply at raw speed, we have to look at speed versus size. Thus it makes sense that optimal L:B decreases with increasing size at a given raw speed (i.e. a 33kn BB has optimally lower L:B than a 33kn CA, which has optimally lower L:B than a 33kn DD).

Extending the reasoning of our BB-CA-DD L:B curve to megaship gets you most of the way to understanding why a fast megaBB has higher L:B than a normal BB. Of course it doesn't cover the even faster MegaBB directly, for that we need step further along our understanding of the fundamentals.

...but I've written enough already, willing to answer additional questions though.

[TheMarcksPlan]

To tie up a few additional points ...

HP/Displacement tradeoff:

Because displacement and volume scale cubically with linear dimensions while the primary driver of friction resistance (wetted area) scales quadratically, a larger ship always has lower HP/displacement than a smaller for a given speed (even absent the wavemaking dynamic discussed above).

This has important design optimization implications:

• The relationship between citadel length and powerplant is effectively severed: unless we want megaship to do >50kn, powerplant occupies a small portion of its hull/citadel (I estimate only 5% of hull).
• 1,000 tons of displacement (e.g. a large turret and its guns) costs far less - in HP terms - on megaship than on a normal ship.

...for those reasons, the megaship designer's tradeoffs differ fundamentally from the normal BB's designer. Whereas the normal BB designer must hew closely to maximizing the HP/ton ratio, for the megaship the HP/ton ratio is a small part of the overall equation. Megaship designers can accept sub-optimal propulsion efficiency in exchange for enormous gains in firepower, protection, aviation, and/or transport capacity.

...so even though a 7:1 L:B might give the megaship better HP/ton (per my calculations it's close), that's a very minor consideration in overall efficiency.

pre-proximity fused AA functionally useless, rather like the supposedly AA capable 18.1" on Yamato.

No idea what you're talking about. Are you assuming I'm using 18in shells akin to Yamato's odd AA shells?

If so, no.

Deeper than many seas where a ship will possibly be needed.

As already conceded, Java Sea and southern North Sea are probably foreclosed.

I has just curious as such a massively variable draught hull and massive speed ambitions presumably isnt relying on 15ft propellors for actual general movement?

I didn't specify main (i.e. stern) propeller size, only their aggregate area (36% of stern cross-section below 10ft depth, as on Iowa). That's 40 25ft props. You can choose different prop #'s and diameters with basic arithmetic.

Would need a very large magazine!

Better build a very large ship then!

Breakdown of volume distribution in my crude spreadsheet:

[TheMarcksPlan]

Re elevation of main guns...

To scale how much power we need let's compare to another long-range, large-caliber, AA-capable gun: the 8in/55RF Mk16, which elevates at 8.2 deg/sec using 25hp motors.

At equal elevation rate, the 18in gun's necessary HP should scale linearly with weight, which we'll scale cubically* with diameter.
*An overestimate as the entire loading apparatus elevates on the Mk16 (thus 10 rds/min), whereas here only the gun rotates (rammers/trays fixed).

...that gives us 285hp. Across 1,000 guns that means 285,000hp for elevating motors.

...a fact included/anticipated in my 1mil HP budget for non-propulsion generating capacity.

This HP requirement dovetails nicely with specifying large rammers for the higher loading angle: we'll never be ramming and elevating at once so the higher of rammer/elevator HP sets the generating capacity parameter. On Mk16 the rammer motor was 35HP. That's for 10/min RoF, however, and we don't need quite that rate of fire. As both elevator and rammer should scale similarly, let's slow the rammer by 15%, adding maybe half a second to firing cycle. As HP ~ speed squared, that equalizes rammer and elevator HP at 285HP/gun and 285k total [35 * .85^2. = ~25]

By comparison, Iowa's used 60hp for elevating and ramming.

So I'm adding >200k HP generating capacity (i.e. a whole Iowa powerplant) and machinery just to attain the higher elevation/ramming rates.

It's expensive but it enables a ~30x delta to our AA firepower, and, as being an invulnerable deathtrap for enemy airpower is at the heart of the megaship concept, it's well worth the expense.

[TheMarcksPlan]

At equal elevation rate, the 18in gun's necessary HP should scale linearly with weight, which we'll scale cubically* with diameter.
*An overestimate as the entire loading apparatus elevates on the Mk16 (thus 10 rds/min), whereas here only the gun rotates (rammers/trays fixed).

Forgot that NavWeaps gives the oscillating weight of the gun+loading apparatus: 51.6 tons.

An 18in gun escalated cubically from the 16in/50 weighs 170.3 tons or 3.3x the 8inRF. So my cubic escalation overestimated by a factor of ~3.4.

Instead of reducing our 285hp elevating motors, however, (they already draw on a reasonable generating capacity constraint for the rammers), let's keep our big elevating motors and increase our elevation rate instead.

Supposing the 8inRF system were elevating the 8in Mk15's oscillating weight of 29.1 tons, and scaling elevating rate with Sqrt(oscillating weight), we'd get 10.9 degrees elevation rate.

Let's stick with that for the megaship.

That means 4.6 seconds to elevate/depress from the 45degree loading angle to 70 degrees.

With a 12-second firing cycle, that leaves 7.4 seconds for ramming to maintain 5 rpm against high AC.

...which is very doable because loading the trays happens in a compartment separated from the gun house and therefore occurs during firing (not the case with all other BB guns afaik - certainly not for Iowas).

As we'll see, 5 rpm can probably be exceeded in short bursts (such as when hundreds of heavy bombers are approaching).

[Terry Duncan]

No idea what you're talking about. Are you assuming I'm using 18in shells akin to Yamato's odd AA shells?

If so, no.

So, what is your intention? To make them crash via air turbulance?

[TheMarcksPlan]

No idea what you're talking about. Are you assuming I'm using 18in shells akin to Yamato's odd AA shells?

If so, no.

So, what is your intention? To make them crash via air turbulance?

Normal AA shells. Just bigger.

[Andy H]

No idea what you're talking about. Are you assuming I'm using 18in shells akin to Yamato's odd AA shells?

If so, no.

So, what is your intention? To make them crash via air turbulance?

Normal AA shells. Just bigger.

Hi TMP

Given that the Yamato's gun barrel life was rather low if memory serves, why waste that limited life shooting at aircraft?

Regards

Andy H

[T. A. Gardner]

That means 4.6 seconds to elevate/depress from the 45degree loading angle to 70 degrees.

With a 12-second firing cycle, that leaves 7.4 seconds for ramming to maintain 5 rpm against high AC.

...which is very doable because loading the trays happens in a compartment separated from the gun house and therefore occurs during firing (not the case with all other BB guns afaik - certainly not for Iowas).

As we'll see, 5 rpm can probably be exceeded in short bursts (such as when hundreds of heavy bombers are approaching).

I assume you are using bagged powder here. That means the loading sequence is (briefly).

Set the loading tray between the rammer and gun in place once it reaches loading position.
Before loading the gun captain looks in the breech to ensure it was evacuated properly after the last shot by compressed air.
Assuming the shell is already on the rammer, once the gun captain knows the tray is correctly in place he orders the shell rammed in at high speed.
Now the powder bags are unloaded from the hoist and placed on the loading tray. This might be a two step process depending on the length of the ramming tray and the size of the powder hoist (eg., it delivers say half the powder at a time due to size constraints)
Ramming the powder occurs at the lowest speed the rammer can move forward at. It is very gently placed in the breech.
The gun captain inspects the positioning of the powder once rammed to ensure the last bag is in the proper position to be ignited.
The loading tray is retracted
The breech is closed.
The gun captain inserts a firing cartridge and connects it up (these are fired electrically).
The gun ready signal is sent to the turret fire control room and to central FC if they're in the loop.
The gun is then fired when the ship reaches a level position in roll, pitch, etc., as the gun isn't stabilized. This ensures a more accurate shot.

I really, seriously, doubt you can do that in 12 seconds and even more so that a crew in the gun turret can keep it up. One slip up with the powder and you are out one turret...

[Richard Anderson]

Um, how do you get a call it 2,900 pound projectile to seat vertically? Why does it not simply fall out when the rammer retracts? Ditto for the probably five or six 125-odd pound bags?

Its almost like the reason why heavy guns were not loaded at high elevation is ignored just to make the neato idea work?

Maybe a giant revolver cannon would work, especially since there is apparently unlimited space, unlimited power, zero engineering constraints, and oodles of bucks to spend on R&D during the Great Depression available to make it work.

[Carl Schwamberger]

Um, how do you get a call it 2,900 pound projectile to seat vertically? Why does it not simply fall out when the rammer retracts? Ditto for the probably five or six 125-odd pound bags?

That is so embarrassing when it happens.

[T. A. Gardner]

Um, how do you get a call it 2,900 pound projectile to seat vertically? Why does it not simply fall out when the rammer retracts? Ditto for the probably five or six 125-odd pound bags?

Its almost like the reason why heavy guns were not loaded at high elevation is ignored just to make the neato idea work?

Maybe a giant revolver cannon would work, especially since there is apparently unlimited space, unlimited power, zero engineering constraints, and oodles of bucks to spend on R&D during the Great Depression available to make it work.

Oh, don't be so hard on him. If you ram the shell in hard enough, it'd seat in the rifling and not fall out. Powder? Velcro or something? I suppose it's possible something could be worked out for that... Besides, it's only 45 degrees, not 90... Of course that loading angle is a little difficult when you're shooting at a surface target...

[Richard Anderson]

Oh, don't be so hard on him. If you ram the shell in hard enough, it'd seat in the rifling and not fall out. Powder? Velcro or something? I suppose it's possible something could be worked out for that... Besides, it's only 45 degrees, not 90... Of course that loading angle is a little difficult when you're shooting at a surface target...

I have my doubts that a hard ram would work properly other than occasionally. I foresee lots of broken driving bands, heavy scoring of the rifling, lack of accuracy...and close enough only counts in horseshoes and hand grenades. What happens when it slips in the second or so just as the powder bags are presented to the breech? Call it a ton and a half of steel crashing down on 800 pounds of powder. Somehow I don't think the powder bags will remain intact.

Velcro? Sure, in the massive American militarism spending of the 1920s and 1930s, an American Ordnance engineer just happens to take a quiet walk in the woods and wonders why burdock seeds stick to his pants, has a eureka moment and realizes, "what a perfect way to attach bagged powder to projectiles!"

I thought this was an AA gun? So firing at angles of elevation between around 60 and 90 degrees? So now after firing, it recoils, returns to 45 degree elevation for reloading, then gets loaded and elevated back to 90 degrees...all within a 12-second firing cycle? Slinging a 177-ton rifle around like a Silent Drill Platoon Marine at 8th & I tosses his M1 Garand about.

[T. A. Gardner]

Instead of a chain rammer, use a real hydraulic cylinder with a pretty high operating pressure. For pushing the powder tone it down with a bleed-off valve. What's a ton or three of rammer compared to less than a ton of chain rammer?

[TheMarcksPlan]

Hi TMP

Given that the Yamato's gun barrel life was rather low if memory serves, why waste that limited life shooting at aircraft?

Good question. A few points:

First, American guns seem have had better life than Japanese (surely due to better metallurgy).

Second and related, IMO the barrel life analogy would be to the Iowa's 16in Mk7's. These had a barrel life of 300 full-charge super-heavy shells ("ESR), for an 18in escalation probably around 270. Mk7's HC rounds, however, were only .43 ESR so barrel life of 697 shooting only those. Furthermore, USN HC rounds were designed heavier than your typical AA because they were expected to be useful against concrete fortifications. So even escalated to 18in, a higher-capacity AA shell should give close to seven hundred rounds.

Third, with 1,000 guns, any reasonably-foreseeable enemy force will no outlive the barrels. At 10 rds/kill against AC, for example, 5min of firing would be expected to splash 2,500 aircraft. Same general point for surface targets: If we assume (IMO very generously) that 5 plunging/penetrating 18in shells will kill a battleship (IMO ~1/3 would blow up with 1 such hit), at 1% long-range accuracy 15min of firing (75,000 rounds) is enough to kill 150 enemy BB's. That's 75 rds/barrel or ~30% of effective life per gun liner.

Fourth Because this ship can use barrage fire techniques against air and land targets, broad shell dispersion has the virtue of ensuring a random spread over a given large area/volume. If shell dispersion doubled you'd aim twice as many barrels at the aiming point when using barrage fire (whereas with tighter spreads you cover the same area with two aiming points). So even past effective barrel life, the fall off in accuracy doesn't seriously compromise the effect of 5,000 massive shells flying about. As the fatigue life of the firing chamber was rated at 1,500 cycles, in a pinch the megaship could keep destroying things well past its barrel's "effective" life (if shooting at a single ship/AC, it would require more ammo of course).

In fact it may optimal to design the megaship's gun chambers for "super charges" at barrel life of 100 or so. You'd only use this capability in critical situations such as when Japan has spent half its GDP on a ship or fleet of aircraft specifically designed to challenge megaship. That'd cost some in some in weight and machinery of course but megaship has room to play with weight and machinery.

[TheMarcksPlan]

If you ram the shell in hard enough, it'd seat in the rifling and not fall out.

Right. Every high-angle gun relies on this (that it's usually a case+shell rammed versus a shell doesn't seem determinative).

Instead of a chain rammer, use a real hydraulic cylinder with a pretty high operating pressure. For pushing the powder tone it down with a bleed-off valve. What's a ton or three of rammer compared to less than a ton of chain rammer?

This is basically what I was thinking, though the switch-over to a hydraulic rammer is an original and good idea.

Even without hydraulics, we've already specified a very powerful rammer. We can therefore specify - if necessary - the bags be rammed while being slightly compressed and therefore creating binding friction against the chamber walls. If necessary, we can specify powder bags with a compressible outer layer (i.e. if powder's compression characteristics aren't sufficient for the job). Of course we don't want to compress the powder sufficiently to ignite it. I don't know what that limit is but intuitively seems far afield of a slight squeeze in the chamber.

That means the loading sequence is (briefly)

Yeah I'm familiar - I too have the internet.

I'll probably get to putting together a firing cycle timeline, depending on other demands.

For now, note again the differences between the normal BB cycle and what's possible under the conditions I've already specified, plus conditions used on other contemporary guns:

• Powder tray loading in Iowa goes in two stages, presumably to save turret room required by a longer tray. Here there's no such space limitation; that alone saves several seconds.
• Powder/shell tray loading does not begin until the captain indicates the breach is clear. Here the trays are separated from the gunhouse by a steel partition (say 1in thick), which opens only for ramming. Therefore breach opening and inspection does not delay tray loading.
• We'll use an automatic breach as on Des Moines.
• There is no separate spanning tray, the loaded tray is rammed up to the breach by the rammer, after which the rammer continues pushing only the powder/shell. As the weight of tray is negligible compared to powder/shell, I haven't included it in my rammer HP calculation yet. Maybe a 5% HP delta.
• We'll use an automatic detonation system, as on Des Moines. Thus no captain sticking things in the breach, saving another few seconds.

Anyway, I've already thought of a lot of this stuff, just as I'd already noticed that a B-25 is bigger than an F4F, even before you pointed that out.

As with B-25's versus F4F's, the answer to a lot of your points regards having and using a lot more space. Because it's a lot bigger boat. [Also using a lot more HP, for which there's a lot more space because...]